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Processes, Threads, and Jobs in the Windows Operating System

Thread Scheduling

This section describes the Windows scheduling policies and algorithms. The first subsection provides a condensed description of how scheduling works on Windows and a definition of key terms. Then Windows priority levels are described from both the Windows API and the Windows kernel points of view. After a review of the relevant Windows functions and Windows utilities and tools that relate to scheduling, the detailed data structures and algorithms that make up the Windows scheduling system are presented, with uniprocessor systems examined first and then multiprocessor systems.

Overview of Windows Scheduling

Windows implements a priority-driven, preemptive scheduling system—the highest-priority runnable (ready) thread always runs, with the caveat that the thread chosen to run might be limited by the processors on which the thread is allowed to run, a phenomenon called processor affinity. By default, threads can run on any available processor, but you can alter processor affinity by using one of the Windows scheduling functions listed in Table 5-15 (shown later in the chapter) or by setting an affinity mask in the image header.

When a thread is selected to run, it runs for an amount of time called a quantum. A quantum is the length of time a thread is allowed to run before another thread at the same priority level (or higher, which can occur on a multiprocessor system) is given a turn to run. Quantum values can vary from system to system and process to process for any of three reasons: system configuration settings (long or short quantums), foreground/background status of the process, or use of the job object to alter the quantum. (Quantums are described in more detail in the Quantum section later in the chapter.) A thread might not get to complete its quantum, however. Because Windows implements a preemptive scheduler, if another thread with a higher priority becomes ready to run, the currently running thread might be preempted before finishing its time slice. In fact, a thread can be selected to run next and be preempted before even beginning its quantum!

The Windows scheduling code is implemented in the kernel. There’s no single “scheduler” module or routine, however—the code is spread throughout the kernel in which scheduling-related events occur. The routines that perform these duties are collectively called the kernel’s dispatcher. The following events might require thread dispatching:

  • A thread becomes ready to execute—for example, a thread has been newly created or has just been released from the wait state.

  • A thread leaves the running state because its time quantum ends, it terminates, it yields execution, or it enters a wait state.

  • A thread’s priority changes, either because of a system service call or because Windows itself changes the priority value.

  • A thread’s processor affinity changes so that it will no longer run on the processor on which it was running.

At each of these junctions, Windows must determine which thread should run next. When Windows selects a new thread to run, it performs a context switch to it. A context switch is the procedure of saving the volatile machine state associated with a running thread, loading another thread’s volatile state, and starting the new thread’s execution.

As already noted, Windows schedules at the thread granularity. This approach makes sense when you consider that processes don’t run but only provide resources and a context in which their threads run. Because scheduling decisions are made strictly on a thread basis, no consideration is given to what process the thread belongs to. For example, if process A has 10 runnable threads, process B has 2 runnable threads, and all 12 threads are at the same priority, each thread would theoretically receive one-twelfth of the CPU time—Windows wouldn’t give 50 percent of the CPU to process A and 50 percent to process B.

Priority Levels

To understand the thread-scheduling algorithms, you must first understand the priority levels that Windows uses. As illustrated in Figure 5-12, internally Windows uses 32 priority levels, ranging from 0 through 31. These values divide up as follows:

  • Sixteen real-time levels (16 through 31)

  • Fifteen variable levels (1 through 15)

  • One system level (0), reserved for the zero page thread

    Figure 5-12

    Figure 5-12. Thread priority levels

Thread priority levels are assigned from two different perspectives: those of the Windows API and those of the Windows kernel. The Windows API first organizes processes by the priority class to which they are assigned at creation (Real-time, High, Above Normal, Normal, Below Normal, and Idle) and then by the relative priority of the individual threads within those processes (Time-critical, Highest, Above-normal, Normal, Below-normal, Lowest, and Idle).

In the Windows API, each thread has a base priority that is a function of its process priority class and its relative thread priority. The mapping from Windows priority to internal Windows numeric priority is shown in Figure 5-13.

Figure 5-13

Figure 5-13. Mapping of Windows kernel priorities to the Windows API

Whereas a process has only a single base priority value, each thread has two priority values: current and base. Scheduling decisions are made based on the current priority. As explained in the following section on priority boosting, the system under certain circumstances increases the priority of threads in the dynamic range (1 through 15) for brief periods. Windows never adjusts the priority of threads in the real-time range (16 through 31), so they always have the same base and current priority.

A thread’s initial base priority is inherited from the process base priority. A process, by default, inherits its base priority from the process that created it. This behavior can be overridden on the CreateProcess function or by using the command-line start command. A process priority can also be changed after being created by using the SetPriorityClass function or various tools that expose that function, such as Task Manager and Process Explorer (by right-clicking on the process and choosing a new priority class). For example, you can lower the priority of a CPU-intensive process so that it does not interfere with normal system activities. Changing the priority of a process changes the thread priorities up or down, but their relative settings remain the same. It usually doesn’t make sense, however, to change individual thread priorities within a process, because unless you wrote the program or have the source code, you don’t really know what the individual threads are doing, and changing their relative importance might cause the program not to behave in the intended fashion.

Normally, the process base priority (and therefore the starting thread base priority) will default to the value at the middle of each process priority range (24, 13, 10, 8, 6, or 4). However, some Windows system processes (such as the Session Manager, service controller, and local security authentication server) have a base process priority slightly higher than the default for the Normal class (8). This higher default value ensures that the threads in these processes will all start at a higher priority than the default value of 8. These system processes use an internal system call (NtSetInformationProcess) to set their process base priority to a numeric value other than the normal default starting base priority.

Windows Scheduling APIs

The Windows API functions that relate to thread scheduling are listed in Table 5-15. (For more information, see the Windows API reference documentation.)

Table 5-15. Scheduling-Related APIs and Their Functions




Suspends or resumes a paused thread from execution.


Returns or sets a process’s priority class (base priority).


Returns or sets a thread’s priority (relative to its process base priority).


Returns or sets a process’s affinity mask.


Sets a thread’s affinity mask (must be a subset of the process’s affinity mask) for a particular set of processors, restricting it to running on those processors.


Sets attributes for a job; some of the attributes affect scheduling, such as affinity and priority. (See the Job Objects section later in the chapter for a description of the job object.)


Returns details about processor hardware configuration (for hyperthreaded and NUMA systems).


Returns or sets the ability for Windows to boost the priority of a thread temporarily. (This ability applies only to threads in the dynamic range.)


Establishes a preferred processor for a particular thread, but doesn’t restrict the thread to that processor.


Returns or sets the default priority boost control state of the current process. (This function is used to set the thread priority boost control state when a thread is created.)


Puts the current thread into a wait state until the specified object(s) is/are satisfied, or until the specified time interval (figured in milliseconds [msec]) expires, if given.


Yields execution to another thread (at priority 1 or higher) that is ready to run on the current processor.


Puts the current thread into a wait state for a specified time interval (figured in milliseconds [msec]). A zero value relinquishes the rest of the thread’s quantum.


Causes the current thread to go into a wait state until either an I/O completion callback is completed, an APC is queued to the thread, or the specified time interval ends.

Relevant Tools

You can change (and view) the base process priority with Task Manager and Process Explorer. You can kill individual threads in a process with Process Explorer (which should be done, of course, with extreme care).

You can view individual thread priorities with the Reliability and Performance Monitor, Process Explorer, or WinDbg. While it might be useful to increase or lower the priority of a process, it typically does not make sense to adjust individual thread priorities within a process because only a person who thoroughly understands the program (in other words, typically only the developer himself) would understand the relative importance of the threads within the process.

The only way to specify a starting priority class for a process is with the start command in the Windows command prompt. If you want to have a program start every time with a specific priority, you can define a shortcut to use the start command by beginning the command with cmd /c. This runs the command prompt, executes the command on the command line, and terminates the command prompt. For example, to run Notepad in the low-process priority, the shortcut would be cmd /c start /low Notepad.exe.

Real-Time Priorities

You can raise or lower thread priorities within the dynamic range in any application; however, you must have the increase scheduling priority privilege to enter the real-time range. Be aware that many important Windows kernel-mode system threads run in the real-time priority range, so if threads spend excessive time running in this range, they might block critical system functions (such as in the memory manager, cache manager, or other device drivers).

Thread States

Before you can comprehend the thread-scheduling algorithms, you need to understand the various execution states that a thread can be in. Figure 5-14 illustrates the state transitions for threads. (The numeric values shown represent the value of the thread state performance counter.) More details on what happens at each transition are included later in this section.

The thread states are as follows:

  • Ready A thread in the ready state is waiting to execute. When looking for a thread to execute, the dispatcher considers only the pool of threads in the ready state.

  • Deferred ready This state is used for threads that have been selected to run on a specific processor but have not yet been scheduled. This state exists so that the kernel can minimize the amount of time the systemwide lock on the scheduling database is held.

  • Standby A thread in the standby state has been selected to run next on a particular processor. When the correct conditions exist, the dispatcher performs a context switch to this thread. Only one thread can be in the standby state for each processor on the system. Note that a thread can be preempted out of the standby state before it ever executes (if, for example, a higher priority thread becomes runnable before the standby thread begins execution).

  • Running Once the dispatcher performs a context switch to a thread, the thread enters the running state and executes. The thread’s execution continues until its quantum ends (and another thread at the same priority is ready to run), it is preempted by a higher priority thread, it terminates, it yields execution, or it voluntarily enters the wait state.

  • Waiting A thread can enter the wait state in several ways: a thread can voluntarily wait for an object to synchronize its execution, the operating system can wait on the thread’s behalf (such as to resolve a paging I/O), or an environment subsystem can direct the thread to suspend itself. When the thread’s wait ends, depending on the priority, the thread either begins running immediately or is moved back to the ready state.

  • Gate Waiting When a thread does a wait on a gate dispatcher object (see Chapter 3 for more information on gates), it enters the gate waiting state instead of the waiting state. This difference is important when breaking a thread’s wait as the result of an APC. Because gates don’t use the dispatcher lock, but a per-object lock, the kernel needs to perform some unique locking operations when breaking the wait of a thread waiting on a gate and a way to differentiate this from a normal wait.

  • Transition A thread enters the transition state if it is ready for execution but its kernel stack is paged out of memory. Once its kernel stack is brought back into memory, the thread enters the ready state.

  • Terminated When a thread finishes executing, it enters the terminated state. Once the thread is terminated, the executive thread block (the data structure in nonpaged pool that describes the thread) might or might not be deallocated. (The object manager sets policy regarding when to delete the object.)

  • Initialized This state is used internally while a thread is being created.

    Figure 5-14

    Figure 5-14. Thread states and transitions

Dispatcher Database

To make thread-scheduling decisions, the kernel maintains a set of data structures known collectively as the dispatcher database, illustrated in Figure 5-15. The dispatcher database keeps track of which threads are waiting to execute and which processors are executing which threads.

To improve scalability, including thread-dispatching concurrency, Windows multiprocessor systems have per-processor dispatcher ready queues, as illustrated in Figure 5-15. In this way each CPU can check its own ready queues for the next thread to run without having to lock the systemwide ready queues. (Versions of Windows before Windows Server 2003 used a global database).

The per-processor ready queues, as well as the per-processor ready summary, are part of the processor control block (PRCB) structure. (To see the fields in the PRCB, type dt nt!_prcb in the kernel debugger.) The names of each component that we will talk about (in italics) are field members of the PRCB structure.

The dispatcher ready queues (DispatcherReadyListHead) contain the threads that are in the ready state, waiting to be scheduled for execution. There is one queue for each of the 32 priority levels. To speed up the selection of which thread to run or preempt, Windows maintains a 32-bit bit mask called the ready summary (ReadySummary). Each bit set indicates one or more threads in the ready queue for that priority level. (Bit 0 represents priority 0, and so on.)

Instead of scanning each ready list to see whether it is empty or not (which would make scheduling decisions dependent on the number of different priority threads), a single bit scan is performed as a native processor command to find the highest bit set. Regardless of the number of threads in the ready queue, this operation takes a constant amount of time, which is why you may sometimes see the Windows scheduling algorithm referred to as an O(1), or constant time, algorithm.

Figure 5-15

Figure 5-15. Windows multiprocessor dispatcher database

Table 5-16 lists the KPRCB fields involved in thread scheduling.

Table 5-16. Thread-Scheduling KPRCB Fields





Bitmask (32 bits)

Bitmask of priority levels that have one or more ready threads


Singly linked list

Single list head for the deferred ready queue


Array of 32 list entries

List heads for the 32 ready queues

The dispatcher database is synchronized by raising IRQL to SYNCH_LEVEL (which is defined as level 2). (For an explanation of interrupt priority levels, see the Trap Dispatching section in Chapter 3.) Raising IRQL in this way prevents other threads from interrupting thread dispatching on the processor because threads normally run at IRQL 0 or 1. However, on a multiprocessor system, more is required than just raising IRQL because other processors can simultaneously raise to the same IRQL and attempt to operate on the dispatcher database. How Windows synchronizes access to the dispatcher database is explained in the Multiprocessor Systems section later in the chapter.


As mentioned earlier in the chapter, a quantum is the amount of time a thread gets to run before Windows checks to see whether another thread at the same priority is waiting to run. If a thread completes its quantum and there are no other threads at its priority, Windows permits the thread to run for another quantum.

On Windows Vista, threads run by default for 2 clock intervals; on Windows Server systems, by default, a thread runs for 12 clock intervals. (We’ll explain how you can change these values later.) The rationale for the longer default value on server systems is to minimize context switching. By having a longer quantum, server applications that wake up as the result of a client request have a better chance of completing the request and going back into a wait state before their quantum ends.

The length of the clock interval varies according to the hardware platform. The frequency of the clock interrupts is up to the HAL, not the kernel. For example, the clock interval for most x86 uniprocessors is about 10 milliseconds, and for most x86 and x64 multiprocessors it is about 15 milliseconds. This clock interval is stored in the kernel variable KeMaximumIncrement as hundreds of nanoseconds.

Because of changes in thread run-time accounting in Windows Vista (briefly mentioned earlier in the thread activity experiment), although threads still run in units of clock intervals, the system does not use the count of clock ticks as the deciding factor for how long a thread has run and whether its quantum has expired. Instead, when the system starts up, a calculation is made whose result is the number of clock cycles that each quantum is equivalent to (this value is stored in the kernel variable KiCyclesPerClockQuantum). This calculation is made by multiplying the processor speed in Hz (CPU clock cycles per second) with the number of seconds it takes for one clock tick to fire (based on the KeMaximumIncrement value described above).

The end result of this new accounting method is that, as of Windows Vista, threads do not actually run for a quantum number based on clock ticks; they instead run for a quantum target, which represents an estimate of what the number of CPU clock cycles the thread has consumed should be when its turn would be given up. This target should be equal to an equivalent number of clock interval timer ticks because, as we’ve just seen, the calculation of clock cycles per quantum is based on the clock interval timer frequency, which you can check using the following experiment. On the other hand, because interrupt cycles are not charged to the thread, the actual clock time may be longer.

Quantum Accounting

Each process has a quantum reset value in the kernel process block. This value is used when creating new threads inside the process and is duplicated in the kernel thread block, which is then used when giving a thread a new quantum target. The quantum reset value is stored in terms of actual quantum units (we’ll discuss what these mean soon), which are then multiplied by the number of clock cycles per quantum, resulting in the quantum target.

As a thread runs, CPU clock cycles are charged at different events (context switches, interrupts, and certain scheduling decisions). If at a clock interval timer interrupt, the number of CPU clock cycles charged has reached (or passed) the quantum target, then quantum end processing is triggered. If there is another thread at the same priority waiting to run, a context switch occurs to the next thread in the ready queue.

Internally, a quantum unit is represented as one third of a clock tick (so one clock tick equals three quantums). This means that on Windows Vista systems, threads, by default, have a quantum reset value of 6 (2 * 3), and that Windows Server 2008 systems have a quantum reset value of 36 (12 * 3). For this reason, the KiCyclesPerClockQuantum value is divided by three at the end of the calculation previously described, since the original value would describe only CPU clock cycles per clock interval timer tick.

The reason a quantum was stored internally as a fraction of a clock tick rather than as an entire tick was to allow for partial quantum decay on wait completion on versions of Windows prior to Windows Vista. Prior versions used the clock interval timer for quantum expiration. If this adjustment were not made, it would have been possible for threads never to have their quantums reduced. For example, if a thread ran, entered a wait state, ran again, and entered another wait state but was never the currently running thread when the clock interval timer fired, it would never have its quantum charged for the time it was running. Because threads now have CPU clock cycles charged instead of quantums, and because this no longer depends on the clock interval timer, these adjustments are not required.

Controlling the Quantum

You can change the thread quantum for all processes, but you can choose only one of two settings: short (2 clock ticks, the default for client machines) or long (12 clock ticks, the default for server systems).

To change this setting, right-click on your computer name’s icon on the desktop, choose Properties, click the Advanced System Settings label, select the Advanced tab, click the Settings button in the Performance section, and finally click the Advanced tab. The dialog box displayed is shown in Figure 5-16.

Figure 5-16

Figure 5-16. Quantum configuration in the Performance Options dialog box

The Programs setting designates the use of short, variable quantums—the default for Windows Vista. If you install Terminal Services on Windows Server 2008 systems and configure the server as an application server, this setting is selected so that the users on the terminal server will have the same quantum settings that would normally be set on a desktop or client system. You might also select this manually if you were running Windows Server as your desktop operating system.

The Background Services option designates the use of long, fixed quantums—the default for Windows Server 2008 systems. The only reason you might select this option on a workstation system is if you were using the workstation as a server system.

One additional difference between the Programs and Background Services settings is the effect they have on the quantum of the threads in the foreground process. This is explained in the next section.

Quantum Boosting

When a window is brought into the foreground on a client system, all the threads in the process containing the thread that owns the foreground window have their quantums tripled. Thus, threads in the foreground process run with a quantum of 6 clock ticks, whereas threads in other processes have the default client quantum of 2 clock ticks. In this way, when you switch away from a CPU-intensive process, the new foreground process will get proportionally more of the CPU, because when its threads run they will have a longer turn than background threads (again, assuming the thread priorities are the same in both the foreground and background processes).

Note that this adjustment of quantums applies only to processes with a priority higher than Idle on systems configured to Programs in the Performance Options settings described in the previous section. Thread quantums are not changed for the foreground process on systems configured to Background Services (the default on Windows Server 2008 systems).

Quantum Settings Registry Value

The user interface to control quantum settings described earlier modifies the registry value HKLM\SYSTEM\CurrentControlSet\Control\PriorityControl\Win32PrioritySeparation. In addition to specifying the relative length of thread quantums (short or long), this registry value also defines whether or not threads in the foreground process should have their quantums boosted (and if so, the amount of the boost). This value consists of 6 bits divided into the three 2-bit fields shown in Figure 5-17.

Figure 5-17

Figure 5-17. Fields of the Win32PrioritySeparation registry value

The fields shown in Figure 5-17 can be defined as follows:

  • Short vs. Long A setting of 1 specifies long, and 2 specifies short. A setting of 0 or 3 indicates that the default will be used (short for Windows Vista, long for Windows Server 2008 systems).

  • Variable vs. Fixed A setting of 1 means to vary the quantum for the foreground process, and 2 means that quantum values don’t change for foreground processes. A setting of 0 or 3 means that the default (which is variable for Windows Vista and fixed for Windows Server 2008 systems) will be used.

  • Foreground Quantum Boost This field (stored in the kernel variable PsPrioritySeperation) must have a value of 0, 1, or 2. (A setting of 3 is invalid and treated as 2.) It is used as an index into a three-element byte array named PspForegroundQuantum to obtain the quantum for the threads in the foreground process. The quantum for threads in background processes is taken from the first entry in this quantum table. Table 5-17 shows the possible settings for PspForegroundQuantum.

Table 5-17. Quantum Values

















Note that when you’re using the Performance Options dialog box described earlier, you can choose from only two combinations: short quantums with foreground quantums tripled, or long quantums with no quantum changes for foreground threads. However, you can select other combinations by modifying the Win32PrioritySeparation registry value directly.

Scheduling Scenarios

Windows bases the question of “Who gets the CPU?” on thread priority; but how does this approach work in practice? The following sections illustrate just how priority-driven preemptive multitasking works on the thread level.

Voluntary Switch

First a thread might voluntarily relinquish use of the processor by entering a wait state on some object (such as an event, a mutex, a semaphore, an I/O completion port, a process, a thread, a window message, and so on) by calling one of the Windows wait functions (such as WaitForSingleObject or WaitForMultipleObjects). Waiting for objects is described in more detail in Chapter 3.

Figure 5-18 illustrates a thread entering a wait state and Windows selecting a new thread to run.

In Figure 5-18, the top block (thread) is voluntarily relinquishing the processor so that the next thread in the ready queue can run (as represented by the halo it has when in the Running column). Although it might appear from this figure that the relinquishing thread’s priority is being reduced, it’s not—it’s just being moved to the wait queue of the objects the thread is waiting for.

Figure 5-18

Figure 5-18. Voluntary switching


In this scheduling scenario, a lower-priority thread is preempted when a higher-priority thread becomes ready to run. This situation might occur for a couple of reasons:

  • A higher-priority thread’s wait completes. (The event that the other thread was waiting for has occurred.)

  • A thread priority is increased or decreased.

In either of these cases, Windows must determine whether the currently running thread should still continue to run or whether it should be preempted to allow a higher-priority thread to run.

When a thread is preempted, it is put at the head of the ready queue for the priority it was running at. Figure 5-19 illustrates this situation.

In Figure 5-19, a thread with priority 18 emerges from a wait state and repossesses the CPU, causing the thread that had been running (at priority 16) to be bumped to the head of the ready queue. Notice that the bumped thread isn’t going to the end of the queue but to the beginning; when the preempting thread has finished running, the bumped thread can complete its quantum.

Figure 5-19

Figure 5-19. Preemptive thread scheduling

Quantum End

When the running thread exhausts its CPU quantum, Windows must determine whether the thread’s priority should be decremented and then whether another thread should be scheduled on the processor.

If the thread priority is reduced, Windows looks for a more appropriate thread to schedule. (For example, a more appropriate thread would be a thread in a ready queue with a higher priority than the new priority for the currently running thread.) If the thread priority isn’t reduced and there are other threads in the ready queue at the same priority level, Windows selects the next thread in the ready queue at that same priority level and moves the previously running thread to the tail of that queue (giving it a new quantum value and changing its state from running to ready). This case is illustrated in Figure 5-20. If no other thread of the same priority is ready to run, the thread gets to run for another quantum.

Figure 5-20

Figure 5-20. Quantum end thread scheduling

As we’ve seen, instead of simply relying on a clock interval timer–based quantum to schedule threads, Windows uses an accurate CPU clock cycle count to maintain quantum targets. One factor we haven’t yet mentioned is that Windows also uses this count to determine whether quantum end is currently appropriate for the thread—something that may have happened previously and is important to discuss.

Under the scheduling model prior to Windows Vista, which relied only on the clock interval timer, the following situation could occur:

  • Threads A and B become ready to run during the middle of an interval (scheduling code runs not just at each clock interval, so this is often the case).

  • Thread A starts running but is interrupted for a while. The time spent handling the interrupt is charged to the thread.

  • Interrupt processing finishes, thread A starts running again, but it quickly hits the next clock interval. The scheduler can only assume that thread A had been running all this time and now switches to thread B.

  • Thread B starts running and has a chance to run for a full clock interval (barring pre-emption or interrupt handling).

In this scenario, thread A was unfairly penalized in two different ways. First of all, the time that it had to spend handling a device interrupt was accounted to its own CPU time, even though the thread had probably nothing to do with the interrupt. (Recall that interrupts are handled in the context of whichever thread had been running at the time.) It was also unfairly penalized for the time the system was idling inside that clock interval before it was scheduled.

Figure 5-21 represents this scenario.

Figure 5-21

Figure 5-21. Unfair time slicing in previous versions of Windows

Because Windows keeps an accurate count of the exact number of CPU clock cycles spent doing work that the thread was scheduled to do (which means excluding interrupts), and because it keeps a quantum target of clock cycles that should have been spent by the thread at the end of its quantum, both of the unfair decisions that would have been made against thread A will not happen in Windows.

Instead, the following situation will occur:

  • Threads A and B become ready to run during the middle of an interval.

  • Thread A starts running but is interrupted for a while. The CPU clock cycles spent handling the interrupt are not charged to the thread.

  • Interrupt processing finishes, thread A starts running again, but it quickly hits the next clock interval. The scheduler looks at the number of CPU clock cycles that have been charged to the thread and compares them to the expected CPU clock cycles that should have been charged at quantum end.

  • Because the former number is much smaller than it should be, the scheduler assumes that thread A started running in the middle of a clock interval and may have additionally been interrupted.

  • Thread A gets its quantum increased by another clock interval, and the quantum target is recalculated. Thread A now has its chance to run for a full clock interval.

  • At the next clock interval, thread A has finished its quantum, and thread B now gets a chance to run.

    Figure 5-22 represents this scenario.

    Figure 5-22

    Figure 5-22. Fair time slicing in current versions of Windows


When a thread finishes running (either because it returned from its main routine, called ExitThread, or was killed with TerminateThread), it moves from the running state to the terminated state. If there are no handles open on the thread object, the thread is removed from the process thread list and the associated data structures are deallocated and released.

Context Switching

A thread’s context and the procedure for context switching vary depending on the processor’s architecture. A typical context switch requires saving and reloading the following data:

  • Instruction pointer

  • Kernel stack pointer

  • A pointer to the address space in which the thread runs (the process’s page table directory)

The kernel saves this information from the old thread by pushing it onto the current (old thread’s) kernel-mode stack, updating the stack pointer, and saving the stack pointer in the old thread’s KTHREAD block. The kernel stack pointer is then set to the new thread’s kernel stack, and the new thread’s context is loaded. If the new thread is in a different process, it loads the address of its page table directory into a special processor register so that its address space is available. (See the description of address translation in Chapter 9.) If a kernel APC that needs to be delivered is pending, an interrupt at IRQL 1 is requested. Otherwise, control passes to the new thread’s restored instruction pointer and the new thread resumes execution.

Idle Thread

When no runnable thread exists on a CPU, Windows dispatches the per-CPU idle thread. Each CPU is allotted one idle thread because on a multiprocessor system one CPU can be executing a thread while other CPUs might have no threads to execute.

Various Windows process viewer utilities report the idle process using different names. Task Manager and Process Explorer call it “System Idle Process,” while Tlist calls it “System Process.” If you look at the EPROCESS structure’s ImageFileName member, you’ll see the internal name for the process is “Idle.” Windows reports the priority of the idle thread as 0 (15 on x64 systems). In reality, however, the idle threads don’t have a priority level because they run only when there are no real threads to run—they are not scheduled and never part of any ready queues. (Remember, only one thread per Windows system is actually running at priority 0—the zero page thread, explained in Chapter 9.)

Apart from priority, there are many other fields in the idle process or its threads that may be reported as 0. This occurs because the idle process is not an actual full-blown object manager process object, and neither are its idle threads. Instead, the initial idle thread and idle process objects are statically allocated and used to bootstrap the system before the process manager initializes. Subsequent idle thread structures are allocated dynamically as additional processors are brought online. Once process management initializes, it uses the special variable PsIdleProcess to refer to the idle process.

Apart from some critical fields provided so that these threads and their process can have a PID and name, everything else is ignored, which means that query APIs may simply return zeroed data.

The idle loop runs at DPC/dispatch level, polling for work to do, such as delivering deferred procedure calls (DPCs) or looking for threads to dispatch to. Although some details of the flow vary between architectures, the basic flow of control of the idle thread is as follows:

  1. Enables and disables interrupts (allowing any pending interrupts to be delivered).

  2. Checks whether any DPCs (described in Chapter 3) are pending on the processor. If DPCs are pending, clears the pending software interrupt and delivers them. (This will also perform timer expiration, as well as deferred ready processing. The latter is explained in the upcoming multiprocessor scheduling section.)

  3. Checks whether a thread has been selected to run next on the processor, and if so, dispatches that thread.

  4. Calls the registered power management processor idle routine (in case any power management functions need to be performed), which is either in the processor power driver (such as intelppm.sys) or in the HAL if such a driver is unavailable.

  5. On debug systems, checks if there is a kernel debugger trying to break into the system and gives it access.

  6. If requested, checks for threads waiting to run on other processors and schedules them locally. (This operation is also explained in the upcoming multiprocessor scheduling section.)

Priority Boosts

In six cases, the Windows scheduler can boost (increase) the current priority value of threads:

  • On completion of I/O operations

  • After waiting for executive events or semaphores

  • When a thread has been waiting on an executive resource for too long

  • After threads in the foreground process complete a wait operation

  • When GUI threads wake up because of windowing activity

  • When a thread that’s ready to run hasn’t been running for some time (CPU starvation)

The intent of these adjustments is to improve overall system throughput and responsiveness as well as resolve potentially unfair scheduling scenarios. Like any scheduling algorithms, however, these adjustments aren’t perfect, and they might not benefit all applications.

Windows Vista adds one more scenario in which a priority boost can occur, multimedia playback. Unlike the other priority boosts, which are applied directly by kernel code, multimedia playback boosts are managed by a user-mode service called the MultiMedia Class Scheduler Service (MMCSS). (Although the boosts are still done in kernel mode, the request to boost the threads is managed by this user-mode service.) We’ll first cover the typical kernel-managed priority boosts and then talk about MMCSS and the kind of boosting it performs.

Priority Boosting after I/O Completion

Windows gives temporary priority boosts upon completion of certain I/O operations so that threads that were waiting for an I/O will have more of a chance to run right away and process whatever was being waited for. Recall that 1 quantum unit is deducted from the thread’s remaining quantum when it wakes up so that I/O bound threads aren’t unfairly favored. Although you’ll find recommended boost values in the Windows Driver Kit (WDK) header files (by searching for “#define IO” in Wdm.h or Ntddk.h), the actual value for the boost is up to the device driver. (These values are listed in Table 5-18.) It is the device driver that specifies the boost when it completes an I/O request on its call to the kernel function IoCompleteRequest. In Table 5-18, notice that I/O requests to devices that warrant better responsiveness have higher boost values.

Table 5-18. Recommended Boost Values



Disk, CD-ROM, parallel, video


Network, mailslot, named pipe, serial


Keyboard, mouse




The boost is always applied to a thread’s current priority, not its base priority. As illustrated in Figure 5-23, after the boost is applied, the thread gets to run for one quantum at the elevated priority level. After the thread has completed its quantum, it decays one priority level and then runs another quantum. This cycle continues until the thread’s priority level has decayed back to its base priority. A thread with a higher priority can still preempt the boosted thread, but the interrupted thread gets to finish its time slice at the boosted priority level before it decays to the next lower priority.

Figure 5-23

Figure 5-23. Priority boosting and decay

As noted earlier, these boosts apply only to threads in the dynamic priority range (0 through 15). No matter how large the boost is, the thread will never be boosted beyond level 15 into the real-time priority range. In other words, a priority 14 thread that receives a boost of 5 will go up to priority 15. A priority 15 thread that receives a boost will remain at priority 15.

Boosts After Waiting for Events and Semaphores

When a thread that was waiting for an executive event or a semaphore object has its wait satisfied (because of a call to the function SetEvent, PulseEvent, or ReleaseSemaphore), it receives a boost of 1. (See the value for EVENT_ INCREMENT and SEMAPHORE_INCREMENT in the WDK header files.) Threads that wait for events and semaphores warrant a boost for the same reason that threads that wait for I/O operations do—threads that block on events are requesting CPU cycles less frequently than CPU-bound threads. This adjustment helps balance the scales.

This boost operates the same as the boost that occurs after I/O completion, as described in the previous section:

  • The boost is always applied to the base priority (not the current priority).

  • The priority will never be boosted above 15.

  • The thread gets to run at the elevated priority for its remaining quantum (as described earlier, quantums are reduced by 1 when threads exit a wait) before decaying one priority level at a time until it reaches its original base priority.

A special boost is applied to threads that are awoken as a result of setting an event with the special functions NtSetEventBoostPriority(used in Ntdll.dll for critical sections) and KeSetEventBoostPriority (used for executive resources) or if a signaling gate is used (such as with pushlocks). If a thread waiting for an event is woken up as a result of the special event boost function and its priority is 13 or below, it will have its priority boosted to be the setting thread’s priority plus one. If its quantum is less than 4 quantum units, it is set to 4 quantum units. This boost is removed at quantum end.

Boosts During Waiting on Executive Resources

When a thread attempts to acquire an executive resource (ERESOURCE; see Chapter 3 for more information on kernel synchronization objects) that is already owned exclusively by another thread, it must enter a wait state until the other thread has released the resource. To avoid deadlocks, the executive performs this wait in intervals of five seconds instead of doing an infinite wait on the resource.

At the end of these five seconds, if the resource is still owned, the executive will attempt to prevent CPU starvation by acquiring the dispatcher lock, boosting the owning thread or threads, and performing another wait. Because the dispatcher lock is held and the thread’s WaitNext flag is set to TRUE, this ensures a consistent state during the boosting process until the next wait is done.

This boost operates in the following manner:

  • The boost is always applied to the base priority (not the current priority) of the owner thread.

  • The boost raises priority to 14.

  • The boost is only applied if the owner thread has a lower priority than the waiting thread, and only if the owner thread’s priority isn’t already 14.

  • The quantum of the thread is reset so that the thread gets to run at the elevated priority for a full quantum, instead of only the quantum it had left. Just like other boosts, at each quantum end, the priority boost will slowly decrease by one level.

Because executive resources can be either shared or exclusive, the kernel will first boost the exclusive owner and then check for shared owners and boost all of them. When the waiting thread enters the wait state again, the hope is that the scheduler will schedule one of the owner threads, which will have enough time to complete its work and release the resource. It’s important to note that this boosting mechanism is used only if the resource doesn’t have the Disable Boost flag set, which developers can choose to set if the priority inversion mechanism described here works well with their usage of the resource.

Additionally, this mechanism isn’t perfect. For example, if the resource has multiple shared owners, the executive will boost all those threads to priority 14, resulting in a sudden surge of high-priority threads on the system, all with full quantums. Although the exclusive thread will run first (since it was the first to be boosted and therefore first on the ready list), the other shared owners will run next, since the waiting thread’s priority was not boosted. Only until after all the shared owners have gotten a chance to run and their priority decreased below the waiting thread will the waiting thread finally get its chance to acquire the resource. Because shared owners can promote or convert their ownership from shared to exclusive as soon as the exclusive owner releases the resource, it’s possible for this mechanism not to work as intended.

Priority Boosts for Foreground Threads After Waits

Whenever a thread in the foreground process completes a wait operation on a kernel object, the kernel function KiUnwaitThread boosts its current (not base) priority by the current value of PsPrioritySeperation. (The windowing system is responsible for determining which process is considered to be in the foreground.) As described in the section on quantum controls, PsPrioritySeperation reflects the quantum-table index used to select quantums for the threads of foreground applications. However, in this case, it is being used as a priority boost value.

The reason for this boost is to improve the responsiveness of interactive applications—by giving the foreground application a small boost when it completes a wait, it has a better chance of running right away, especially when other processes at the same base priority might be running in the background.

Unlike other types of boosting, this boost applies to all Windows systems, and you can’t disable this boost, even if you’ve disabled priority boosting using the Windows SetThreadPriorityBoost function.

Priority Boosts After GUI Threads Wake Up

Threads that own windows receive an additional boost of 2 when they wake up because of windowing activity such as the arrival of window messages. The windowing system (Win32k.sys) applies this boost when it calls KeSetEvent to set an event used to wake up a GUI thread. The reason for this boost is similar to the previous one—to favor interactive applications.

Priority Boosts for CPU Starvation

Imagine the following situation: you have a priority 7 thread that’s running, preventing a priority 4 thread from ever receiving CPU time; however, a priority 11 thread is waiting for some resource that the priority 4 thread has locked. But because the priority 7 thread in the middle is eating up all the CPU time, the priority 4 thread will never run long enough to finish whatever it’s doing and release the resource blocking the priority 11 thread. What does Windows do to address this situation?

We have previously seen how the executive code responsible for executive resources manages this scenario by boosting the owner threads so that they can have a chance to run and release the resource. However, executive resources are only one of the many synchronization constructs available to developers, and the boosting technique will not apply to any other primitive. Therefore, Windows also includes a generic CPU starvation relief mechanism as part of a thread called the balance set manager (a system thread that exists primarily to perform memory management functions and is described in more detail in Chapter 9).

Once per second, this thread scans the ready queues for any threads that have been in the ready state (that is, haven’t run) for approximately 4 seconds. If it finds such a thread, the balance set manager boosts the thread’s priority to 15 and sets the quantum target to an equivalent CPU clock cycle count of 4 quantum units. Once the quantum is expired, the thread’s priority decays immediately to its original base priority. If the thread wasn’t finished and a higher priority thread is ready to run, the decayed thread will return to the ready queue, where it again becomes eligible for another boost if it remains there for another 4 seconds.

The balance set manager doesn’t actually scan all ready threads every time it runs. To minimize the CPU time it uses, it scans only 16 ready threads; if there are more threads at that priority level, it remembers where it left off and picks up again on the next pass. Also, it will boost only 10 threads per pass—if it finds 10 threads meriting this particular boost (which would indicate an unusually busy system), it stops the scan at that point and picks up again on the next pass.

Will this algorithm always solve the priority inversion issue? No—it’s not perfect by any means. But over time, CPU-starved threads should get enough CPU time to finish whatever processing they were doing and reenter a wait state.

Priority Boosts for MultiMedia Applications and Games (MMCSS)

As we’ve just seen in the last experiment, although Windows’s CPU starvation priority boosts may be enough to get a thread out of an abnormally long wait state or potential deadlock, they simply cannot deal with the resource requirements imposed by a CPU-intensive application such as Windows Media Player or a 3D computer game.

Skipping and other audio glitches have been a common source of irritation among Windows users in the past, and the user-mode audio stack in Windows Vista would have only made the situation worse since it offers even more chances for preemption. To address this, Windows Vista incorporates a new service (called MMCSS, described earlier in this chapter) whose purpose is to ensure “glitch-free” multimedia playback for applications that register with it.

MMCSS works by defining several tasks, including:

  • Audio

  • Capture

  • Distribution

  • Games

  • Playback

  • Pro Audio

  • Window Manager

In turn, each of these tasks includes information about the various properties that differentiate them. The most important one for scheduling is called the Scheduling Category, which is the primary factor determining the priority of threads registered with MMCSS. Table 5-19 shows the various scheduling categories.

Table 5-19. Scheduling Categories






Pro Audio threads running at a higher priority than any other thread on the system except for critical system threads.



Threads part of a foreground application such as Windows Media Player.



All other threads not part of the previous categories.



Threads that have exhausted their share of the CPU and will only continue running if no other higher priority threads are ready to run.

The main mechanism behind MMCSS boosts the priority of threads inside a registered process to the priority level matching their scheduling category and relative priority within this category for a guaranteed period of time. It then lowers those threads to the Exhausted category so that other, nonmultimedia threads on the system can also get a chance to execute.

By default, multimedia threads will get 80 percent of the CPU time available, while other threads will receive 20 percent (based on a sample of 10 ms; in other words, 8 ms and 2 ms). MMCSS itself runs at priority 27, since it needs to preempt any Pro Audio threads in order to lower their priority to the Exhausted category.

It is important to emphasize that the kernel still does the actual boosting of the values inside the KTHREAD (MMCSS simply makes the same kind of system call any other application would do), and the scheduler is still in control of these threads. It is simply their high priority that makes them run almost uninterrupted on a machine, since they are in the real-time range and well above threads that most user applications would be running in.

As was discussed earlier, changing the relative thread priorities within a process does not usually make sense, and no tool allows this because only developers understand the importance of the various threads in their programs.

On the other hand, because applications must manually register with MMCSS and provide it with information about what kind of thread this is, MMCSS does have the necessary data to change these relative thread priorities (and developers are well aware that this will be happening).

MMCSS’s functionality does not stop at simple priority boosting, however. Because of the nature of network drivers on Windows and the NDIS stack, DPCs are quite common mechanisms for delaying work after an interrupt has been received from the network card. Because DPCs run at an IRQL level higher than user-mode code (see Chapter 3 for more information on DPCs and IRQLs), long-running network card driver code could still interrupt media playback during network transfers, or when playing a game for example.

Therefore, MMCSS also sends a special command to the network stack, telling it to throttle network packets during the duration of the media playback. This throttling is designed to maximize playback performance, at the cost of some small loss in network throughput (which would not be noticeable for network operations usually performed during playback, such as playing an online game). The exact mechanisms behind it do not belong to any area of the scheduler, so we will leave them out of this description.

Multiprocessor Systems

On a uniprocessor system, scheduling is relatively simple: the highest-priority thread that wants to run is always running. On a multiprocessor system, it is more complex, as Windows attempts to schedule threads on the most optimal processor for the thread, taking into account the thread’s preferred and previous processors, as well as the configuration of the multiprocessor system. Therefore, while Windows attempts to schedule the highest-priority runnable threads on all available CPUs, it only guarantees to be running the (single) highest-priority thread somewhere.

Before we describe the specific algorithms used to choose which threads run where and when, let’s examine the additional information Windows maintains to track thread and processor state on multiprocessor systems and the two different types of multiprocessor systems supported by Windows (hyperthreaded, multicore, and NUMA).

Multiprocessor Considerations in the Dispatcher Database

In addition to the ready queues and the ready summary, Windows maintains two bitmasks that track the state of the processors on the system. (How these bitmasks are used is explained in the upcoming section Multiprocessor Thread-Scheduling Algorithms.) Following are the two bitmasks that Windows maintains:

  • The active processor mask (KeActiveProcessors), which has a bit set for each usable processor on the system (This might be less than the number of actual processors if the licensing limits of the version of Windows running supports less than the number of available physical processors.)

  • The idle summary (KiIdleSummary), in which each set bit represents an idle processor

Whereas on uniprocessor systems, the dispatcher database is locked by raising IRQL to both DPC/dispatch level and Synch level, on multiprocessor systems more is required, because each processor could, at the same time, raise IRQL and attempt to operate on the dispatcher database. (This is true for any systemwide structure accessed from high IRQL.) (See Chapter 3 for a general description of kernel synchronization and spinlocks.)

Because on a multiprocessor system one processor might need to modify another processor’s per-CPU scheduling data structures (such as inserting a thread that would like to run on a certain processor), these structures are synchronized by using a new per-PRCB queued spinlock, which is held at IRQL SYNCH_LEVEL. (See Table 5-20 for the various values of SYNCH_LEVEL.) Thus, thread selection can occur while locking only an individual processor’s PRCB, in contrast to doing this on Windows XP, where the systemwide dispatcher spinlock had to be held.

Table 5-20. IRQL SYNCH_LEVEL on Multiprocessor Systems

CPU Type


Systems running on x86


Systems running on x64


Systems running on IA64


There is also a per-CPU list of threads in the deferred ready state. These represent threads that are ready to run but have not yet been readied for execution; the actual ready operation has been deferred to a more appropriate time. Because each processor manipulates only its own per-processor deferred ready list, this list is not synchronized by the PRCB spinlock. The deferred ready thread list is processed before exiting the thread dispatcher, before performing a context switch, and after processing a DPC. Threads on the deferred ready list are either dispatched immediately or are moved to the per-processor ready queue for their priority level.

Note that the systemwide dispatcher spinlock still exists and is used, but it is held only for the time needed to modify systemwide state that might affect which thread runs next. For example, changes to synchronization objects (mutexes, events, and semaphores) and their wait queues require holding the dispatcher lock to prevent more than one processor from changing the state of such objects (and the consequential action of possibly readying threads for execution). Other examples include changing the priority of a thread, timer expiration, and swapping of thread kernel stacks.

Thread context switching is also synchronized by using a finer-grained per-thread spinlock, whereas in Windows XP context switching was synchronized by holding a systemwide context swap spinlock.

Hyperthreaded and Multicore Systems

As described in the Symmetric Multiprocessing section in Chapter 2, Windows supports hyperthreaded and multicore multiprocessor systems in two primary ways:

  1. Logical processors as well as per-package cores do not count against physical processor licensing limits. For example, Windows Vista Home Basic, which has a licensed processor limit of 1, will use all four cores on a single processor system.

  2. When choosing a processor for a thread, if there is a physical processor with all logical processors idle, a logical processor from that physical processor will be selected, as opposed to choosing an idle logical processor on a physical processor that has another logical processor running a thread.

NUMA Systems

Another type of multiprocessor system supported by Windows is one with a nonuniform memory access (NUMA) architecture. In a NUMA system, processors are grouped together in smaller units called nodes. Each node has its own processors and memory and is connected to the larger system through a cache-coherent interconnect bus. These systems are called “nonuniform” because each node has its own local high-speed memory. While any processor in any node can access all of memory, node-local memory is much faster to access.

The kernel maintains information about each node in a NUMA system in a data structure called KNODE. The kernel variable KeNodeBlock is an array of pointers to the KNODE structures for each node. The format of the KNODE structure can be shown using the dt command in the kernel debugger, as shown here:

lkd> dt nt!_knode
   +0x000 PagedPoolSListHead : _SLIST_HEADER
   +0x008 NonPagedPoolSListHead : [3] _SLIST_HEADER
   +0x020 PfnDereferenceSListHead : _SLIST_HEADER
   +0x028 ProcessorMask    : Uint4B
   +0x02c Color            : UChar
   +0x02d Seed             : UChar
   +0x02e NodeNumber       : UChar
   +0x02f Flags            : _flags
   +0x030 MmShiftedColor   : Uint4B
   +0x034 FreeCount        : [2] Uint4B
   +0x03c PfnDeferredList  : Ptr32 _SINGLE_LIST_ENTRY
   +0x040 CachedKernelStacks : _CACHED_KSTACK_LIST

Applications that want to gain the most performance out of NUMA systems can set the affinity mask to restrict a process to the processors in a specific node. This information can be obtained using the functions listed in Table 5-21. Functions that can alter thread affinity are listed in Table 5-13.

Table 5-21. NUMA-Related Functions




Retrieves the node that currently has the highest number.


Retrieves the processor mask for the specified node.


Returns the NUMA node number for the given proximity ID.


Retrieves the node number for the specified processor.

How the scheduling algorithms take into account NUMA systems will be covered in the upcoming section Multiprocessor Thread-Scheduling Algorithms (and the optimizations in the memory manager to take advantage of node-local memory are covered in Chapter 9).


Each thread has an affinity mask that specifies the processors on which the thread is allowed to run. The thread affinity mask is inherited from the process affinity mask. By default, all processes (and therefore all threads) begin with an affinity mask that is equal to the set of active processors on the system—in other words, the system is free to schedule all threads on any available processor.

However, to optimize throughput and/or partition workloads to a specific set of processors, applications can choose to change the affinity mask for a thread. This can be done at several levels:

  • Calling the SetThreadAffinityMask function to set the affinity for an individual thread

  • Calling the SetProcessAffinityMask function to set the affinity for all the threads in a process. Task Manager and Process Explorer provide a GUI to this function if you right-click a process and choose Set Affinity. The Psexec tool (from Sysinternals) provides a command-line interface to this function. (See the –a switch.)

  • By making a process a member of a job that has a jobwide affinity mask set using the SetInformationJobObject function (Jobs are described in the upcoming Job Objects section.)

  • By specifying an affinity mask in the image header when compiling the application (For more information on the detailed format of Windows images, search for “Portable Executable and Common Object File Format Specification” on www.microsoft.com.)

You can also set the “uniprocessor” flag for an image (at compile time). If this flag is set, the system chooses a single processor at process creation time and assigns that as the process affinity mask, starting with the first processor and then going round-robin across all the processors. For example, on a dual-processor system, the first time you run an image marked as uniprocessor, it is assigned to CPU 0; the second time, CPU 1; the third time, CPU 0; the fourth time, CPU 1; and so on. This flag can be useful as a temporary workaround for programs that have multithreaded synchronization bugs that, as a result of race conditions, surface on multiprocessor systems but that don’t occur on uniprocessor systems. (This has actually saved the authors of this book on two different occasions.)

Windows won’t move a running thread that could run on a different processor from one CPU to a second processor to permit a thread with an affinity for the first processor to run on the first processor. For example, consider this scenario: CPU 0 is running a priority 8 thread that can run on any processor, and CPU 1 is running a priority 4 thread that can run on any processor. A priority 6 thread that can run on only CPU 0 becomes ready. What happens? Windows won’t move the priority 8 thread from CPU 0 to CPU 1 (preempting the priority 4 thread) so that the priority 6 thread can run; the priority 6 thread has to wait.

Therefore, changing the affinity mask for a process or a thread can result in threads getting less CPU time than they normally would, as Windows is restricted from running the thread on certain processors. Therefore, setting affinity should be done with extreme care—in most cases, it is optimal to let Windows decide which threads run where.

Ideal and Last Processor

Each thread has two CPU numbers stored in the kernel thread block:

  • Ideal processor, or the preferred processor that this thread should run on

  • Last processor, or the processor on which the thread last ran

The ideal processor for a thread is chosen when a thread is created using a seed in the process block. The seed is incremented each time a thread is created so that the ideal processor for each new thread in the process will rotate through the available processors on the system. For example, the first thread in the first process on the system is assigned an ideal processor of 0. The second thread in that process is assigned an ideal processor of 1. However, the next process in the system has its first thread’s ideal processor set to 1, the second to 2, and so on. In that way, the threads within each process are spread evenly across the processors.

Note that this assumes the threads within a process are doing an equal amount of work. This is typically not the case in a multithreaded process, which normally has one or more housekeeping threads and then a number of worker threads. Therefore, a multithreaded application that wants to take full advantage of the platform might find it advantageous to specify the ideal processor numbers for its threads by using the SetThreadIdealProcessor function.

On hyperthreaded systems, the next ideal processor is the first logical processor on the next physical processor. For example, on a dual-processor hyperthreaded system with four logical processors, if the ideal processor for the first thread is assigned to logical processor 0, the second thread would be assigned to logical processor 2, the third thread to logical processor 1, the fourth thread to logical process 3, and so forth. In this way, the threads are spread evenly across the physical processors.

On NUMA systems, when a process is created, an ideal node for the process is selected. The first process is assigned to node 0, the second process to node 1, and so on. Then, the ideal processors for the threads in the process are chosen from the process’s ideal node. The ideal processor for the first thread in a process is assigned to the first processor in the node. As additional threads are created in processes with the same ideal node, the next processor is used for the next thread’s ideal processor, and so on.

Dynamic Processor Addition and Replacement

As we’ve seen, developers can fine-tune which threads are allowed to (and in the case of the ideal processor, should) run on which processor. This works fine on systems that have a constant number of processors during their run time (for example, desktop machines require shutting down the computer to make any sort of hardware changes to the processor or their count).

Today’s server systems, however, cannot afford the downtime that CPU replacement or addition normally requires. In fact, one of the times when adding a CPU is required for a server is at times of high load that is above what the machine can support at its current level of performance. Having to shut down the server during a period of peak usage would defeat the purpose. To meet this requirement, the latest generation of server motherboards and systems support the addition of processors (as well as their replacement) while the machine is still running. The ACPI BIOS and related hardware on the machine have been specifically built to allow and be aware of this need, but operating system participation is required for full support.

Dynamic processor support is provided through the HAL, which will notify the kernel of a new processor on the system through the function KeStartDynamicProcessor. This routine does similar work to that performed when the system detects more than one processor at startup and needs to initialize the structures related to them. When a dynamic processor is added, a variety of system components perform some additional work. For example, the memory manager allocates new pages and memory structures optimized for the CPU. It also initializes a new DPC kernel stack while the kernel initializes the Global Descriptor Table (GDT), the Interrupt Descriptor Table ( IDT), the processor control region (PCR), the processor control block (PRCB), and other related structures for the processor.

Other executive parts of the kernel are also called, mostly to initialize the per-processor lookaside lists for the processor that was added. For example, the I/O manager, the executive lookaside list code, the cache manager, and the object manager all use per-processor look-aside lists for their frequently allocated structures.

Finally, the kernel initializes threaded DPC support for the processor and adjusts exported kernel variables to report the new processor. Different memory manager masks and process seeds based on processor counts are also updated, and processor features need to be updated for the new processor to match the rest of the system (for example, enabling virtualization support on the newly added processor). The initialization sequence completes with the notification to the Windows Hardware Error Architecture (WHEA) component that a new processor is online.

The HAL is also involved in this process. It is called once to start the dynamic processor after the kernel is aware of it, and it is called again after the kernel has finished initialization of the processor. However, these notifications and callbacks only make the kernel aware and respond to processor changes. Although an additional processor increases the throughput of the kernel, it does nothing to help drivers.

To handle drivers, the system has a new default executive callback, the processor add callback, that drivers can register with for notifications. Similar to the callbacks that notify drivers of power state or system time changes, this callback allows driver code to, for example, create a new worker thread if desirable so that it can handle more work at the same time.

Once drivers are notified, the final kernel component called is the Plug and Play manager, which adds the processor to the system’s device node and rebalances interrupts so that the new processor can handle interrupts that were already registered for other processors. Unfortunately, until now, CPU-hungry applications have still been left out of this process, but Windows Server 2008 and Windows Vista Service Pack 1 have improved the process to allow applications to be able to take advantage of newer processors as well.

However, a sudden change of affinity can have potentially breaking changes for a running application (especially when going from a single-processor to a multiprocessor environment) through the appearance of potential race conditions or simply misdistribution of work (since the process might have calculated the perfect ratios at startup, based on the number of CPUs it was aware of). As a result, applications do not take advantage of a dynamically added processor by default—they must request it.

The Windows APIs SetProcessAffinityUpdateMode and QueryProcessAffinityMode (which use the undocumented NtSet/QueryInformationProcess system call) tell the process manager that these applications should have their affinity updated (by setting the AffinityUpdateEnable flag in EPROCESS), or that they do not want to deal with affinity updates (by setting the AffinityPermanent flag in EPROCESS). Once an application has told the system that its affinity is permanent, it cannot later change its mind and request affinity updates, so this is a one-time change.

As part of KeStartDynamicProcessor, a new step has been added after interrupts are rebalanced, which is to call the process manager to perform affinity updates through PsUpdateActiveProcessAffinity. Some Windows core processes and services already have affinity updates enabled, while third-party software will need to be recompiled to take advantage of the new API call. The System process, Svchost processes, and Smss are all compatible with dynamic processor addition.

Multiprocessor Thread-Scheduling Algorithms

Now that we’ve described the types of multiprocessor systems supported by Windows as well as the thread affinity and ideal processor settings, we’re ready to examine how this information is used to determine which threads run where. There are two basic decisions to describe:

  • Choosing a processor for a thread that wants to run

  • Choosing a thread on a processor that needs something to do

Choosing a Processor for a Thread When There Are Idle Processors

When a thread becomes ready to run, Windows first tries to schedule the thread to run on an idle processor. If there is a choice of idle processors, preference is given first to the thread’s ideal processor, then to the thread’s previous processor, and then to the currently executing processor (that is, the CPU on which the scheduling code is running).

To select the best idle processor, Windows starts with the set of idle processors that the thread’s affinity mask permits it to run on. If the system is NUMA and there are idle CPUs in the node containing the thread’s ideal processor, the list of idle processors is reduced to that set. If this eliminates all idle processors, the reduction is not done. Next, if the system is running hyperthreaded processors and there is a physical processor with all logical processors idle, the list of idle processors is reduced to that set. If that results in an empty set of processors, the reduction is not done.

If the current processor (the processor trying to determine what to do with the thread that wants to run) is in the remaining idle processor set, the thread is scheduled on it. If the current processor is not in the remaining set of idle processors, it is a hyperthreaded system, and there is an idle logical processor on the physical processor containing the ideal processor for the thread, the idle processors are reduced to that set. If not, the system checks whether there are any idle logical processors on the physical processor containing the thread’s previous processor. If that set is nonzero, the idle processors are reduced to that list. Finally, the lowest numbered CPU in the remaining set is selected as the processor to run the thread on.

Once a processor has been selected for the thread to run on, that thread is put in the standby state and the idle processor’s PRCB is updated to point to this thread. When the idle loop on that processor runs, it will see that a thread has been selected to run and will dispatch that thread.

Choosing a Processor for a Thread When There Are No Idle Processors

If there are no idle processors when a thread wants to run, Windows compares the priority of the thread running (or the one in the standby state) on the thread’s ideal processor to determine whether it should preempt that thread.

If the thread’s ideal processor already has a thread selected to run next (waiting in the standby state to be scheduled) and that thread’s priority is less than the priority of the thread being readied for execution, the new thread preempts that first thread out of the standby state and becomes the next thread for that CPU. If there is already a thread running on that CPU, Windows checks whether the priority of the currently running thread is less than the thread being readied for execution. If so, the currently running thread is marked to be preempted and Windows queues an interprocessor interrupt to the target processor to preempt the currently running thread in favor of this new thread.

If the ready thread cannot be run right away, it is moved into the ready state where it awaits its turn to run. Note that threads are always put on their ideal processor’s per-processor ready queues.

Selecting a Thread to Run on a Specific CPU

Because each processor has its own list of threads waiting to run on that processor, when a thread finishes running, the processor can simply check its per-processor ready queue for the next thread to run. If the per-processor ready queues are empty, the idle thread for that processor is scheduled. The idle thread then begins scanning other processor’s ready queues for threads it can run. Note that on NUMA systems, the idle thread first looks at processors on its node before looking at other nodes’ processors.

CPU Rate Limits

As part of the new hard quota management system added in Windows Vista (which builds on previous quota support present since the first version of Windows NT, but adds hard limits instead of soft hints), support for limiting CPU usage was added to the system in three different ways: per-session, per-user, or per-system. Unfortunately, information on enabling these new limits has not yet been documented, and no tool that is part of the operating system allows you to set these limits: you must modify the registry settings manually. Because all the quotas—save one—are memory quotas, we will cover those in Chapter 9, which deals with the memory manager, and focus our attention on the CPU rate limit.

The new quota system can be accessed through the registry key HKLM\SYSTEM\Current-ControlSet\Control\Session Manager\QuotaSystem, as well as through the standard NtSet-InformationProcess system call. CPU rate limits can therefore be set in one of three ways:

  • By creating a new value called CpuRateLimit and entering the rate information.

  • By creating a new key with the security ID (SID) of the account you want to limit, and creating a CpuRateLimit value inside that key.

  • By calling NtSetInformationProcess and giving it the process handle of the process to limit and the CPU rate limiting information.

In all three cases, the CPU rate limit data is not a straightforward value; it is based on a compressed bitfield, documented in the WDK as part of the RATE_QUOTA_LIMIT structure. The bottom four bits define the rate phase, which can be expressed either as one, two, or three seconds—this value defines how often the rate limiting should be applied and is called the PS_RATE_PHASE. The rest of the bits are used for the actual rate, as a value representing a percentage of maximum CPU usage. Because any number from 0 to 100 can be represented with only 7 bits, the rest of the bits are unused. Therefore, a rate limit of 40 percent every 2 seconds would be defined by the value 0x282, or 101000 0010 in binary.

The process manager, which is responsible for enforcing the CPU rate limit, uses a variety of system mechanisms to do its job. First of all, rate limiting is able to reliably work because of the CPU cycle count improvements discussed earlier, which allow the process manager to accurately determine how much CPU time a process has taken and know whether the limit should be enforced. It then uses a combination of DPC and APC routines to throttle down DPC and APC CPU usage, which are outside the direct control of user-mode developers but still result in CPU usage in the system (in the case of a systemwide CPU rate limit).

Finally, the main mechanism through which rate limiting works is by creating an artificial wait on a kernel gate object (making the thread uniquely bound to this object and putting it in a wait state, which does not consume CPU cycles). This mechanism operates through the normal routine of an APC object queued to the thread or threads inside the process currently responsible for the work. The gate is signaled by an internal worker thread inside the process manager responsible for replenishment of the CPU usage, which is queued by a DPC responsible for replenishing systemwide CPU usage requests.