linux-kernel - Re: VolanoMark regression with 2.6.27-rc1

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Date:	Fri, 08 Aug 2008 09:30:05 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
Cc:	Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Aneesh Kumar KV <aneesh.kumar@...ux.vnet.ibm.com>,
	Balbir Singh <balbir@...ibm.com>
Subject: Re: VolanoMark regression with 2.6.27-rc1

On Tue, 2030-08-06 at 11:26 +0800, Zhang, Yanmin wrote:
> On Mon, 2008-08-04 at 09:12 +0200, Peter Zijlstra wrote:
> > On Mon, 2008-08-04 at 12:35 +0530, Dhaval Giani wrote:
> > > On Mon, Aug 04, 2008 at 08:26:11AM +0200, Peter Zijlstra wrote:
> > > > On Mon, 2008-08-04 at 11:23 +0530, Dhaval Giani wrote:
> > > > 
> > > > > Peter, vatsa, any ideas?
> > > > 
> > > > ---
> > > > 
> > > > Revert:
> > > >   a7be37ac8e1565e00880531f4e2aff421a21c803  sched: revert the revert of: weight calculations
> > > >   c9c294a630e28eec5f2865f028ecfc58d45c0a5a  sched: fix calc_delta_asym()
> > > >   ced8aa16e1db55c33c507174c1b1f9e107445865  sched: fix calc_delta_asym, #2
> > > > 
> > > 
> > > Did we not fix those? :) 
> > 
> > Works for me,.. just guessing here.
> I did more investigation on 16-core tigerton.
> 
> Firstly, let's focus on CONFIG_GROUP_SCHED=n. With 2.6.26, the result
> has little difference
> between with and without CONFIG_GROUP_SCHED.
> 
> 1) I tried different sched_features and found AFFINE_WAKEUPS has big
> impact on volanoMark. Other
> features have little impact.
> 
> 2) With kernel 2.6.26, if disabling AFFINE_WAKEUPS, the result is
> 260000; if enabling AFFINE_WAKEUPS,
> the result is 515000, so the improvement caused by AFFINE_WAKEUPS is
> about 100%. With kernel 2.6.27-rc1,
> the improvement is only about 25%.
> 
> 3) I turned on CONFIG_SCHETSTATS in kernel and collect
> ttwu_move_affine. Mostly, collect ttwu_move_affine,
> then recollect it after 30 seconds and calculate the difference. With
> 2.6.26, I got below data:

<snip data>

> So with kernel 2.6.27-rc1, the successful wakeup_affine is about
> double of the one of 2.6.27-rc1
> on domain 0, but about 10 times on domain 1. That means more tasks are
> woken up on waker cpus.
> 
> Does that mean it doesn't follow cache-hot checking?

I'm a bit puzzled, but you're right - I too noticed that volanomark is
_very_ sensitive to affine wakeups.

I'll try and find what changed in that code for GROUP=n.

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