How to make thread sleep less than a millisecond o

Question

On Windows I have a problem I never encountered on Unix. That is how to get a thread to sleep for less than one millisecond. On Unix you typically have a number of choices (sleep, usleep and nanosleep) to fit your needs. On Windows, however, there is only Sleep with millisecond granularity.

On Unix, I can use the use the select system call to create a microsecond sleep which is pretty straightforward:

int usleep(long usec)
{
    struct timeval tv;
    tv.tv_sec = usec/1000000L;
    tv.tv_usec = usec%1000000L;
    return select(0, 0, 0, 0, &tv);
}

How can I achieve the same on Windows?

Answer 1

As Joel says, you can't meaningfully 'sleep' (i.e. relinquish your scheduled CPU) for such short periods. If you want to delay for some short time, then you need to spin, repeatedly checking a suitably high-resolution timer (e.g. the 'performance timer') and hoping that something of high priority doesn't pre-empt you anyway.

If you really care about accurate delays of such short times, you should not be using Windows.

Answer 2

This indicates a mis-understanding of sleep functions. The parameter you pass is a minimum time for sleeping. There's no guarantee that the thread will wake up after exactly the time specified. In fact, threads don't 'wake up' at all, but are rather chosen for execution by the scheduler. The scheduler might choose to wait much longer than the requested sleep duration to activate a thread, especially if another thread is still active at that moment.

Answer 3

Just use Sleep(0). 0 is clearly less than a millisecond. Now, that sounds funny, but I'm serious. Sleep(0) tells Windows that you don't have anything to do right now, but that you do want to be reconsidered as soon as the scheduler runs again. And since obviously the thread can't be scheduled to run before the scheduler itself runs, this is the shortest delay possible.

Note that you can pass in a microsecond number to your usleep, but so does void usleep(__int64 t) { Sleep(t/1000); } - no guarantees to actually sleeping that period.

Answer 4

Sleep function that is way less than a millisecond-maybe

I found that sleep(0) worked for me. On a system with a near 0% load on the cpu in task manager, I wrote a simple console program and the sleep(0) function slept for a consistent 1-3 microseconds, which is way less than a millisecond.

But from the above answers in this thread, I know that the amount sleep(0) sleeps can vary much more wildly than this on systems with a large cpu load.

But as I understand it, the sleep function should not be used as a timer. It should be used to make the program use the least percentage of the cpu as possible and execute as frequently as possible. For my purposes, such as moving a projectile across the screen in a videogame much faster than one pixel a millisecond, sleep(0) works, I think.

You would just make sure the sleep interval is way smaller than the largest amount of time it would sleep. You don't use the sleep as a timer but just to make the game use the minimum amount of cpu percentage possible. You would use a separate function that has nothing to do is sleep to get to know when a particular amount of time has passed and then move the projectile one pixel across the screen-at a time of say 1/10th of a millisecond or 100 microseconds.

The pseudo-code would go something like this.

while (timer1 < 100 microseconds) {
sleep(0);
}

if (timer2 >=100 microseconds) {
move projectile one pixel
}

//Rest of code in iteration here

I know the answer may not work for advanced issues or programs but may work for some or many programs.

Answer 5

#include <Windows.h>

static NTSTATUS(__stdcall *NtDelayExecution)(BOOL Alertable, PLARGE_INTEGER DelayInterval) = (NTSTATUS(__stdcall*)(BOOL, PLARGE_INTEGER)) GetProcAddress(GetModuleHandle("ntdll.dll"), "NtDelayExecution");

static NTSTATUS(__stdcall *ZwSetTimerResolution)(IN ULONG RequestedResolution, IN BOOLEAN Set, OUT PULONG ActualResolution) = (NTSTATUS(__stdcall*)(ULONG, BOOLEAN, PULONG)) GetProcAddress(GetModuleHandle("ntdll.dll"), "ZwSetTimerResolution");




static void SleepShort(float milliseconds) {
    static bool once = true;
    if (once) {
        ULONG actualResolution;
        ZwSetTimerResolution(1, true, &actualResolution);
        once = false;
    }

    LARGE_INTEGER interval;
    interval.QuadPart = -1 * (int)(milliseconds * 10000.0f);
    NtDelayExecution(false, &interval);
}

yes it uses some undocumented kernel functions, but it works very well, I use SleepShort(0.5); in some of my threds

Answer 6

Try using SetWaitableTimer...