GetSystemTimePreciseAsFileTime (was Re: [ANNOUNCEMENT] Updated: Cygwin 1.7.19)

Sun Jun 9 11:53:00 GMT 2013

On Jun 5 18:28, Corinna Vinschen wrote:
> On Jun 6 01:02, Shaddy Baddah wrote:
> > On 2013年06月05日 19:39+1000, Corinna Vinschen wrote:
> > >- Drop support for Windows 2000 and Windows XP pre-SP3.
> > 
> > I find this change interesting. In no way am I complaining about this,
> > releases matching the above are well past end-of-life anyway.
> > 
> > But I am curious about the technical reason (and perhaps discussion)
> > that ruled Win 2K out.
> > [...]
> [...]
> There's also stuff we still not use a lot, foremost the Win32 API call
> CancelSynchronousIo which, if it had been introduced in NT4 already,
> would be probably heavily used by Cygwin (think signal handling).
> The next big thing developement-wise is not Windows 7, but Windows 8 (of
> all things!), because of the new GetSystemTimePreciseAsFileTime call,
> which I'm going to introduce into Cygwin pretty soon.

Or not. I just tested the GetSystemTimePreciseAsFileTime call and it's
a wonderful performance killer.
Below I pasted my simple testcase. It computes the average number of
CPU cycles per call to compare GetSystemTimeAsFileTime and
GetSystemTimePreciseAsFileTime with each other.
I tested the call on two 64 bit Windows 8 systems, one real machine,
one QEMU/KVM based virtual machine. On real hardware I get:
On the virtual machine, the results are:
 Best case:
 8 cycles for GetSystemTimeAsFileTime
 15557 cycles for GetSystemTimePreciseAsFileTime
 Worst case under load:
 20 cycles for GetSystemTimeAsFileTime
 17443 cycles for GetSystemTimePreciseAsFileTime
On real hardware, the results are much better, but the difference between
GetSystemTimeAsFileTime and GetSystemTimePreciseAsFileTime are still
terrible:
 Best case:
 9 cycles for GetSystemTimeAsFileTime
 2761 cycles for GetSystemTimePreciseAsFileTime
 Worst case under load:
 18 cycles for GetSystemTimeAsFileTime
 5874 cycles for GetSystemTimePreciseAsFileTime
Corinna
#include <stdio.h>
#include <stdint.h>
#include <windows.h>
void (WINAPI *pGetSystemTimeAsFileTime) (LPFILETIME);
void (WINAPI *pGetSystemTimePreciseAsFileTime) (LPFILETIME);
static inline uint64_t
rdtsc(void)
{
 uint32_t low, high;
 __asm __volatile("rdtsc" : "=a" (low), "=d" (high));
 return (low | ((uint64_t)high << 32));
}
static void
cycle (void (WINAPI *f) (LPFILETIME), const char *name)
{
 const int iter = 1000000;
 int i;
 uint64_t v;
 FILETIME l;
 v = rdtsc ();
 for (i = 0; i < iter; i++)
 f (&l);
 v = rdtsc () - v;
 v /= iter;
 printf ("%6lld cycles for %s\n", v, name);
}
int main ()
{
 HMODULE h = GetModuleHandle ("kernel32.dll");
 pGetSystemTimeAsFileTime = (void (WINAPI *)(LPFILETIME))
 			GetProcAddress (h, "GetSystemTimeAsFileTime");
 pGetSystemTimePreciseAsFileTime = (void (WINAPI *)(LPFILETIME))
 			GetProcAddress (h, "GetSystemTimePreciseAsFileTime");
 if (!pGetSystemTimePreciseAsFileTime)
 {
 fputs ("GetSystemTimePreciseAsFileTime unsupported\n", stderr);
 return 1;
 }
 cycle (pGetSystemTimeAsFileTime, "GetSystemTimeAsFileTime");
 cycle (pGetSystemTimePreciseAsFileTime, "GetSystemTimePreciseAsFileTime");
 return 0;
}
-- 
Corinna Vinschen Please, send mails regarding Cygwin to
Cygwin Maintainer cygwin AT cygwin DOT com
Red Hat
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