On 05/06/10 13:33, cronfy wrote:
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.

Everything the kernel does when requested by the user is counted as kernel time - file system access, network access, getpid(), gettimeofday(), process scheduling, memory management, etc.

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).

As I told you before - monitor the "top" line for the processes you suspect and you will get a fairly good idea what they are doing. Look at the "STATE" column.

When you are looking at per-process statistics, the system time is also accounted. For example, if a process takes 50% of a CPU, it is possible that it takes 25% in userspace and 25% in kernel (the reverse is not true - kernel can take system CPU time without it being accounted on behalf of processes).

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?

Everything can be accounted for by enough statistics :)

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