linux-kernel - Re: fair clock use in CFS

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Date:	Mon, 14 May 2007 18:34:12 +0530
From:	Srivatsa Vaddagiri <vatsa@...ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	efault@....de, tingy@...umass.edu, wli@...omorphy.com,
	linux-kernel@...r.kernel.org
Subject: Re: fair clock use in CFS

On Mon, May 14, 2007 at 01:10:51PM +0200, Ingo Molnar wrote:
> but let me give you some more CFS design background:

Thanks for this excellent explanation. Things are much clearer now to
me. I just want to clarify one thing below:

> > 2. Preemption granularity - sysctl_sched_granularity

[snip]

> This granularity value does not depend on the number of tasks running. 

Hmm ..so does sysctl_sched_granularity represents granularity in
real/wall-clock time scale then? AFAICS that doesnt seem to be the case.

__check_preempt_curr_fair() compares for the distance between the two
task's (current and next-to-be-run task) fair_key values for deciding
preemption point.

Let's say that to begin with, at real time t0, both current task Tc and next 
task Tn's fair_key values are same, at value K. Tc will keep running until its 
fair_key value reaches atleast K + 2000000. The *real/wall-clock* time taken 
for Tc's fair_key value to reach K + 2000000 - is surely dependent on N,
the number of tasks on the queue (more the load, more slowly the fair
clock advances)?

This is what I meant by my earlier remark: "If there a million cpu hungry tasks,
then the (real/wall-clock) time taken to switch between two tasks is more 
compared to the case where just two cpu hungry tasks are running".

-- 
Regards,
vatsa
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