User cpu time VS system cpu time

[cronfy]

Hello,

>> I want to understand difference between user CPU time and system CPU
>> time in system accounting.
> But keep in mind that "kernel time" is a broad category - while IO time in
> itself does not count as CPU time, file system operations for example do,
> because they really can be CPU intensive.

Ivan, thanks for the great explanation.

I think that I can measure user filesystem usage with sa - it reports
number of IO operations per user/command. In which other cases kernel
time is used instead of user time for a process? I do not mean all of
them - just that usually occur in practice.

I've noticed that there are moments when system load in top for system
time is very high (60-80% while user load is 15-25%, this produces
very high LA also). All processes that were run at this time show high
kernel time usage, although they usually do not. System is getting
back to normal after Apache restart (I think this is related to Apache
shared memory somehow, but not sure).

This makes me suspect that system time in sa can not be relied on
while measuring user system usage, because it notably varies under
some circumstances for same operations. Am I wrong?


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// cronfy