linux-kernel - Re: CFS scheduler unfairly prefers pinned tasks

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Message-Id: <201510062044.t96KiGXd021728@como.maths.usyd.edu.au>
Date:	Wed, 7 Oct 2015 07:44:16 +1100
From:	paul.szabo@...ney.edu.au
To:	umgwanakikbuti@...il.com
Cc:	linux-kernel@...r.kernel.org
Subject: Re: CFS scheduler unfairly prefers pinned tasks

Dear Mike,

>> ... the CFS is meant to be fair, using things like vruntime
>> to preempt, and throttling. Why are those pinned tasks not preempted or
>> throttled?
>
> Imagine you own a 8192 CPU box for a moment, all CPUs having one pinned
> task, plus one extra unpinned task, and ponder what would have to happen
> in order to meet your utilization expectation. ...

Sorry but the kernel contradicts. As per my original report, things are
"fair" in the case of:
 - with CGROUP controls and the two kinds of processes run by
   different users, when there is just one un-pinned process
and that is so on my quad-core i5-3470 baby or my 32-core 4*E5-4627v2
server (and everywhere that I tested). The kernel is smart and gets it
right for one un-pinned process: why not for two?

Now re-testing further (on some machines with CGROUP): on the i5-3470
things are fair still with one un-pinned (become un-fair with two), on
the 4*E5-4627v2 are fair still with 4 un-pinned (become un-fair with 5).
Does this suggest that the kernel does things right within each physical
CPU, but breaks across several (or exact contrary)? Maybe not: on a
2*E5530 machine, things are fair with just one un-pinned and un-fair
with 2 already.

> What you're seeing is not a bug.  No task can occupy more than one CPU
> at a time, making space reservation on multiple CPUs a very bad idea.

I agree that pinning may be bad... should not the kernel penalize the
badly pinned processes?

Cheers, Paul

Paul Szabo   psz@...hs.usyd.edu.au   http://www.maths.usyd.edu.au/u/psz/
School of Mathematics and Statistics   University of Sydney    Australia
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