linux-kernel - Re: [RFC PATCH 0/4] Gang scheduling in CFS

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Message-ID: <1324309901.24621.14.camel@twins>
Date:	Mon, 19 Dec 2011 16:51:41 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>
Cc:	mingo@...e.hu, linux-kernel@...r.kernel.org,
	vatsa@...ux.vnet.ibm.com, bharata@...ux.vnet.ibm.com,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	paulus <paulus@...ba.org>
Subject: Re: [RFC PATCH 0/4] Gang scheduling in CFS

On Mon, 2011-12-19 at 14:03 +0530, Nikunj A. Dadhania wrote:
> The following patches implements gang scheduling. These patches
>     are *highly* experimental in nature and are not proposed for
>     inclusion at this time.

Nor will they ever be, I've always strongly opposed the whole concept
and I'm not about to change my mind. Gang scheduling is a scalability
nightmare. 

>     Gang scheduling can be helpful in virtualization scenario. It will
>     help in avoiding the lock-holder-preemption[1] problem and other
>     benefits include improved lock-acquisition times. This feature
>     will help address some limitations of KVM on Power

Use paravirt ticket locks or a pause-loop-filter like thing.

>     On Power, we have an interesting hardware restriction on guests
>     running across SMT theads: on any single core, we can only run one
>     mm context at any given time. 

OMFG are your hardware engineers insane?

Anyway, I had a look at your patches and I don't see how could ever
work. You gang-schedule cgroup entities, but there's no guarantee the
load-balancer will have at least one task for each group on every cpu.


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