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

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Date:	Wed, 04 Jan 2012 15:56:49 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Nikunj A Dadhania <nikunj@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>, peterz@...radead.org,
	linux-kernel@...r.kernel.org, vatsa@...ux.vnet.ibm.com,
	bharata@...ux.vnet.ibm.com
Subject: Re: [RFC PATCH 0/4] Gang scheduling in CFS

On 01/04/2012 12:16 PM, Avi Kivity wrote:
> On 01/04/2012 06:47 PM, Rik van Riel wrote:
>>> So it looks like the default is optimal, at least wrt the cases you
>>> tested and your test workload.
>>
>>
>> It depends on the workload.
>>
>> I believe ebizzy synchronously bounces messages around between
>> userland threads, and may benefit from lower latency preemption
>> and re-scheduling.
>>
>> Workloads like AMQP do asynchronous messaging, and are likely
>> to benefit from having a lower number of switches.
>>
>> I do not know which kind of workload is more prevalent.
>>
>> Another worry with gang scheduling is scalability.  One of
>> the reasons Linux scales well to larger systems is that a
>> lot of things are done CPU local, without communicating
>> things with other CPUs.  Making the scheduling algorithm
>> system-global has the potential to add in a lot of overhead.
>>
>> Likewise, removing the ability to migrate workloads to idle
>> CPUs is likely to hurt a lot of real world workloads.
>>
>> Benchmarks don't care, because they run full-out. However,
>> users do not run benchmarks nearly as much as they run
>> actual workloads...
>>
>
> I think we can solve it at the guest level.  The paravirt ticketlock
> stuff introduces wait/wake calls (actually wait is just a HLT
> instruction); we could spin for a while, then HLT until the other side
> wakes us.  We should do this for all sites that busy wait.

Agreed, that would probably be the best (and nicest) solution.

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