From: James Bottomley <James.Bottomley@HansenPartnership.com>
Subject: Re: [PATCH 3/3] mm: slub: Default slub_max_order to 0
Date: Thu, 12 May 2011 10:43:13 -0500
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Cc: Mel Gorman <mgorman@suse.de>, Andrew Morton
 <akpm@linux-foundation.org>, Colin King <colin.king@canonical.com>,
 Raghavendra D Prabhu <raghu.prabhu13@gmail.com>, Jan Kara <jack@suse.cz>,
 Chris Mason <chris.mason@oracle.com>, Pekka Enberg <penberg@kernel.org>,
 Rik van Riel <riel@redhat.com>, Johannes Weiner <hannes@cmpxchg.org>,
 linux-fsdevel <linux-fsdevel@vger.kernel.org>, linux-mm
 <linux-mm@kvack.org>, linux-kernel <linux-kernel@vger.kernel.org>,
 linux-ext4 <linux-ext4@vger.kernel.org>
To: Christoph Lameter <cl@linux.com>
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On Thu, 2011-05-12 at 10:27 -0500, Christoph Lameter wrote:
> On Thu, 12 May 2011, James Bottomley wrote:
> 
> > > >   */
> > > >  static int slub_min_order;
> > > > -static int slub_max_order = PAGE_ALLOC_COSTLY_ORDER;
> > > > +static int slub_max_order;
> > >
> > > If we really need to do this then do not push this down to zero please.
> > > SLAB uses order 1 for the meax. Lets at least keep it theere.
> >
> > 1 is the current value.  Reducing it to zero seems to fix the kswapd
> > induced hangs.  The problem does look to be some shrinker/allocator
> > interference somewhere in vmscan.c, but the fact is that it's triggered
> > by SLUB and not SLAB.  I really think that what's happening is some type
> > of feedback loops where one of the shrinkers is issuing a
> > wakeup_kswapd() so kswapd never sleeps (and never relinquishes the CPU
> > on non-preempt).
> 
> The current value is PAGE_ALLOC_COSTLY_ORDER which is 3.
> 
> > > We have been using SLUB for a long time. Why is this issue arising now?
> > > Due to compaction etc making reclaim less efficient?
> >
> > This is the snark argument (I've said it thrice the bellman cried and
> > what I tell you three times is true).  The fact is that no enterprise
> > distribution at all uses SLUB.  It's only recently that the desktop
> > distributions started to ... the bugs are showing up under FC15 beta,
> > which is the first fedora distribution to enable it.  I'd say we're only
> > just beginning widespread SLUB testing.
> 
> Debian and Ubuntu have been using SLUB for a long time

Only from Squeeze, which has been released for ~3 months.  That doesn't
qualify as a "long time" in my book.

>  (and AFAICT from my
> archives so has Fedora).

As I said above, no released fedora version uses SLUB.  It's only just
been enabled for the unreleased FC15; I'm testing a beta copy.

>  I have been running those here for a couple of
> years and the issues that I see here seem to be only with the most
> recent kernels that now do compaction and other reclaim tricks.

but a sample of one doeth not great testing make.

However, since you admit even you see problems, let's concentrate on
fixing them rather than recriminations?

James


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