Scheduling Priorities

Threads are scheduled to run based on their scheduling priority. Each thread is assigned a scheduling priority. The priority levels range from zero (lowest priority) to 31 (highest priority). Only the zero-page thread can have a priority of zero. (The zero-page thread is a system thread responsible for zeroing any free pages when there are no other threads that need to run.)

The system treats all threads with the same priority as equal. The system assigns time slices in a round-robin fashion to all threads with the highest priority. If none of these threads are ready to run, the system assigns time slices in a round-robin fashion to all threads with the next highest priority. If a higher-priority thread becomes available to run, the system ceases to execute the lower-priority thread (without allowing it to finish using its time slice) and assigns a full time slice to the higher-priority thread. For more information, see Context Switches.

The priority of each thread is determined by the following criteria:

• The priority class of its process
• The priority level of the thread within the priority class of its process

The priority class and priority level are combined to form the base priority of a thread. For information on the dynamic priority of a thread, see Priority Boosts.

Priority Class

Each process belongs to one of the following priority classes: IDLE_PRIORITY_CLASS
BELOW_NORMAL_PRIORITY_CLASS
NORMAL_PRIORITY_CLASS
ABOVE_NORMAL_PRIORITY_CLASS
HIGH_PRIORITY_CLASS
REALTIME_PRIORITY_CLASS

By default, the priority class of a process is NORMAL_PRIORITY_CLASS. Use the CreateProcess function to specify the priority class of a child process when you create it. If the calling process is IDLE_PRIORITY_CLASS or BELOW_NORMAL_PRIORITY_CLASS, the new process will inherit this class. Use the GetPriorityClass function to determine the current priority class of a process and the SetPriorityClass function to change the priority class of a process.

Processes that monitor the system, such as screen savers or applications that periodically update a display, should use IDLE_PRIORITY_CLASS. This prevents the threads of this process, which do not have high priority, from interfering with higher priority threads.

Use HIGH_PRIORITY_CLASS with care. If a thread runs at the highest priority level for extended periods, other threads in the system will not get processor time. If several threads are set at high priority at the same time, the threads lose their effectiveness. The high-priority class should be reserved for threads that must respond to time-critical events. If your application performs one task that requires the high-priority class while the rest of its tasks are normal priority, use SetPriorityClass to raise the priority class of the application temporarily; then reduce it after the time-critical task has been completed. Another strategy is to create a high-priority process that has all of its threads blocked most of the time, awakening threads only when critical tasks are needed. The important point is that a high-priority thread should execute for a brief time, and only when it has time-critical work to perform.

You should almost never use REALTIME_PRIORITY_CLASS, because this interrupts system threads that manage mouse input, keyboard input, and background disk flushing. This class can be appropriate for applications that «talk» directly to hardware or that perform brief tasks that should have limited interruptions.

Priority Level

The following are priority levels within each priority class: THREAD_PRIORITY_IDLE
THREAD_PRIORITY_LOWEST
THREAD_PRIORITY_BELOW_NORMAL
THREAD_PRIORITY_NORMAL
THREAD_PRIORITY_ABOVE_NORMAL
THREAD_PRIORITY_HIGHEST
THREAD_PRIORITY_TIME_CRITICAL

All threads are created using THREAD_PRIORITY_NORMAL. This means that the thread priority is the same as the process priority class. After you create a thread, use the SetThreadPriority function to adjust its priority relative to other threads in the process.

A typical strategy is to use THREAD_PRIORITY_ABOVE_NORMAL or THREAD_PRIORITY_HIGHEST for the process’s input thread, to ensure that the application is responsive to the user. Background threads, particularly those that are processor intensive, can be set to THREAD_PRIORITY_BELOW_NORMAL or THREAD_PRIORITY_LOWEST, to ensure that they can be preempted when necessary. However, if you have a thread waiting for another thread with a lower priority to complete some task, be sure to block the execution of the waiting high-priority thread. To do this, use a wait function, critical section, or the Sleep function, SleepEx, or SwitchToThread function. This is preferable to having the thread execute a loop. Otherwise, the process may become deadlocked, because the thread with lower priority is never scheduled.

To determine the current priority level of a thread, use the GetThreadPriority function.

Base Priority

The process priority class and thread priority level are combined to form the base priority of each thread.

The following table shows the base priority for combinations of process priority class and thread priority value.

Process and Thread Functions

This topic describes the process and thread functions.

Dispatch Queue Function

The following function creates a DispatcherQueueController.

Function	Description
CreateDispatcherQueueController	Creates a DispatcherQueueController which manages the lifetime of a DispatcherQueue that runs queued tasks in priority order on another thread.

Process Functions

The following functions are used with processes.

Function	Description
CreateProcess	Creates a new process and its primary thread.
CreateProcessAsUser	Creates a new process and its primary thread. The new process runs in the security context of the user represented by the specified token.
CreateProcessWithLogonW	Creates a new process and its primary thread. The new process then runs the specified executable file in the security context of the specified credentials (user, domain, and password).
CreateProcessWithTokenW	Creates a new process and its primary thread. The new process runs in the security context of the specified token.
ExitProcess	Ends the calling process and all its threads.
FlushProcessWriteBuffers	Flushes the write queue of each processor that is running a thread of the current process.
FreeEnvironmentStrings	Frees a block of environment strings.
GetCommandLine	Retrieves the command-line string for the current process.
GetCurrentProcess	Retrieves a pseudo handle for the current process.
GetCurrentProcessId	Retrieves the process identifier of the calling process.
GetCurrentProcessorNumber	Retrieves the number of the processor the current thread was running on during the call to this function.
GetEnvironmentStrings	Retrieves the environment block for the current process.
GetEnvironmentVariable	Retrieves the value of the specified variable from the environment block of the calling process.
GetExitCodeProcess	Retrieves the termination status of the specified process.
GetGuiResources	Retrieves the count of handles to graphical user interface (GUI) objects in use by the specified process.
GetLogicalProcessorInformation	Retrieves information about logical processors and related hardware.
GetPriorityClass	Retrieves the priority class for the specified process.
GetProcessAffinityMask	Retrieves a process affinity mask for the specified process and the system affinity mask for the system.
GetProcessGroupAffinity	Retrieves the processor group affinity of the specified process.
GetProcessHandleCount	Retrieves the number of open handles that belong to the specified process.
GetProcessId	Retrieves the process identifier of the specified process.
GetProcessIdOfThread	Retrieves the process identifier of the process associated with the specified thread.
GetProcessIoCounters	Retrieves accounting information for all I/O operations performed by the specified process.
GetProcessMitigationPolicy	Retrieves mitigation policy settings for the calling process.
GetProcessPriorityBoost	Retrieves the priority boost control state of the specified process.
GetProcessShutdownParameters	Retrieves shutdown parameters for the currently calling process.
GetProcessTimes	Retrieves timing information about for the specified process.
GetProcessVersion	Retrieves the major and minor version numbers of the system on which the specified process expects to run.
GetProcessWorkingSetSize	Retrieves the minimum and maximum working set sizes of the specified process.
GetProcessWorkingSetSizeEx	Retrieves the minimum and maximum working set sizes of the specified process.
GetProcessorSystemCycleTime	Retrieves the cycle time each processor in the specified group spent executing deferred procedure calls (DPCs) and interrupt service routines (ISRs).
GetStartupInfo	Retrieves the contents of the STARTUPINFO structure that was specified when the calling process was created.
IsImmersiveProcess	Determines whether the process belongs to a Windows Store app.
NeedCurrentDirectoryForExePath	Determines whether the current directory should be included in the search path for the specified executable.
OpenProcess	Opens an existing local process object.
QueryFullProcessImageName	Retrieves the full name of the executable image for the specified process.
QueryProcessAffinityUpdateMode	Retrieves the affinity update mode of the specified process.
QueryProcessCycleTime	Retrieves the sum of the cycle time of all threads of the specified process.
SetEnvironmentVariable	Sets the value of an environment variable for the current process.
SetPriorityClass	Sets the priority class for the specified process.
SetProcessAffinityMask	Sets a processor affinity mask for the threads of a specified process.
SetProcessAffinityUpdateMode	Sets the affinity update mode of the specified process.
SetProcessInformation	Sets information for the specified process.
SetProcessMitigationPolicy	Sets the mitigation policy for the calling process.
SetProcessPriorityBoost	Disables the ability of the system to temporarily boost the priority of the threads of the specified process.
SetProcessRestrictionExemption	Exempts the calling process from restrictions preventing desktop processes from interacting with the Windows Store app environment. This function is used by development and debugging tools.
SetProcessShutdownParameters	Sets shutdown parameters for the currently calling process.
SetProcessWorkingSetSize	Sets the minimum and maximum working set sizes for the specified process.
SetProcessWorkingSetSizeEx	Sets the minimum and maximum working set sizes for the specified process.
TerminateProcess	Terminates the specified process and all of its threads.

Process Enumeration Functions

The following functions are used to enumerate processes.

Function	Description
EnumProcesses	Retrieves the process identifier for each process object in the system.
Process32First	Retrieves information about the first process encountered in a system snapshot.
Process32Next	Retrieves information about the next process recorded in a system snapshot.
WTSEnumerateProcesses	Retrieves information about the active processes on the specified terminal server.

Policy Functions

The following functions are used with process wide policy.

Function	Description
QueryProtectedPolicy	Queries the value associated with a protected policy.
SetProtectedPolicy	Sets a protected policy.

Thread Functions

The following functions are used with threads.

Function	Description
AttachThreadInput	Attaches the input processing mechanism of one thread to that of another thread.
CreateRemoteThread	Creates a thread that runs in the virtual address space of another process.
CreateRemoteThreadEx	Creates a thread that runs in the virtual address space of another process and optionally specifies extended attributes such as processor group affinity.
CreateThread	Creates a thread to execute within the virtual address space of the calling process.
ExitThread	Ends the calling thread.
GetCurrentThread	Retrieves a pseudo handle for the current thread.
GetCurrentThreadId	Retrieves the thread identifier of the calling thread.
GetExitCodeThread	Retrieves the termination status of the specified thread.
GetThreadDescription	Retrieves the description that was assigned to a thread by calling SetThreadDescription.
GetThreadGroupAffinity	Retrieves the processor group affinity of the specified thread.
GetThreadId	Retrieves the thread identifier of the specified thread.
GetThreadIdealProcessorEx	Retrieves the processor number of the ideal processor for the specified thread.
GetThreadInformation	Retrieves information about the specified thread.
GetThreadIOPendingFlag	Determines whether a specified thread has any I/O requests pending.
GetThreadPriority	Retrieves the priority value for the specified thread.
GetThreadPriorityBoost	Retrieves the priority boost control state of the specified thread.
GetThreadTimes	Retrieves timing information for the specified thread.
OpenThread	Opens an existing thread object.
QueryIdleProcessorCycleTime	Retrieves the cycle time for the idle thread of each processor in the system.
QueryThreadCycleTime	Retrieves the cycle time for the specified thread.
ResumeThread	Decrements a thread’s suspend count.
SetThreadAffinityMask	Sets a processor affinity mask for the specified thread.
SetThreadDescription	Assigns a description to a thread.
SetThreadGroupAffinity	Sets the processor group affinity for the specified thread.
SetThreadIdealProcessor	Specifies a preferred processor for a thread.
SetThreadIdealProcessorEx	Sets the ideal processor for the specified thread and optionally retrieves the previous ideal processor.
SetThreadInformation	Sets information for the specified thread.
SetThreadPriority	Sets the priority value for the specified thread.
SetThreadPriorityBoost	Disables the ability of the system to temporarily boost the priority of a thread.
SetThreadStackGuarantee	Sets the stack guarantee for the calling thread.
Sleep	Suspends the execution of the current thread for a specified interval.
SleepEx	Suspends the current thread until the specified condition is met.
SuspendThread	Suspends the specified thread.
SwitchToThread	Causes the calling thread to yield execution to another thread that is ready to run on the current processor.
TerminateThread	Terminates a thread.
ThreadProc	An application-defined function that serves as the starting address for a thread.
TlsAlloc	Allocates a thread local storage (TLS) index.
TlsFree	Releases a TLS index.
TlsGetValue	Retrieves the value in the calling thread’s TLS slot for a specified TLS index.
TlsSetValue	Stores a value in the calling thread’s TLS slot for a specified TLS index.
WaitForInputIdle	Waits until the specified process is waiting for user input with no input pending, or until the time-out interval has elapsed.

Process and Thread Extended Attribute Functions

The following functions are used to set extended attributes for process and thread creation.

Function	Description
DeleteProcThreadAttributeList	Deletes the specified list of attributes for process and thread creation.
InitializeProcThreadAttributeList	Initializes the specified list of attributes for process and thread creation.
UpdateProcThreadAttribute	Updates the specified attribute in the specified list of attributes for process and thread creation.

WOW64 Functions

The following functions are used with WOW64.

Function	Description
IsWow64Message	Determines whether the last message read from the current thread’s queue originated from a WOW64 process.
IsWow64Process	Determines whether the specified process is running under WOW64.
IsWow64Process2	Determines whether the specified process is running under WOW64; also returns additional machine process and architecture information.
Wow64SuspendThread	Suspends the specified WOW64 thread.

Job Object Functions

The following functions are used with job objects.

Function	Description
AssignProcessToJobObject	Associates a process with an existing job object.
CreateJobObject	Creates or opens a job object.
IsProcessInJob	Determines whether the process is running in the specified job.
OpenJobObject	Opens an existing job object.
QueryInformationJobObject	Retrieves limit and job state information from the job object.
SetInformationJobObject	Set limits for a job object.
TerminateJobObject	Terminates all processes currently associated with the job.
UserHandleGrantAccess	Grants or denies access to a handle to a User object to a job that has a user-interface restriction.

Thread Pool Functions

The following functions are used with thread pools.

Function	Description
CallbackMayRunLong	Indicates that the callback may not return quickly.
CancelThreadpoolIo	Cancels the notification from the StartThreadpoolIo function.
CloseThreadpool	Closes the specified thread pool.
CloseThreadpoolCleanupGroup	Closes the specified cleanup group.
CloseThreadpoolCleanupGroupMembers	Releases the members of the specified cleanup group, waits for all callback functions to complete, and optionally cancels any outstanding callback functions.
CloseThreadpoolIo	Releases the specified I/O completion object.
CloseThreadpoolTimer	Releases the specified timer object.
CloseThreadpoolWait	Releases the specified wait object.
CloseThreadpoolWork	Releases the specified work object.
CreateThreadpool	Allocates a new pool of threads to execute callbacks.
CreateThreadpoolCleanupGroup	Creates a cleanup group that applications can use to track one or more thread pool callbacks.
CreateThreadpoolIo	Creates a new I/O completion object.
CreateThreadpoolTimer	Creates a new timer object.
CreateThreadpoolWait	Creates a new wait object.
CreateThreadpoolWork	Creates a new work object.
DestroyThreadpoolEnvironment	Deletes the specified callback environment. Call this function when the callback environment is no longer needed for creating new thread pool objects.
DisassociateCurrentThreadFromCallback	Removes the association between the currently executing callback function and the object that initiated the callback. The current thread will no longer count as executing a callback on behalf of the object.
FreeLibraryWhenCallbackReturns	Specifies the DLL that the thread pool will unload when the current callback completes.
InitializeThreadpoolEnvironment	Initializes a callback environment.
IsThreadpoolTimerSet	Determines whether the specified timer object is currently set.
LeaveCriticalSectionWhenCallbackReturns	Specifies the critical section that the thread pool will release when the current callback completes.
QueryThreadpoolStackInformation	Retrieves the stack reserve and commit sizes for threads in the specified thread pool.
ReleaseMutexWhenCallbackReturns	Specifies the mutex that the thread pool will release when the current callback completes.
ReleaseSemaphoreWhenCallbackReturns	Specifies the semaphore that the thread pool will release when the current callback completes.
SetEventWhenCallbackReturns	Specifies the event that the thread pool will set when the current callback completes.
SetThreadpoolCallbackCleanupGroup	Associates the specified cleanup group with the specified callback environment.
SetThreadpoolCallbackLibrary	Ensures that the specified DLL remains loaded as long as there are outstanding callbacks.
SetThreadpoolCallbackPersistent	Specifies that the callback should run on a persistent thread.
SetThreadpoolCallbackPool	Sets the thread pool to be used when generating callbacks.
SetThreadpoolCallbackPriority	Specifies the priority of a callback function relative to other work items in the same thread pool.
SetThreadpoolCallbackRunsLong	Indicates that callbacks associated with this callback environment may not return quickly.
SetThreadpoolStackInformation	Sets the stack reserve and commit sizes for new threads in the specified thread pool.
SetThreadpoolThreadMaximum	Sets the maximum number of threads that the specified thread pool can allocate to process callbacks.
SetThreadpoolThreadMinimum	Sets the minimum number of threads that the specified thread pool must make available to process callbacks.
SetThreadpoolTimerEx	Sets the timer object. A worker thread calls the timer object’s callback after the specified timeout expires.
SetThreadpoolTimer	Sets the timer object. A worker thread calls the timer object’s callback after the specified timeout expires.
SetThreadpoolWait	Sets the wait object. A worker thread calls the wait object’s callback function after the handle becomes signaled or after the specified timeout expires.
SetThreadpoolWaitEx	Sets the wait object. A worker thread calls the wait object’s callback function after the handle becomes signaled or after the specified timeout expires.
StartThreadpoolIo	Notifies the thread pool that I/O operations may possibly begin for the specified I/O completion object. A worker thread calls the I/O completion object’s callback function after the operation completes on the file handle bound to this object.
SubmitThreadpoolWork	Posts a work object to the thread pool. A worker thread calls the work object’s callback function.
TpInitializeCallbackEnviron	Initializes a callback environment for the thread pool.
TpDestroyCallbackEnviron	Deletes the specified callback environment. Call this function when the callback environment is no longer needed for creating new thread pool objects.
TpSetCallbackActivationContext	Assigns an activation context to the callback environment.
TpSetCallbackCleanupGroup	Associates the specified cleanup group with the specified callback environment.
TpSetCallbackFinalizationCallback	Indicates a function to call when the callback environment is finalized.
TpSetCallbackLongFunction	Indicates that callbacks associated with this callback environment may not return quickly.
TpSetCallbackNoActivationContext	Indicates that the callback environment has no activation context.
TpSetCallbackPersistent	Specifies that the callback should run on a persistent thread.
TpSetCallbackPriority	Specifies the priority of a callback function relative to other work items in the same thread pool.
TpSetCallbackRaceWithDll	Ensures that the specified DLL remains loaded as long as there are outstanding callbacks.
TpSetCallbackThreadpool	Assigns a thread pool to a callback environment.
TrySubmitThreadpoolCallback	Requests that a thread pool worker thread call the specified callback function.
WaitForThreadpoolIoCallbacks	Waits for outstanding I/O completion callbacks to complete and optionally cancels pending callbacks that have not yet started to execute.
WaitForThreadpoolTimerCallbacks	Waits for outstanding timer callbacks to complete and optionally cancels pending callbacks that have not yet started to execute.
WaitForThreadpoolWaitCallbacks	Waits for outstanding wait callbacks to complete and optionally cancels pending callbacks that have not yet started to execute.
WaitForThreadpoolWorkCallbacks	Waits for outstanding work callbacks to complete and optionally cancels pending callbacks that have not yet started to execute.

The following functions are part of the original thread pooling API.

Function	Description
BindIoCompletionCallback	Associates the I/O completion port owned by the thread pool with the specified file handle. On completion of an I/O request involving this file, a non-I/O worker thread will execute the specified callback function.
QueueUserWorkItem	Queues a work item to a worker thread in the thread pool.
RegisterWaitForSingleObject	Directs a wait thread in the thread pool to wait on the object.
UnregisterWaitEx	Waits until one or all of the specified objects are in the signaled state or the time-out interval elapses.

Thread Ordering Service Functions

The following functions are used with the thread ordering service.

Function	Description
AvQuerySystemResponsiveness	Retrieves the system responsiveness setting used by the multimedia class scheduler service.
AvRtCreateThreadOrderingGroup	Creates a thread ordering group.
AvRtCreateThreadOrderingGroupEx	Creates a thread ordering group and associates the server thread with a task.
AvRtDeleteThreadOrderingGroup	Deletes the specified thread ordering group created by the caller.
AvRtJoinThreadOrderingGroup	Joins client threads to a thread ordering group.
AvRtLeaveThreadOrderingGroup	Enables client threads to leave a thread ordering group.
AvRtWaitOnThreadOrderingGroup	Enables client threads of a thread ordering group to wait until they should execute.

Multimedia Class Scheduler Service Functions

The following functions are used with the multimedia class scheduler service.

Function	Description
AvRevertMmThreadCharacteristics	Indicates that a thread is no longer performing work associated with the specified task.
AvSetMmMaxThreadCharacteristics	Associates the calling thread with the specified tasks.
AvSetMmThreadCharacteristics	Associates the calling thread with the specified task.
AvSetMmThreadPriority	Adjusts the thread priority of the calling thread relative to other threads performing the same task.

Fiber Functions

The following functions are used with fibers.

Function	Description
ConvertFiberToThread	Converts the current fiber into a thread.
ConvertThreadToFiber	Converts the current thread into a fiber.
ConvertThreadToFiberEx	Converts the current thread into a fiber.
CreateFiber	Allocates a fiber object, assigns it a stack, and sets up execution to begin at the specified start address.
CreateFiberEx	Allocates a fiber object, assigns it a stack, and sets up execution to begin at the specified start address.
DeleteFiber	Deletes an existing fiber.
FiberProc	An application-defined function used with the CreateFiber function.
FlsAlloc	Allocates a fiber local storage (FLS) index.
FlsFree	Releases an FLS index.
FlsGetValue	Retrieves the value in the calling fiber’s FLS slot for a specified FLS index.
FlsSetValue	Stores a value in the calling fiber’s FLS slot for a specified FLS index.
IsThreadAFiber	Determines whether the current thread is a fiber.
SwitchToFiber	Schedules a fiber.

NUMA Support Functions

The following functions provide NUMA support.

Function	Description
AllocateUserPhysicalPagesNuma	Reserves or commits a region of memory within the virtual address space of the specified process, and specifies the NUMA node for the physical memory.
GetLogicalProcessorInformation	Retrieves information about logical processors and related hardware.
GetNumaAvailableMemoryNode	Retrieves the amount of memory available in the specified node.
GetNumaAvailableMemoryNodeEx	Retrieves the amount of memory that is available in the specified node as a USHORT value.
GetNumaHighestNodeNumber	Retrieves the node that currently has the highest number.
GetNumaNodeNumberFromHandle	Retrieves the NUMA node associated with the underlying device for a file handle.
GetNumaNodeProcessorMask	Retrieves the processor mask for the specified node.
GetNumaNodeProcessorMaskEx	Retrieves the processor mask for the specified NUMA node as a USHORT value.
GetNumaProcessorNode	Retrieves the node number for the specified processor.
GetNumaProcessorNodeEx	Retrieves the node number of the specified logical processor as a USHORT value.
GetNumaProximityNode	Retrieves the node number for the specified proximity identifier.
GetNumaProximityNodeEx	Retrieves the node number as a USHORT value for the specified proximity identifier.
VirtualAllocExNuma	Reserves or commits a region of memory within the virtual address space of the specified process, and specifies the NUMA node for the physical memory.

Processor Functions

The following functions are used with logical processors and processor groups.

Function	Description
GetActiveProcessorCount	Returns the number of active processors in a processor group or in the system.
GetActiveProcessorGroupCount	Returns the number of active processor groups in the system.
GetCurrentProcessorNumber	Retrieves the number of the processor the current thread was running on during the call to this function.
GetCurrentProcessorNumberEx	Retrieves the processor group and number of the logical processor in which the calling thread is running.
GetLogicalProcessorInformation	Retrieves information about logical processors and related hardware.
GetLogicalProcessorInformationEx	Retrieves information about the relationships of logical processors and related hardware.
GetMaximumProcessorCount	Returns the maximum number of logical processors that a processor group or the system can have.
GetMaximumProcessorGroupCount	Returns the maximum number of processor groups that the system can have.
QueryIdleProcessorCycleTime	Retrieves the cycle time for the idle thread of each processor in the system.
QueryIdleProcessorCycleTimeEx	Retrieves the accumulated cycle time for the idle thread on each logical processor in the specified processor group.

User-Mode Scheduling Functions

The following functions are used with user-mode scheduling (UMS).