linux-kernel - Re: hackbench regression with 2.6.26-rc2 on tulsa machine

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Date:	Tue, 20 May 2008 11:22:32 +0200
From:	Mike Galbraith <efault@....de>
To:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: hackbench regression with 2.6.26-rc2 on tulsa machine


On Tue, 2008-05-20 at 16:09 +0800, Zhang, Yanmin wrote:
> Comparing with 2.6.26-rc1, hackbench has about 30% regression with 2.6.26-rc2 on my tulsa machine
> which is a netburst architecure hyper-threading x86_64.
> 
> Command line to test: #hackbench 100 process 2000
> 
> With 2.6.26-rc1, it takes 30 seconds. But with 2.6.26-rc2/rc3, it takes 40 seconds.
> 
> Bisect located below patch:
> 46151122e0a2e80e5a6b2889f595e371fe2b600d is first bad commit
> commit 46151122e0a2e80e5a6b2889f595e371fe2b600d
> Author: Mike Galbraith <efault@....de>
> Date:   Thu May 8 17:00:42 2008 +0200
> 
>     sched: fix weight calculations
>     
>     The conversion between virtual and real time is as follows:
>     
>       dvt = rw/w * dt <=> dt = w/rw * dvt
>     
>     Since we want the fair sleeper granularity to be in real time, we actually
>     need to do:
>     
>       dvt = - rw/w * l
> 
> 
> 
> The bisect steps look stable.
> 
> On my core architecure machines(stoakley and tigerton), I do see improvement instead of regression,
> like result from 31 seconds down to 28 seconds.
> 
> I'm not sure if hyper-threading need more cares in this situation.

Oh joy.  I'll update my poor old P4 and see what I can duplicate this.

Do you still have group scheduling enabled?  If so, can you turn it off
and try again?  (when in doubt, grasp at any straw within reach;)

	-Mike

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