linux-kernel - RE: Volanomark slows by 80% under CFS

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Message-ID: <MDEHLPKNGKAHNMBLJOLKGEEIFLAC.davids@webmaster.com>
Date:	Sat, 28 Jul 2007 11:55:35 -0700
From:	"David Schwartz" <davids@...master.com>
To:	"Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>
Subject: RE: Volanomark slows by 80% under CFS

> > Volanomark runs better
> > and is only 40% (instead of 80%) down from old scheduler
> > without CFS.

> 40 or 80 % is still a huge regression.
> Dmitry Adamushko

Can anyone explain precisely what Volanomark is doing? If it's something
dumb like "looping on sched_yield until the 'right' thread runs and finishes
what we're waiting for" then I think any regression can be ignored.

This applies if and only if CFS' sched_yield behavior is sane and Volano's
is insane.

A sane sched_yield implementation must do two things:

1) Reward processes that actually do yield most of their CPU time to another
process.

2) Make an effort to run every ready-to-run process at the same or higher
static priority level before re-scheduling this process. (That won't always
be possible due to SMP issues, but a reasonable effort is needed.)

If CFS is doing these two things, and Volanomark is looping on sched_yield
until the 'right thread' runs, then CFS is doing the right and Volanomark
isn't. Volanomark deserves to lose.

If CFS binds processes to processors more tightly and thus sched_yield can't
yield to a process that was planned to run on another CPU in the future,
that would be a legitimate complaint about CFS.

DS

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