Why user time of the process depends on machine load?

[Artem Belevich]

Hi,

On Wed, Jun 15, 2011 at 2:55 PM, Dan Nelson <dnelson at allantgroup.com> wrote:
> In the last episode (Jun 15), Yuri said:
>> When I test performance of the code, I always observe dependency of CPU
>> user time on the presence of other CPU intense processes.  Same CPU-only
>> deterministic process that on the quiet machine completes in 220 user
>> seconds in the presence of, for example, kde rebuild would complete in
>> 261, 266 or even 379 user seconds.  I am talking about times shown by
>> time(1), not actual an execution time.  It's the same time as getrusage(2)
>> returns in ru_utime field.
>>
>> Why time that process takes in user seconds depends on what other
>> processes are running?
>>
>> FreeBSD-8.2 STABLE on i7 CPU @ 9200 @ 2.67GHz.
>
> Some possible factors:
>
> o Intel Turbo Boost, which raises the clock rate of a single core if the
>  other cores are idle.  A single process on an idle system will run faster.
>
> o i7 chips have a shared L3 cache across all cores, so a single process on
>  an idle system will tend to have more of its data in cache compared to a
>  system with multiple processes, so it spends less time waiting for slower
>  physical memory lookups.
>
> o Process accounting isn't exact.  I may be wrong, but I don't think
>  timestamps are taken every time a syscall is invoked and returns.  Some
>  time marked as "user" may actually be "system" time, in which case you may
>  be seeing the effect of contention in the kernel as more processes are
>  run.

I would add hyper-threading to the list. Once you don't have enough
real cores available to do the job, things do tend to slow down,
unless your process is heavily i/o bound. The time process spends on a
hyper-thread when it's not active still counts towards total time
consumed by the process.

--Artem

>
> You may be able to disable Turbo Boot in your BIOS, which you can use to
> determine how much of the single-process speedup is due to that.
>
> Unrelated but still interesting note on your particular CPU:
> http://www.passmark.com/forum/showthread.php?t=2256
>
> --
>        Dan Nelson
>        dnelson at allantgroup.com
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