Re: [RFC PATCH v3 0/3] sched: simplify the select_task_rq_fair()

[Michael Wang <wangyun@linux.vnet.ibm.com>]

On 02/21/2013 05:43 PM, Mike Galbraith wrote:
> On Thu, 2013-02-21 at 17:08 +0800, Michael Wang wrote:
> 
>> But is this patch set really cause regression on your Q6600? It may
>> sacrificed some thing, but I still think it will benefit far more,
>> especially on huge systems.
> 
> We spread on FORK/EXEC, and will no longer will pull communicating tasks
> back to a shared cache with the new logic preferring to leave wakee
> remote, so while no, I haven't tested (will try to find round tuit) it
> seems  it _must_ hurt.  Dragging data from one llc to the other on Q6600
> hurts a LOT.  Every time a client and server are cross llc, it's a huge
> hit.  The previous logic pulled communicating tasks together right when
> it matters the most, intermittent load... or interactive use.

I agree that this is a problem need to be solved, but don't agree that
wake_affine() is the solution.

According to my understanding, in the old world, wake_affine() will only
be used if curr_cpu and prev_cpu share cache, which means they are in
one package, whatever search in llc sd of curr_cpu or prev_cpu, we won't
have the chance to spread the task out of that package.

I'm going to recover the logical that only do select_idle_sibling() when
prev_cpu and curr_cpu are affine, so now the new logical will only
prefer leaving task in old package if both prev_cpu and curr_cpu are in
that package, I think this could solve the problem, isn't it?

Regards,
Michael Wang



> 
> -Mike
> 
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