Hello, I'm interested in knowing how the cpu data from /proc/stat is gathered. Following my way from this function: http://lxr.linux.no/linux+v2.6.25.10/fs/proc/proc_misc.c#L459 I've figured that the time is probably gathered using those account_*_time on sched.c. I'm not sure where the times are read from, though. Anyhow, I thought I'd do a little test. I expected that if I added together all the values on the cpu line of /proc/stat, waited 1 second and added together all the values again, then their difference would be a constant value. That is, the value should be the same any time I would repeat the experiment. My reasoning was that those values accounted for some unit of time and that the amount of time units between seconds should always be the same. I expected a small variation due to the kernel not always being able to wake the process that was doing the measurement in exactly one second. What I noticed, though, was that sometimes there's a very big variation. For this experiment I used the following bash code: t=`head -n1 /proc/stat | awk '{ print $2 + $3 + $4 + $5 + $6 + $7 + $8 + $9 }'` sleep 1 for (( i=0; i < 1800; i=i+1 )); do nt=`head -n1 /proc/stat | awk '{ print $2 + $3 + $4 + $5 + $6 + $7 + $8 + $9 }'` echo $(( $nt - $t )) >> /tmp/values t=$nt sleep 1 done The file with all the values can be reached at: http://homepages.dcc.ufmg.br/~rafaelc/values The mean of the numbers in the values file was: 109.85 The standard deviation was: 29.47 The maximum was: 589 The minimum was: 99 I found those values rather odd. There were 40 values above 200 and 7 above 300. I didn't expect those big values to show up. I think there might be a relation between the system utilization and the variation, but I'm not sure. Why does it happen? Should I ask this on lkml as well? A little info about my system (Debian etch): $ uname -a Linux gaz 2.6.18-6-686 #1 SMP Fri Jun 6 22:22:11 UTC 2008 i686 GNU/Linux $ cat /proc/cpuinfo processor : 0 vendor_id : GenuineIntel cpu family : 6 model : 8 model name : Pentium III (Coppermine) stepping : 10 cpu MHz : 898.087 cache size : 256 KB fdiv_bug : no hlt_bug : no f00f_bug : no coma_bug : no fpu : yes fpu_exception : yes cpuid level : 2 wp : yes flags : fpu vme de pse tsc msr pae mce cx8 sep mtrr pge mca cmov pat pse36 mmx fxsr sse up bogomips : 1797.57 $ []'s Rafael -- To unsubscribe from this list: send an email with "unsubscribe kernelnewbies" to ecartis@xxxxxxxxxxxx Please read the FAQ at http://kernelnewbies.org/FAQ
