linux-kernel - Re: [RFC] cfq: adapt slice to number of processes doing I/O

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Message-ID: <x49ocprx4on.fsf@segfault.boston.devel.redhat.com>
Date:	Thu, 03 Sep 2009 14:29:28 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Corrado Zoccolo <czoccolo@...il.com>
Cc:	Linux-Kernel <linux-kernel@...r.kernel.org>,
	Jens Axboe <jens.axboe@...cle.com>
Subject: Re: [RFC] cfq: adapt slice to number of processes doing I/O

Corrado Zoccolo <czoccolo@...il.com> writes:

> Hi Jeff,
>
>> How did you come to this magic number of 3, both for the number of
>> competing tasks and the multiplier for the slice time?  Did you
>> experiment with this number at all?
>
> The number is quickly explained. The base slice is 100ms, and on the
> mailing list it was mentioned that the latency becomes annoing for an
> user when it is above 300ms.  This means that, up to 3 processes, the
> current thresholds are good, and for more we have to scale.  This is
> good, because we don't change the behaviour unless we have many
> competing processes.

OK, then in your next patch, could you make this more clear?  Maybe
define a LATENCY_MAX and derive the number of processes from that,
instead of assuming that the base slice will always and forever be
100ms?

Cheers,
Jeff
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