On 05/03/10 18:00, cronfy wrote:
Hello,
I want to understand difference between user CPU time and system CPU
time in system accounting.
When some process uses many system CPU, does it really mean that
process prouduces heavy load on server and takes up resources that
could be used by other tasks instead? Or it only means that this
process performs many waits for, say, I/O operations?
In contrast to Linux (luckily), IO wait in itself in FreeBSD does not
count as CPU time. Linux's "iowait" state is supposedly defined as "idle
but having outstanding IO requests" but judging from performance curves
in some cases I think some polling is involved.
System time is time spent in the kernel - it is practically always the
result of issuing kernel syscalls from userland processes. Whether it
influences the overall performance of the system depends on what the
processes are doing - if the machine has multiple CPU cores and the
syscalls exercise those parts of the system that are parallelized then
no, your process would not influence the rest of the system anymore than
it would by using "user" time.
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. And by using the file
system in certain ways, you really can block other processes from using
it with the performance they would get otherwise. You can find out at a
glance approximately what the process is doing simply by monitoring the
"top" line for the process.
There is one more thing - if the machine in question is a virtual
machine, it is normal for the "system" time to, unfortunately, be
significantly higher than it would be otherwise because of the nature of
the emulation.
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