I wrote a small micro-benchmark utility[1] to test various time
syscalls and the results were a bit surprising to me. The results
were from a UP machine and I believe that the difference between
gettimeofday(2) and clock_gettime(CLOCK_REALTIME_FAST) would've been
bigger on an SMP system and performance would've degraded further with
each additional core.
clock_gettime(CLOCK_REALTIME_FAST) is likely the ideal function for
most authors (CLOCK_REALTIME_FAST is supposed to be precise to +/-
10ms of CLOCK_REALTIME's value[2]). In fact, I'd assume that
CLOCK_REALTIME_FAST is just as accurate as Linux's gettimeofday(2) (a
statement I can't back up, but believe is likely to be correct) and
therefore there isn't much harm (if any) in seeing clock_gettime(2) +
CLOCK_REALTIME_FAST receive more widespread use vs. gettimeofday(2).
FYI. -sc
PS Is there a reason that time(3) can't be implemented in terms of
clock_gettime(CLOCK_SECOND)? 10ms seems precise enough compared to
time_t's whole second resolution.
% ./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)
Last value from gettimeofday(2): 1212380080.620649
Last value from time(3): 1212380161
Last value from clock_gettime(2/CLOCK_VIRTUAL): 2.296430000
Last value from clock_gettime(2/CLOCK_SECOND): 1212380338.000000000
Last value from clock_gettime(2/CLOCK_REALTIME_FAST):
1212380243.461081040
Last value from clock_gettime(2/CLOCK_REALTIME): 1212380240.459788612
Last value from clock_gettime(2/CLOCK_PROF): 185.560343000
Last value from clock_gettime(2/CLOCK_MONOTONIC_FAST): 5747219.271879584
Last value from clock_gettime(2/CLOCK_MONOTONIC): 5747216.886509281
[1] http://sean.chittenden.org/pubfiles/freebsd/bench_time.c
[2] sys/time.h comments about precision.
http://fxr.watson.org/fxr/source/sys/time.h#L269
--
Sean Chittenden
[EMAIL PROTECTED]
http://sean.chittenden.org/
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