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

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Date:	Wed, 20 Aug 2008 15:24:49 +0800
From:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
To:	Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>,
	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 Mon, 2008-08-18 at 10:51 +0530, Dhaval Giani wrote: 
> > > > > > 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.
> > > > 
> > > > hi Yanmin,
> > > > 
> > > > I was wondering if you could send me your config and what sysctls you
> > > > have set. I have not been able to reproduce the 2.6.26 -> 2.6.27-rc1
> > > > GROUP=n regression.
> > > Pls. see the attachment. As for sysctl, I just set /proc/sys/kernel/sched_compat_yield=1.
> > > 
> > > I am wondering if the load balance causes the regression when group=n. I manually delete
> > > all GROUP codes and do a diff against 26 and 27-rc1.
> > > 
> > 
> > You can disable load balancing by being in uniprocessor mode.
> > 
> 
> Hi,
> 
> I can see this regression only with sched_compat_yield=1. Some numbers
> though, I see a 5% regression with max_cpus=1 whereas close to 50% with
> SMP on a 8 way.
After reverting below patch, volanoMark regression becomes less than 2% with CONFIG_GROUP_SCHED=n
on my 8-core stoakely. The improvement on 16-core tigerton is about 44%, but there is still about
 20% regression, comparing with 2.6.26_nogroup.


commit 93b75217df39e6d75889cc6f8050343286aff4a5
Author: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Date:   Fri Jun 27 13:41:33 2008 +0200

    sched: disable source/target_load bias
    
    The bias given by source/target_load functions can be very large, disable
    it by default to get faster convergence.
    
    Signed-off-by: Peter Zijlstra <peterz@...radead.org>
    Cc: Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
    Cc: Mike Galbraith <efault@....de>
    Signed-off-by: Ingo Molnar <mingo@...e.hu>



This patch adds a new feature LB_BIAS, but uses it with a NOT, so I lost it when I tested
single sched feature one by one. That also explains why wake_affine and load_balance_newidle
have more successful task pulling with kernel 2.6.27-rc, because MC and CPU domain's wake_idx
is 1, so this patch has impact on them.

Dhaval, could you test it on your 8-way machine?

> 
> Peter do you have any patches already, which I can try?
> 
> Thanks,

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