Liu Hao wrote:
在 2021-04-22 20:35, Christian Franke 写道:
This patch significantly increases the precision of gettimeofday() on
Windows 8 or later. It is then similar to Cygwin or Linux.
It also affects C++11 std::chrono:*_clock as
_GLIBCXX_USE_GETTIMEOFDAY is defined but not _GLIBCXX_USE_CLOCK_*.
A similar change (or a call to getntptimeofday()) would possibly also
make sense for clock_gettime(CLOCK_REALTIME,...).
Drawback: GetSystemTimePreciseAsFileTime() is slower than the legacy
function.
But it is still reasonably fast. Average time per call on an old
machine with i7-2600K:
GetSystemTimePreciseAsFileTime(): 23ns
GetSystemTimeAsFileTime(): <3ns
I don't think the complexity is worth.
Which complexity do you mean - the extra cost of the system call or the
extra 15 lines of code?
If users want precision they should use a monotonic clock such as
`QueryPerformanceCounter()`. I can't think of any real usage of
`gettimeofday()` which needs accurate results.
- The API of gettimeofday() provides microsecond precision.
- All other gettimeofday() implementations I tested (Cygwin, Linux,
FreeBSD, ...) provide a resolution better than 1 millisecond.
- GetSystemTimePreciseAsFileTime() combines high resolution with
wall-clock accuracy for long term measurements,
QueryPerformanceCounter() does not.
- The resolution of GetSystemTimeAsFileTime() may change unpredictably
between >15ms to 1ms because it depends on the minimum of all current
"Multimedia Timer" resolutions (timeBegin/EndPeriod()).
- This problem is inherited to all C++11 std::chrono::*clock, in
particular high_resolution_clock - which definitely is an indirect usage
of gettimeofday() where high resolution is expected.
System calls and resolutions of C++11 clocks:
Mingw-w64 MSVC16 Cygwin
system_clock::now() 1 2 2
steady_clock::now() 1 3 3
high_resolution_clock::now() 1 3 3
where
1: GetSystemTimeAsFileTime: >15ms to 1ms (unpredictable)
2: GetSystemTimePreciseAsFileTime: <1us
3: QueryPerformanceCounter: <500ns
AFAICS, my patch would switch the Mingw-w64 column to '2'.
BTW: None of the above provide an extra high_resolution_clock:
GNU Libstdc++ Headers: using high_resolution_clock = system_clock;
Cygwin (Clang headers), MSVC16: using high_resolution_clock = steady_clock;
--
Best regards,
Christian Franke
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