linux-kernel - Re: [PATCH v1] sched: steer waking task to empty cfs

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Message-ID: <20120502140117.GC22740@linux.vnet.ibm.com>
Date:	Wed, 2 May 2012 19:31:17 +0530
From:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Mike Galbraith <efault@....de>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Paul Turner <pjt@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] sched: steer waking task to empty cfs_rq for better
 latencies

* Peter Zijlstra <peterz@...radead.org> [2012-04-24 19:09:14]:

> On Tue, 2012-04-24 at 18:58 +0200, Peter Zijlstra wrote:
> > On Tue, 2012-04-24 at 22:26 +0530, Srivatsa Vaddagiri wrote:
> > > Steer a waking task towards a cpu where its cgroup has zero tasks (in
> > > order to provide it better sleeper credits and hence reduce its wakeup
> > > latency). 
> > 
> > That's just vile.. pjt could you post your global vruntime stuff so
> > vatsa can have a go at that?
> 
> That is, you're playing a deficiency we should fix, not exploit.
> 
> Also, you do a second loop over all those cpus right after we've already
> iterated them..
> 
> furthermore, that 100%+ gain is still way insane, what else is broken?
> Did you try those paravirt tlb-flush patches and other such weirdness?

I got around to try pv-tlb-flush patches and it showed >100%
improvement for sysbench (without the balance-on-wake patch on host). This
was what readprofile showed (when pv-tlb-flush patches were absent in
guest):

1135704 total                                   0.3265
636737 native_cpuid                             18192.4857
283201 __bitmap_empty                           2832.0100
137853 flush_tlb_others_ipi                     569.6405

I will try out how much they help Trade workload (which got me started
on this originally) and report back (part of the problem in trying out
is that pv-tlb-flush platches are throwing wierd problems - which Nikunj
is helping investigate). 

In any case, we can't expect users to be able to easily upgrade their
guest VM kernels and so I am still looking for a solution that works for
older guest VM kernels. Paul, hope I can get the global vruntime patches
from you soon to test how much it helps!

- vatsa

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