linux-kernel - Re: fair clock use in CFS

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Message-Id: <200705141802.05158.a1426z@gawab.com>
Date:	Mon, 14 May 2007 18:02:05 +0300
From:	Al Boldi <a1426z@...ab.com>
To:	linux-kernel@...r.kernel.org
Subject: Re: fair clock use in CFS

Ingo Molnar wrote:
> the current task is recalculated at scheduler tick time and put into the
> tree at its new position. At a million tasks the fair-clock will advance
> little (or not at all - which at these load levels is our smallest
> problem anyway) so during a scheduling tick in kernel/sched_fair.c
> update_curr() we will have a 'delta_mine' and 'delta_fair' of near zero
> and a 'delta_exec' of ~1 million, so curr->wait_runtime will be
> decreased at 'full speed': delta_exec-delta_mine, by almost a full tick.
> So preemption will occur every sched_granularity (rounded up to the next
> tick) points in time, in wall-clock time.

The only problem I have with this fairness is the server workload that 
services requests by fork/thread creation.  In such a case, this fairness is 
completely counter-productive, as running tasks unfairly inhibit the 
creation of peers.

Giving fork/thread creation special priority may alleviate this problem.


Thanks!

--
Al

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