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. They were gathered while I was using
the system like I usually do. There were 40 values above 200 and 7
above 300. I didn't expect those big values to show up. Why does it happen?
It looks like that when I run:
% dd if=/dev/zero of=/tmp/foo
things get more variable. I'm not sure why that would happen. Running
CPU-bound process added for some variation, but not nearly as much as if
I used the dd command (remembert that my computer is rather old).
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
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