cpu usage in 7.0
jroberson at chesapeake.net
Sun Feb 24 22:46:11 UTC 2008
On Sun, 24 Feb 2008, Ivan Voras wrote:
> Kris Kennaway wrote:
>> Hugo Silva wrote:
>>> Kris Kennaway wrote:
>>>> Barney Cordoba wrote:
>>>>> I have a dual core system running 7.0 and I can't get
>>>>> top to show more than 100% usage no matter how I
>>>>> hammer it. My MAC shows over 100% often, but its not
>>>>> clear if top is averaging the 2 cpus or just not going
>>>>> over 100, or just showing 1 of the cpus.
>>>> 100% in FreeBSD means "all of your CPUs are completely active". It is
>>>> hard to exceed this amount :-)
>>> You should see my production mysql going over 458% on service startup, on
>>> a quad core server :-)
>> That is a multithreaded process using multiple CPUs, not the total CPU
>> statistics (first line of top(1)).
> So how does a multithreaded process get 458% CPU on a quad-core machine? :)
> (Really, I want to know; I thought thread CPU accounting was fixed in 7.x.
> Unless I'm mistaken, 4 CPU-intensive threads in a single process should
> account as 4 CPU-intensive single-thread processes; i.e. each could only take
> up to 100% of a core/CPU, accounting for NCPU*100% total).
It is possible for the sum of all threads in the system to exceed 100%
cpu. This is because the decay function is not precise. 15% over is a
bit more than I would expect but I suppose it's possible. We also inhert
pcpu information from the parent on fork/thread creation so the child
isn't created with a priority as if it had been idle. So for a moment the
utilization is duplicated.
Penalizing the child for an expensive parent is an important optimization
that prevents batch jobs from overwhelming the system under load.
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