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[Bug libstdc++/63400] [C++11]precision of std::chrono::high_resolution_clock
- From: "frankhb1989 at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: Mon, 29 Sep 2014 21:00:23 +0000
- Subject: [Bug libstdc++/63400] [C++11]precision of std::chrono::high_resolution_clock
- Auto-submitted: auto-generated
- References: <bug-63400-4 at http dot gcc dot gnu dot org/bugzilla/>
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63400
--- Comment #4 from frankhb1989 at gmail dot com ---
(In reply to Jonathan Wakely from comment #3)
> What libstdc++ is doing is sensible, why is the realtime clock so much
> coarser than the monotonic clock on mingw-w64?
It is not always true that the real time clock would have a higher resolution
than monotonic clock. At least I found nothing about resolution guaranteed by
POSIX, in fact, "clock resolutions are implementation-defined".
With mingw-w64, the following program shows that the monotonic clock is far
more precise:
#include <pthread_time.h>
#include <iostream>
int main()
{
using namespace std;
timespec ts;
if(clock_getres(CLOCK_REALTIME, &ts) == 0)
cout << "CLOCK_REALTIME: " << ts.tv_sec << ',' << ts.tv_nsec << endl;
if(clock_getres(CLOCK_MONOTONIC, &ts) == 0)
cout << "CLOCK_MONOTONIC: " << ts.tv_sec << ',' << ts.tv_nsec << endl;
}
The result on my machine:
CLOCK_REALTIME: 0,15625000
CLOCK_MONOTONIC: 0,489
I then found the actual implementation in winpthreads (clock.c):
int clock_gettime(clockid_t clock_id, struct timespec *tp)
{
unsigned __int64 t;
LARGE_INTEGER pf, pc;
union {
unsigned __int64 u64;
FILETIME ft;
} ct, et, kt, ut;
switch(clock_id) {
case CLOCK_REALTIME:
{
GetSystemTimeAsFileTime(&ct.ft);
t = ct.u64 - DELTA_EPOCH_IN_100NS;
tp->tv_sec = t / POW10_7;
tp->tv_nsec = ((int) (t % POW10_7)) * 100;
return 0;
}
case CLOCK_MONOTONIC:
{
if (QueryPerformanceFrequency(&pf) == 0)
return lc_set_errno(EINVAL);
if (QueryPerformanceCounter(&pc) == 0)
return lc_set_errno(EINVAL);
tp->tv_sec = pc.QuadPart / pf.QuadPart;
tp->tv_nsec = (int) (((pc.QuadPart % pf.QuadPart) * POW10_9 +
(pf.QuadPart >> 1)) / pf.QuadPart);
if (tp->tv_nsec >= POW10_9) {
tp->tv_sec ++;
tp->tv_nsec -= POW10_9;
}
return 0;
}
case CLOCK_PROCESS_CPUTIME_ID:
{
if(0 == GetProcessTimes(GetCurrentProcess(), &ct.ft, &et.ft, &kt.ft,
&ut.ft))
return lc_set_errno(EINVAL);
t = kt.u64 + ut.u64;
tp->tv_sec = t / POW10_7;
tp->tv_nsec = ((int) (t % POW10_7)) * 100;
return 0;
}
case CLOCK_THREAD_CPUTIME_ID:
{
if(0 == GetThreadTimes(GetCurrentThread(), &ct.ft, &et.ft, &kt.ft,
&ut.ft))
return lc_set_errno(EINVAL);
t = kt.u64 + ut.u64;
tp->tv_sec = t / POW10_7;
tp->tv_nsec = ((int) (t % POW10_7)) * 100;
return 0;
}
default:
break;
}
return lc_set_errno(EINVAL);
}
For CLOCK_REALTIME, the Windows API GetSystemTimeAsFileTime is used.
GetSystemTimePreciseAsFileTime is an improved version which provide "the
highest possible level of precision (<1us)". Unfortunately, the latter is only
available since Windows 8/Windows 2012, which is not suited for winpthreads for
compatibility reason IMO. See this MSDN page for details:
http://msdn.microsoft.com/en-us/library/windows/desktop/hh706895(v=vs.85).aspx
For CLOCK_MONOTONIC, QPC(Query Performance Counter) APIs are used. This method
is reasonably portable (among different versions of Windows, since Windows
2000), and Microsoft strongly suggested it when high-resolution time stamps
needed, see
http://msdn.microsoft.com/en-us/library/windows/desktop/dn553408(v=vs.85).aspx.
However, QPC is not suited for a system-wide clock.