Sleep Less Than One Millisecond

Question

On windows you have a problem you typically never encounter 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. You can however use the select system call to create a microsecond sleep. On Unix this is pretty straight forward:

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

---

On windows however, the use of select forces you to include the winsock library which has to be initialized like this in your application:

WORD wVersionRequested = MAKEWORD(1,0);
WSADATA wsaData;
WSAStartup(wVersionRequested, &wsaData);

And then the select won't allow you to be called without any socket so you have to do a little more to create a microsleep method:

int usleep(long usec)
{
    struct timeval tv;
    fd_set dummy;
    SOCKET s = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP);
    FD_ZERO(&dummy);
    FD_SET(s, &dummy);
    tv.tv_sec = usec/1000000L;
    tv.tv_usec = usec%1000000L;
    return select(0, 0, 0, &dummy, &tv);
}

All these created usleep methods return zero when successful and non-zero for errors.

Answer 1

On windows however, the use of select forces you to include the winsock library which has to be initialized like this in your application:

WORD wVersionRequested = MAKEWORD(1,0);
WSADATA wsaData;
WSAStartup(wVersionRequested, &wsaData);

And then the select won't allow you to be called without any socket so you have to do a little more to create a microsleep method:

int usleep(long usec)
{
    struct timeval tv;
    fd_set dummy;
    SOCKET s = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP);
    FD_ZERO(&dummy);
    FD_SET(s, &dummy);
    tv.tv_sec = usec/1000000L;
    tv.tv_usec = usec%1000000L;
    return select(0, 0, 0, &dummy, &tv);
}

All these created usleep methods return zero when successful and non-zero for errors.

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

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 4

Use the high resolution timers available in winmm.lib. See this for an example.

Answer 5

What are you waiting for that requires such precision? In general if you need to specify that level of precision (e.g. because of a dependency on some external hardware) you are on the wrong platform and should look at a real time OS.

Otherwise you should be considering if there is an event you can synchronize on, or in the worse case just busy wait the CPU and use the high performance counter API to measure the elapsed time.

Answer 6

Yes, you need to understand your OS' time quantums. On Windows, you won't even be getting 1ms resolution times unless you change the time quantum to 1ms. (Using for example timeBeginPeriod()/timeEndPeriod()) That still won't really guarantee anything. Even a little load or a single crappy device driver will throw everything off.

SetThreadPriority() helps, but is quite dangerous. Bad device drivers can still ruin you.

You need an ultra-controlled computing environment to make this ugly stuff work at all.

Answer 7

Try boost::xtime and a timed_wait()

has nanosecond accuracy.

Answer 8

As several people have pointed out, sleep and other related functions are by default dependent on the "system tick". This is the minimum unit of time between OS tasks; the scheduler, for instance, will not run faster than this. Even with a realtime OS, the system tick is not usually less than 1 ms. While it is tunable, this has implications for the entire system, not just your sleep functionality, because your scheduler will be running more frequently, and potentially increasing the overhead of your OS (amount of time for the scheduler to run, vs. amount of time a task can run).

The solution to this is to use an external, high-speed clock device. Most Unix systems will allow you to specify to your timers and such a different clock to use, as opposed to the default system clock.

Answer 9

If you want so much granularity you are in the wrong place (in user space). Remember that if you are in user space your time is not always précis. The scheduler can start your thread (or app), and schedule it, so you are depending by the OS scheduler . If you are looking for something precisely you have to go: 1) In kernel space (like drivers) 2) Choose an RTOS. Anyway if you are looking for some granularity (but remember the problem with user space ) look to QueryPerformanceCounter Function and QueryPerformanceFrequency function in MSDN.

Answer 10

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 11

I have the same problem and nothing seems to be faster than a ms, even the Sleep(0). My problem is the communication between a client and a server application where I use the _InterlockedExchange function to test and set a bit and then I Sleep(0).

I really need to perform thousands of operations per second this way and it doesn't work as fast as I planned.

Since I have a thin client dealing with the user, which in turn invokes an agent which then talks to a thread, I will move soon to merge the thread with the agent so that no event interface will be required.

Just to give you guys an idea how slow this Sleep is, I ran a test for 10 seconds performing an empty loop (getting something like 18,000,000 loops) whereas with the event in place I only got 180,000 loops. That is, 100 times slower!

Answer 12

Actually using this usleep function will cause a big memory/resource leak. (depending how often called)

use this corrected version (sorry can't edit?)

int usleep(long usec)
{
    struct timeval tv;
    fd_set dummy;
    SOCKET s = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP);
    FD_ZERO(&dummy);
    FD_SET(s, &dummy);
    tv.tv_sec = usec/1000000L;
    tv.tv_usec = usec%1000000L;

    bool sucess = 0 == select(0, 0, 0, &dummy, &tv);
    closesocket(dummy);

    return sucess;
}