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

[Avi Kivity <avi@redhat.com>]

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.

-- 
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