Micro-benchmark for various time syscalls...

[Sean Chittenden]

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
sean at chittenden.org
http://sean.chittenden.org/