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

[Rik van Riel]

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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