linux-kernel - Re: sched: Performance of Trade workload running inside VM

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Date:	Mon, 20 Feb 2012 15:56:30 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
Cc:	mingo@...e.hu, pjt@...gle.com, efault@....de, venki@...gle.com,
	suresh.b.siddha@...el.com, linux-kernel@...r.kernel.org,
	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>
Subject: Re: sched: Performance of Trade workload running inside VM

On Sat, 2012-02-18 at 13:11 +0530, Srivatsa Vaddagiri wrote:
> * Peter Zijlstra <a.p.zijlstra@...llo.nl> [2012-02-15 18:45:02]:

> > why does your patch sort it etc.
> 
> The patch does result in a hunt for "least" busy cpu when the target cpu
> returned by select_idle_sibling() is not idle - thus resulting in better
> scheduling latencies for the task (and in turn better benchmark scores).
> 
> Another variant of the patch could be to have select_idle_sibling() look
> for any idle cpu that is in same cache domain (rather than looking for a
> whole group of cpus to be idle)?

Right, so I looked over select_idle_sibling() again and it made my head
hurt :/ I can't immediately tell if its actually doing the right thing
or not (it _should_ try and avoid using SMT siblings if possible).

It would be very nice not to have both select_idle_sibling() and
SD_BALANCE_WAKE iterate the domain tree. So merging them if at all
possible would be goodness I think.

We'd have WAKE_AFFINE to decide which cache domain etc to stuff the task
on and then use select_idle_sibling() to find the most appropriate cpu
within that cache domain.

There was talk of modifying select_idle_sibling() to also consider the
C-state the cpu was in, preferring shallower over deeper C-states where
there's choice, this is very similar to what you propose, taking the
least loaded cpu when there isn't a proper idle one around.

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