linux-kernel - Re: [PATCH 0/2] RFC: readd fair sleepers for server systems

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Date:	Tue, 22 May 2012 11:01:08 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Martin Schwidefsky <schwidefsky@...ibm.com>
Cc:	Ingo Molnar <mingo@...nel.org>, Mike Galbraith <efault@....de>,
	linux-kernel@...r.kernel.org,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Christian Ehrhardt <ehrhardt@...ux.vnet.ibm.com>
Subject: Re: [PATCH 0/2] RFC: readd fair sleepers for server systems

On Mon, 2012-05-21 at 17:45 +0200, Martin Schwidefsky wrote:
> our performance team found a performance degradation with a recent
> distribution update in regard to fair sleepers (or the lack of fair
> sleepers). On s390 we used to run with fair sleepers disabled.

This change was made a very long time ago.. tell your people to mind
what upstream does if they want us to mind them.

Also, reports like this make me want to make /debug/sched_features a
patch in tip/out-of-tree so that its never available outside
development.

> We see the performance degradation with our network benchmark and fair
> sleepers enabled, the largest hit is on virtual connections:
> 
> VM guest Hipersockets 
>    Throughput degrades up to 18% 
>    CPU load/cost increase up to 17%
> VM stream 
>    Throughput degrades up to 15% 
>    CPU load/cost increase up to 22%
> LPAR Hipersockets
>    Throughput degrades up to 27% 
>    CPU load/cost increase up to 20%

Why is this, is this some weird interaction with your hypervisor?

> In short, we want the fair sleepers tunable back. I understand that on
> x86 we want to avoid the cost of a branch on the hot path in place_entity,
> therefore add a compile time config option for the fair sleeper control.

I'm very much not liking this... this makes s390 schedule completely
different from all the other architectures.
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