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SubjectRe: [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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