On Wed, Jul 27, 2011 at 11:00:45PM -0400, Nick Mathewson wrote:
> On Wed, Jul 27, 2011 at 10:35 PM, William Ahern
> <will...@25thandclement.com> wrote:
>
> If you happen to know, is it the same story with clock_gettime()
> performance? I ask because Libevent uses that function in preference
> to gettimeofday() when it's available.
I quickly ran some tests. Results:
OpenBSD AMD64 4.8: No difference between gettimeofday and clock_gettime.
Both just end up calling bintime() in kern_tc.c. Crazy slow.
Linux x86_64 2.6.35: No difference. In the 3.0 tree gettimeofday() and
CLOCK_REALTIME both just call do_realtime() in
arch/x86/vdso/vclock_gettime.c. CLOCK_MONOTONIC calls do_monotonic() which
is similar code.
Linux x86_64 2.6.30: No difference
Linux i686 2.6.30: gettimeofday() was slightly faster than clock_gettime(),
but I had to crank the iterations up to 20M to really see it. No difference
between CLOCK_REALTIME and CLOCK_MONOTONIC. All of this code seems to reside
in kernel/time/ and kernel/posix-timers.c in the 3.0 tree. clock_gettime()
calls a bunch of function pointers but it _seems_ that it probably ends up
calling getnstimeofday() like gettimeofday(), so the cost difference might
just be because of the indirection of the POSIX timers implementation.
OS X 10.8.0: gettimeofday() as fast as on x86_64 Linux, with all the time
spent in userspace, so they must be doing something similar with reading a
mapped, cached value and offsetting with the CPU TSC.
Here's my code. Linux requires -lrt to find clock_getttime(). Usage:
time ./gtod -i 10M [gtod|rt|mt]
/* gtod.c */
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <sys/time.h>
#include <unistd.h>
#include <err.h>
int main(int argc, char *argv[]) {
extern char *optarg;
extern int optind;
int opt;
struct timeval tv;
struct timespec ts;
unsigned i = 1U<<20;
const char *mode;
while (-1 != (opt = getopt(argc, argv, "i:"))) {
switch (opt) {
case 'i':
i = 0;
for (; *optarg; optarg++) {
switch (*optarg) {
case '0' ... '9':
i *= 10;
i += *optarg - '0';
break;
case 'M': case 'm':
i *= 1U<<20;
break;
case 'K': case 'k':
i *= 1U<<10;
break;
}
}
break;
}
}
mode = (argv[optind])? : "gtod";
if (!strcmp(mode, "gtod")) {
while (i--) {
gettimeofday(&tv, 0);
__asm__("");
}
#if defined(CLOCK_REALTIME)
} else if (!strcmp(mode, "rt")) {
while (i--) {
clock_gettime(CLOCK_REALTIME, &ts);
__asm__("");
}
#endif
#if defined(CLOCK_MONOTONIC)
} else if (!strcmp(mode, "mt")) {
while (i--) {
clock_gettime(CLOCK_MONOTONIC, &ts);
__asm__("");
}
#endif
} else {
errx(EXIT_FAILURE, "%s: unknown mode", mode);
}
return 0;
} /* main() */
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