I was wondering if anyone here has experience with FreeBSD 6.2? I
was compiling 1.2.5 for one of the groups at Yahoo and they saw
dramatic performance decreases with 1.2.5 over 1.2.2. Has there been
an introduction of code I am forgetting that may cause this?
Summary: For 90% of all cases: "clock_gettime(CLOCK_MONOTONIC, ...)"
== good. "gettimeofday(2)" == bad.
I haven't benched 1.2.5, but I'd bet a beer that the culprit is the
following 1.2.5 change:
* Use gettimeofday(2) instead of time(2).
For background on why gettimeofday(2) is more expensive, see the
following thread:
http://lists.freebsd.org/mailman/htdig/freebsd-current/2005-October/057280.html
or for a brief discussion of this in a white paper format:
http://dsd.lbl.gov/publications/imc-2003.pdf
Essentially, gettimeofday(2) is supposed to be highly accurate
according to POSIX, FreeBSD guarantees accuracy and Linux makes no
such guarantees (hence the performance hit when going from time(2) to
gettimeofday(2) for BSD and not Linux). Proper implementation of
gettimeofday(2) requires synchronization across CPUs, whereas time(2)
does not. memcached *should not* use gettimeofday(2) unless highly
accurate and sub-second resolution is a requirement. If highly
precise, but inaccurate time keeping is acceptable,
clock_gettime(CLOCK_MONOTONIC, ...) should be used instead (using a
system call that requires CPU synchronization is the opposite of good
for a performance oriented daemon).
The above thread has all the gory details, but the performance of time
system calls is an FAQ that should be widely understood at this point.
-sc
--
Sean Chittenden
[EMAIL PROTECTED]
