From: Andrea Arcangeli <aarcange@redhat.com>
Subject: Re: [PATCH 3/3] mm: slub: Default slub_max_order to 0
Date: Thu, 12 May 2011 20:18:31 +0200
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Cc: James Bottomley <James.Bottomley@HansenPartnership.com>,
        Dave Jones <davej@redhat.com>, 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, May 12, 2011 at 01:00:10PM -0500, Christoph Lameter wrote:
> On Thu, 12 May 2011, Andrea Arcangeli wrote:
> 
> > order 1 should work better, because it's less likely we end up here
> > (which leaves RECLAIM_MODE_LUMPYRECLAIM on and then see what happens
> > at the top of page_check_references())
> >
> >    else if (sc->order && priority < DEF_PRIORITY - 2)
> 
> Why is this DEF_PRIORITY - 2? Shouldnt it be DEF_PRIORITY? An accomodation
> for SLAB order 1 allocs?

That's to allow a few loops of the shrinker (i.e. not take down
everything in the way regardless of any aging information in pte/page
if there's no memory pressure). This "- 2" is independent of the
allocation order. If it was < DEF_PRIORITY it'd trigger lumpy already
at the second loop (in do_try_to_free_pages). So it'd make things
worse. Like it'd make things worse decreasing the
PAGE_ALLOC_COSTLY_ORDER define to 2 and keeping slub at 3.

> May I assume that the case of order 2 and 3 allocs in that case was not
> very well tested after the changes to introduce compaction since people
> were focusing on RHEL testing?

Not really, I had to eliminate lumpy before compaction was
developed. RHEL6 has zero lumpy code (not even at compile time) and
compaction enabled by default, so even if we enabled SLUB=y it should
work ok (not sure why James still crashes with patch 2 applied that
clears __GFP_WAIT, that crash likely has nothing to do with compaction
or lumpy as both are off with __GFP_WAIT not set).

Lumpy is also eliminated upstream now (but only at runtime when
COMPACTION=y), unless __GFP_REPEAT is set, in which case I think lumpy
will still work upstream too but few unfrequent things like increasing
nr_hugepages uses that.

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