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From: Paul Turner <pjt@google.com>
Date: Mon, 24 Aug 2015 14:19:29 -0700
Message-ID: <CAPM31R+vt1oAPCB-q6570RqZtJVBu5MH=gcQD2m9mjauStV7MA@mail.gmail.com>
Subject: Re: [PATCH 3/3] sched: Implement interface for cgroup unified hierarchy
To: Tejun Heo <tj@kernel.org>
Cc: Austin S Hemmelgarn <ahferroin7@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@redhat.com>,
        Johannes Weiner <hannes@cmpxchg.org>, lizefan@huawei.com,
        cgroups <cgroups@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        kernel-team <kernel-team@fb.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>
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On Mon, Aug 24, 2015 at 2:17 PM, Tejun Heo <tj@kernel.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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