Re: Some ideas about % of CPU for a process

From: Valdis . Kletnieks
Date: Fri Nov 12 2004 - 14:33:16 EST

On Fri, 12 Nov 2004 19:55:33 +0100, Ed Schouten said:

> On Fri 12 Nov 2004 03:47 PM, Diego wrote:
> > I'm trying to define a % of cpu for a process, but i don?t have idea
> > about how i can do it. For example, i said that my process need 40% of
> > CPU during its lifetime, how can i do it in kernel 2.6?
> > Thanks for ideas.
> Why don't you 'ps' the process and divide the running time through the
> starting time?

I think he's trying to make a 40% *allotment* - as in "this number cruncher
is authorized to take 40% of the CPU for the next 6 hours". The devil always
ends up being in the details however - things I've seen scheduling systems
fail with in the past:

1) Process is allowed to take 40% of the CPU, but so overcommits memory that
it can't actually *get* it because 65% of the time, the system is in a page
wait from the swap partition. (iterate across any *other* shared resource
that isn't directly tied to CPU).

2) Process is allowed to take 40% of the CPU, and no more - and somebody gets
torqued off because the system is 60% idle.

3) Process is allowed to take 40% at all times, and more if available - and
somebody gets torqued off because it's slow to notice the system has bogged
down - so it's still getting 60% for another 2-3 minutes when we're loaded
before the scheduler catches up.

4) And you always get the whiner who complains it's getting 39% or 41%. ;)

Saying "40% of the CPU" is easy - the fun is in the corner cases...

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