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

[Peter Zijlstra]

On Mon, 2011-12-19 at 14:03 +0530, Nikunj A. Dadhania wrote:
> The following patches implements gang scheduling. These patches
>     are *highly* experimental in nature and are not proposed for
>     inclusion at this time.

Nor will they ever be, I've always strongly opposed the whole concept
and I'm not about to change my mind. Gang scheduling is a scalability
nightmare. 

>     Gang scheduling can be helpful in virtualization scenario. It will
>     help in avoiding the lock-holder-preemption[1] problem and other
>     benefits include improved lock-acquisition times. This feature
>     will help address some limitations of KVM on Power

Use paravirt ticket locks or a pause-loop-filter like thing.

>     On Power, we have an interesting hardware restriction on guests
>     running across SMT theads: on any single core, we can only run one
>     mm context at any given time. 

OMFG are your hardware engineers insane?

Anyway, I had a look at your patches and I don't see how could ever
work. You gang-schedule cgroup entities, but there's no guarantee the
load-balancer will have at least one task for each group on every cpu.


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