[ale] jiffies & cat /proc & mysql - oh, my!

[Sid Lane]

all,

I'm playing w/a script that monitors cpu utilization (mysqld in my case) by
taking the delta of user & system jiffies in /proc/$PID between loops at
regular intervals.  since individual jiffy mileage may vary I did a
calibration test by watching a gzip -c bigarsefile.gz > /dev/null.  100% of
the time the delta between loops is 100 jiffies/sec while top reports that
PID @ 100% cpu utilization (i.e. saturating one core - the goal for the
benchmark).  this leads me to conclude that a jiffy on my box (dual
quad-core Dell 2950 running openSuSE 11 FWIW) is .01 CPU*s so I should have
a theoretical 800 jiffies/sec on this box (8 cores).  the problem (or ? I
can't explain) is that I have a data point for my mysqld PID where the 10
second delta is 9,402 (7,586 user/1,834 sys) which is almost 20% above what
should be possible.  it would make sense if a jiffy were 1/120th of a second
(=> 9,600 would be available in 10 sec) but (again) my gzip test
consistently produces 500 per 5 seconds (=> 100/s) so I don't know what to
make of this data point.  I though about lack of time precision in my
measurements but 18%?  for a 30 second sample?  when the lowest 5 second
value for the gzip test was 498/highest 502?

any idea what to make of this?  I need to have confidence in this data for
scaling testing/planning I'm doing...
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://mail.ale.org/pipermail/ale/attachments/20090130/6a93608c/attachment.html