linux-kernel - Re: [PATCH 3/3] mm: slub: Default slub_max

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Date:	Thu, 12 May 2011 15:31:35 -0500
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Pekka Enberg <penberg@...nel.org>,
	Christoph Lameter <cl@...ux.com>, Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Colin King <colin.king@...onical.com>,
	Raghavendra D Prabhu <raghu.prabhu13@...il.com>,
	Jan Kara <jack@...e.cz>, Chris Mason <chris.mason@...cle.com>,
	Rik van Riel <riel@...hat.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-ext4 <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH 3/3] mm: slub: Default slub_max_order to 0

On Thu, 2011-05-12 at 22:29 +0200, Johannes Weiner wrote:
> On Thu, May 12, 2011 at 03:04:12PM -0500, James Bottomley wrote:
> > On Thu, 2011-05-12 at 14:44 -0500, James Bottomley wrote:
> > > On Thu, 2011-05-12 at 13:37 -0500, James Bottomley wrote:
> > > > On Thu, 2011-05-12 at 18:55 +0300, Pekka Enberg wrote:
> > > > > On Thu, 2011-05-12 at 10:43 -0500, James Bottomley wrote:
> > > > > > However, since you admit even you see problems, let's concentrate on
> > > > > > fixing them rather than recriminations?
> > > > > 
> > > > > Yes, please. So does dropping max_order to 1 help?
> > > > > PAGE_ALLOC_COSTLY_ORDER is set to 3 in 2.6.39-rc7.
> > > > 
> > > > Just booting with max_slab_order=1 (and none of the other patches
> > > > applied) I can still get the machine to go into kswapd at 99%, so it
> > > > doesn't seem to make much of a difference.
> > > > 
> > > > Do you want me to try with the other two patches and max_slab_order=1?
> > > 
> > > OK, so patches 1 + 2 plus setting slub_max_order=1 still manages to
> > > trigger the problem (kswapd spinning at 99%).  This is still with
> > > PREEMPT; it's possible that non-PREEMPT might be better, so I'll try
> > > patches 1+2+3 with PREEMPT just to see if the perturbation is caused by
> > > it.
> > 
> > Confirmed, I'm afraid ... I can trigger the problem with all three
> > patches under PREEMPT.  It's not a hang this time, it's just kswapd
> > taking 100% system time on 1 CPU and it won't calm down after I unload
> > the system.
> 
> That is kind of expected, though.  If one CPU is busy with a streaming
> IO load generating new pages, kswapd is busy reclaiming the old ones
> so that the generator does not have to do the reclaim itself.
> 
> By unload, do you mean stopping the generator? 

Correct.

>  And if so, how quickly
> after you stop the generator does kswapd go back to sleep?

It doesn't.  At least not on its own; the CPU stays pegged.  If I start
other work (like a kernel compile), then sometimes it does go back to
nothing.

I'm speculating that this is the hang case for non-PREEMPT.

James


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