linux-kernel - Re: [patch] sched: schedstat needs a diet

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Date:	Thu, 18 Oct 2007 15:57:35 -0700
From:	"Ken Chen" <kenchen@...gle.com>
To:	"Mathieu Desnoyers" <mathieu.desnoyers@...ymtl.ca>
Cc:	"Peter Zijlstra" <peterz@...radead.org>,
	"Ingo Molnar" <mingo@...e.hu>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: Re: [patch] sched: schedstat needs a diet

On 10/18/07, Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> wrote:
> Good question indeed. How large is this memory footprint exactly ? If it
> is as small as you say, I suspect that the real issue could be that
> these variable are accessed by the scheduler critical paths and
> therefore trash the caches.

Maybe my wording was ambiguous, I meant to reduce cache line pollution
when accessing these schedstat fields.

With unsigned long, on x86_64, schedstat consumes 288 bytes for each
sched_domain and 128 bytes in struct rq.  On a extremely small system
that has a couple of CPU sockets with one level of numa node, there
will be 704 bytes per CPU for schedstat.  Given the sparseness of
them, we are probably talking about 11-12 cache line eviction on
several heavily used scheduler functions.  Reduce cache line pollution
is the primary goal, actual memory consumption isn't really a concern.

- Ken
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