nanosleep on Windows with waitable timers?

[Dwedit]

It says on https://justine.lol/cosmopolitan/functions.html that the windows version of "nanosleep" uses SleepEx, which uses milliseconds.

On Windows, "Waitable Timers" allow you to request due times in units of 100 nanoseconds. API functions for these: "CreateWaitableTimerEx", "SetWaitableTimer", then use "WaitForSingleObjectEx" to actually wait for the timer to expire. Also "CloseHandle" to destroy the timer.

But as for what precision and accuracy you'll actually get from Windows though, that is a mystery. The last time I tried anything with waitable timers, they didn't seem any different than calling sleep(1). But I also wasn't using the functions to change the timer resolution. So while you can use make requests using precise units for your waitable timers, you probably won't actually get anything that accurate.

There is a documented function "timeBeginPeriod" from Winmm.dll to set timer precision in units of milliseconds. I used Ghidra to peek at its implementation in Kernel32.dll. It wraps things in a mutual exclusion, and calls an undocumented function "NtSetTimerResolution" from NTDLL.DLL. Ntinternals.net has documentation on the function. Units are 100ns, so to convert from milliseconds, you multiply by 10000.

[jart]

OK so I looked into things, and I'm proud to announce I managed to bring the accuracy of clock_nanosleep down from ~15ms to ~20μs in b490e23.

testing: clock_nanosleep(CLOCK_REALTIME) with relative timeout
           1 ns sleep took       19,730 ns delta       19,729 ns
           4 ns sleep took       35,310 ns delta       35,306 ns
          16 ns sleep took       45,450 ns delta       45,434 ns
          64 ns sleep took       47,950 ns delta       47,886 ns
         256 ns sleep took       46,760 ns delta       46,504 ns
       1,024 ns sleep took       43,120 ns delta       42,096 ns
       4,096 ns sleep took       43,090 ns delta       38,994 ns
      16,384 ns sleep took      486,500 ns delta      470,116 ns
      65,536 ns sleep took      520,630 ns delta      455,094 ns
     262,144 ns sleep took      522,680 ns delta      260,536 ns
   1,048,576 ns sleep took    1,531,310 ns delta      482,734 ns
   4,194,304 ns sleep took    4,775,200 ns delta      580,896 ns
  16,777,216 ns sleep took   17,077,640 ns delta      300,424 ns
  67,108,864 ns sleep took   67,285,750 ns delta      176,886 ns

testing: clock_nanosleep(CLOCK_REALTIME_COARSE) with relative timeout
           1 ns sleep took   14,607,150 ns delta   14,607,149 ns
           4 ns sleep took   15,189,510 ns delta   15,189,506 ns
          16 ns sleep took   15,125,670 ns delta   15,125,654 ns
          64 ns sleep took   15,051,370 ns delta   15,051,306 ns
         256 ns sleep took   15,162,860 ns delta   15,162,604 ns
       1,024 ns sleep took   15,260,680 ns delta   15,259,656 ns
       4,096 ns sleep took   15,351,210 ns delta   15,347,114 ns
      16,384 ns sleep took   15,238,830 ns delta   15,222,446 ns
      65,536 ns sleep took   15,250,730 ns delta   15,185,194 ns
     262,144 ns sleep took   15,189,300 ns delta   14,927,156 ns
   1,048,576 ns sleep took   15,248,730 ns delta   14,200,154 ns
   4,194,304 ns sleep took   15,299,620 ns delta   11,105,316 ns
  16,777,216 ns sleep took   30,498,170 ns delta   13,720,954 ns
  67,108,864 ns sleep took   76,749,120 ns delta    9,640,256 ns

testing: clock_nanosleep(CLOCK_MONOTONIC) with relative timeout
           1 ns sleep took       23,730 ns delta       23,729 ns
           4 ns sleep took       21,260 ns delta       21,256 ns
          16 ns sleep took       19,870 ns delta       19,854 ns
          64 ns sleep took       19,550 ns delta       19,486 ns
         256 ns sleep took       19,200 ns delta       18,944 ns
       1,024 ns sleep took       19,450 ns delta       18,426 ns
       4,096 ns sleep took      119,270 ns delta      115,174 ns
      16,384 ns sleep took      521,880 ns delta      505,496 ns
      65,536 ns sleep took      562,910 ns delta      497,374 ns
     262,144 ns sleep took      516,260 ns delta      254,116 ns
   1,048,576 ns sleep took    1,115,390 ns delta       66,814 ns
   4,194,304 ns sleep took    4,310,770 ns delta      116,466 ns
  16,777,216 ns sleep took   17,090,830 ns delta      313,614 ns
  67,108,864 ns sleep took   67,294,520 ns delta      185,656 ns

testing: clock_nanosleep(CLOCK_BOOTTIME) with relative timeout
           1 ns sleep took       23,620 ns delta       23,619 ns
           4 ns sleep took       19,940 ns delta       19,936 ns
          16 ns sleep took       21,840 ns delta       21,824 ns
          64 ns sleep took       20,550 ns delta       20,486 ns
         256 ns sleep took       19,740 ns delta       19,484 ns
       1,024 ns sleep took       20,440 ns delta       19,416 ns
       4,096 ns sleep took       19,960 ns delta       15,864 ns
      16,384 ns sleep took      549,080 ns delta      532,696 ns
      65,536 ns sleep took      541,520 ns delta      475,984 ns
     262,144 ns sleep took      563,800 ns delta      301,656 ns
   1,048,576 ns sleep took    1,356,980 ns delta      308,404 ns
   4,194,304 ns sleep took    4,378,500 ns delta      184,196 ns
  16,777,216 ns sleep took   17,030,940 ns delta      253,724 ns
  67,108,864 ns sleep took   67,506,250 ns delta      397,386 ns

testing: clock_nanosleep(CLOCK_MONOTONIC_COARSE) with relative timeout
           1 ns sleep took   14,171,530 ns delta   14,171,529 ns
           4 ns sleep took   15,083,890 ns delta   15,083,886 ns
          16 ns sleep took   15,224,290 ns delta   15,224,274 ns
          64 ns sleep took   15,123,010 ns delta   15,122,946 ns
         256 ns sleep took   15,137,050 ns delta   15,136,794 ns
       1,024 ns sleep took   15,214,680 ns delta   15,213,656 ns
       4,096 ns sleep took   15,083,310 ns delta   15,079,214 ns
      16,384 ns sleep took   15,219,230 ns delta   15,202,846 ns
      65,536 ns sleep took   15,288,440 ns delta   15,222,904 ns
     262,144 ns sleep took   15,242,240 ns delta   14,980,096 ns
   1,048,576 ns sleep took   15,287,410 ns delta   14,238,834 ns
   4,194,304 ns sleep took   15,354,790 ns delta   11,160,486 ns
  16,777,216 ns sleep took   30,592,030 ns delta   13,814,814 ns
  67,108,864 ns sleep took   76,511,390 ns delta    9,402,526 ns

So the way this works, is if you call a function like sleep() or usleep() then we'll now use the SetWaitableTimer API. In order for this API to actually wake up the process at the correct time, we need to use NTDLL to put the process into maximum timing accuracy mode. We now do that automatically only during the time you're sleeping, since it'll probably waste power and resources to have it always be enabled. If you don't care about precision sleeps, then simply pass a coarse timer to clock_nanosleep although be warned that coarse timers might cause the sleep to not sleep for the full duration of time specified. This is even the case on other platforms. However Cosmo guarantees with the normal timers that the sleep will sleep for at least the requested time, or longer.

Take a look at the change. Please test it and report back. Most importantly, enjoy it!