On Mon, 2 Jun 2008, Claus Guttesen wrote:
% ./bench_time 9079882 | sort -rnk1 Timing micro-benchmark. 9079882 syscall iterations. Avg. us/call Elapsed Name 9.322484 84.647053 gettimeofday(2) 8.955324 81.313291 time(3) 8.648315 78.525684 clock_gettime(2/CLOCK_REALTIME) 8.598495 78.073325 clock_gettime(2/CLOCK_MONOTONIC) 0.674194 6.121600 clock_gettime(2/CLOCK_PROF) 0.648083 5.884515 clock_gettime(2/CLOCK_VIRTUAL) 0.330556 3.001412 clock_gettime(2/CLOCK_REALTIME_FAST) 0.306514 2.783111 clock_gettime(2/CLOCK_SECOND) 0.262788 2.386085 clock_gettime(2/CLOCK_MONOTONIC_FAST)
In previous mail, I said that these were very slow.
rozetta~/devel/c%>sysctl hw.model hw.model: Intel(R) Xeon(R) CPU E5345 @ 2.33GHz rozetta~/devel/c%>./bench_time 9079882 | sort -rnk1 Timing micro-benchmark. 9079882 syscall iterations. Avg. us/call Elapsed Name 1.405469 12.761494 clock_gettime(2/CLOCK_REALTIME) 1.313101 11.922799 time(3) 1.305518 11.853953 clock_gettime(2/CLOCK_MONOTONIC) 1.303947 11.839681 gettimeofday(2) 0.442908 4.021557 clock_gettime(2/CLOCK_PROF) 0.436484 3.963223 clock_gettime(2/CLOCK_VIRTUAL) 0.217718 1.976851 clock_gettime(2/CLOCK_MONOTONIC_FAST) 0.215264 1.954571 clock_gettime(2/CLOCK_REALTIME_FAST) 0.211779 1.922932 clock_gettime(2/CLOCK_SECOND)
These seem about right for a normal untuned ~2GHz system: - there is a syscall overhead of about 200 nsec - the hardware parts of the ACPI (?) timecounter are very slow, so they add 1100 nsec - anomalous extra 100 nsec for CLOCK_REALTIME. CLOCK_REALTIME does less than gettimeofday(). - CLOCK_PROF and CLOCK_VIRTUAL use the slow function calcru() in the kernel. This apparently takes about the same time as a syscall. calcru() uses cpu_ticks() (which normally uses the TSC on i386 and amd64) to determine the time spent since the thread was last context switched, so it is more accurate than CLOCK_REALTIME_FAST but less accurate than CLOCK_REALTIME; using the TSC makes it faster than a non-TSC timecounter. calcru() still seems to have broken accounting for the current timeslice in other running threads in the process.
gettimeofday is 6 times slower on this system, 28 times slower on your system.
1.epsilon times slower on my system :-). Bruce _______________________________________________ freebsd-performance@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-performance To unsubscribe, send any mail to "[EMAIL PROTECTED]"
