[Openais] clock_gettime(2) notes...

[Sean Chittenden]

re: clock_gettime(2).  I have a few random things to add there that  
came out of some benchmarking that I did about a year ago for FreeBSD  
and is of some relevance.  Issues regarding the correctness of yet- 
another-clock_id aside, on FreeBSD CLOCK_REALTIME_FAST is ridiculously  
fast compared to CLOCK_REALTIME.  I don't know what degree of  
precision you're looking for in these time calls since  
CLOCK_REALTIME_FAST only gets updated once a HZ, but it's a pretty  
significant speedup.

The URL to the full post is below, but the specific excerpts from the  
benchmark can be seen here:

% ./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

http://sean.chittenden.org/news/2008/06/01/

Random drive-by $0.02. -sc


--
Sean Chittenden
sean at chittenden.org






--
Sean Chittenden
sean at chittenden.org