linux-kernel - Re: [PATCH 3/3] sched: Implement interface for cgroup unified hierarchy

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Date:	Mon, 24 Aug 2015 14:19:29 -0700
From:	Paul Turner <pjt@...gle.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Austin S Hemmelgarn <ahferroin7@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>, lizefan@...wei.com,
	cgroups <cgroups@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	kernel-team <kernel-team@...com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 3/3] sched: Implement interface for cgroup unified hierarchy

On Mon, Aug 24, 2015 at 2:17 PM, Tejun Heo <tj@...nel.org> wrote:
> Hello,
>
> On Mon, Aug 24, 2015 at 02:10:17PM -0700, Paul Turner wrote:
>> Suppose that we have 10 vcpu threads and 100 support threads.
>> Suppose that we want the support threads to receive up to 10% of the
>> time available to the VM as a whole on that machine.
>>
>> If I have one particular support thread that is busy, I want it to
>> receive that entire 10% (maybe a guest is pounding on scsi for
>> example, or in the thread-pool case, I've passed a single expensive
>> computation).  Conversely, suppose the guest is doing lots of
>> different things and several support threads are active, I want the
>> time to be shared between them.
>>
>> There is no way to implement this with nice.  Either a single thread
>> can consume 10%, and the group can dominate, or the group cannot
>> dominate and the single thread can be starved.
>
> Would it be possible for you to give realistic and concrete examples?
> I'm not trying to play down the use cases but concrete examples are
> usually helpful at putting things in perspective.

I don't think there's anything that's not realistic or concrete about
the example above.  The "suppose" parts were only for qualifying the
pool sizes for vcpu and non-vcpu threads above since discussion of
implementation using nice is dependent on knowing these counts.


>
> Thanks.
>
> --
> tejun
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