Forcing context switch in Windows

Question

Is there a way to force a context switch in C++ to a specific thread, assuming I have the thread handle or thread ID?

Answer 1

No, you won't be able to force operating system to run the thread you want. You can use yield to force a context switch though...

yield in Win32 API is function SwitchToThread. If there is no other thread available for running, then a ZERO value will be returned and current thread will keep running anyway.

Answer 2

You can only encourage the Windows thread scheduler to pick a certain thread, you can't force it. You do so first by making the thread block on a synchronization object and signaling it. Secondary by bumping up its priority.

Explicit context switching is supported, you'll have to use fibers. Review SwitchToFiber(). A fiber is not a thread by a long shot, it is similar to a co-routine of old. Fibers' heyday has come and gone, they are not competitive with threads anymore. They have very crappy cpu cache locality and cannot take advantage of multiple cores.

Answer 3

The only way to force a particular thread to run is by using process/thread affinity, but I can't imagine ever having a problem for which this was a reasonable solution.

The only way to force a context switch is to force a thread onto a different processor using affinity.

In other words, what you are trying to do isn't really viable.

Calling SwitchToThread() will result in a context switch if there is another thread ready to run that are eligible to run on this processor. The documentation states it as follows:

If calling the SwitchToThread function causes the operating system to switch execution to another thread, the return value is nonzero.

If there are no other threads ready to execute, the operating system does not switch execution to another thread, and the return value is zero.

Answer 4

You can temporarily bump the priority of the other thread, while looping with Sleep(0) calls: this passes control to other threads. Suppose that the other thread has increased a lock variable and you need to wait until it becomes zero again:

// Wait until other thread releases lock
SetThreadPriority(otherThread, THREAD_PRIORITY_HIGHER);
while (InterlockedRead(&lock) != 0)
  Sleep(0);
SetThreadPriority(otherThread, THREAD_PRIORITY_NORMAL);

Answer 5

I would check out the book Concurrent Programming for Windows. The scheduler seems to do a few things worth noting.

Sleep(0) only yields to higher priority threads (or possibly others at the same priority). This means you cannot fix priority inversion situations with just a Sleep(0), where other lower priority threads need to run. You must use SwitchToThread, Sleep a non-zero duration, or fully block on some kernel HANDLE.

Answer 6

You can create two synchronization objects (such as two events) and use the API SignalObjectAndWait.

If the hObjectToWaitOn is non-signaled and your other thread is waiting on the hObjectToSignal, the OS can theoretically perform quick context switch inside this API, before end of time slice.

And if you want the current thread to automatically resume, simply inform a small value (such as 50 or 100) on the dwMilliseconds.