strange performance differences of 1.2.2 vs 1.2.5
sean at chittenden.org
Mon Jun 2 04:34:36 UTC 2008
>> That's a good guess (Brian? You know about that?:) ... but a bit
> When I submitted the first patch using clock_gettime() there was a
> push back for this, so I moved back to gettimeofday(). I don't
> remember the argument now, but gettimeofday() is at least better
> then time() (which was what was there first).
> One point, the time value is not being properly stored anyway.
> Technically it requires a lock around it (volatile is not enough to
> guarantee synchronous update).
> I would argue along with Sean that clock_gettime() would be better,
> but I can not remember any longer what the objections were.
When in doubt, benchmark (and I'm glad I did). I wasn't present/
involved with the discussion, but I'd guess it had something to do
with POSIX being lax in its definition of clockid_t, the meaning of
its values and various clockid_t values not being defined universally.
I wrote a small micro-benchmark utility to see what's the fastest time
syscall. I cribbed phk@'s recommendation for CLOCK_MONOTONIC/
MONOTONIC_FAST in a post which I blindly repeated earlier in this
thread... and that information happened to be either old or
incorrect. time(3) is just as expensive as gettimeofday(2) (which
surprised the hell out of me - why it's not implemented in terms of
CLOCK_SECOND, I'm not sure). clock_gettime(CLOCK_REALTIME_FAST) is
the fastest time syscall for non-monotomic time retrieval.
% ./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):
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
For the kernel curious:
sean at chittenden.org
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