From: Johannes Weiner <jweiner@redhat.com>
Subject: Re: [patch 3/5] mm: try to distribute dirty pages fairly across zones
Date: Fri, 30 Sep 2011 10:55:39 +0200
Message-ID: <20110930085539.GD30857@redhat.com>
References: <1317367044-475-1-git-send-email-jweiner@redhat.com>
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 <CAOJsxLFWfH5zDG8ui=yQyOcZY_nXhK6r+ziapLg9Zhmb3ibuWQ@mail.gmail.com>
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Cc: Andrew Morton <akpm@linux-foundation.org>, Mel Gorman <mgorman@suse.de>,
        Christoph Hellwig <hch@infradead.org>,
        Dave Chinner <david@fromorbit.com>,
        Wu Fengguang <fengguang.wu@intel.com>, Jan Kara <jack@suse.cz>,
        Rik van Riel <riel@redhat.com>, Minchan Kim <minchan.kim@gmail.com>,
        Chris Mason <chris.mason@oracle.com>, "Theodore Ts'o" <tytso@mit.edu>,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        Shaohua Li <shaohua.li@intel.com>, xfs@oss.sgi.com,
        linux-btrfs@vger.kernel.org, linux-ext4@vger.kernel.org,
        linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org
To: Pekka Enberg <penberg@cs.helsinki.fi>
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On Fri, Sep 30, 2011 at 10:35:25AM +0300, Pekka Enberg wrote:
> Hi Johannes!
>=20
> On Fri, Sep 30, 2011 at 10:17 AM, Johannes Weiner <jweiner@redhat.com> =
wrote:
> > But there is a flaw in that we have a zoned page allocator which does
> > not care about the global state but rather the state of individual
> > memory zones. =A0And right now there is nothing that prevents one zon=
e
> > from filling up with dirty pages while other zones are spared, which
> > frequently leads to situations where kswapd, in order to restore the
> > watermark of free pages, does indeed have to write pages from that
> > zone's LRU list. =A0This can interfere so badly with IO from the flus=
her
> > threads that major filesystems (btrfs, xfs, ext4) mostly ignore write
> > requests from reclaim already, taking away the VM's only possibility
> > to keep such a zone balanced, aside from hoping the flushers will soo=
n
> > clean pages from that zone.
>=20
> The obvious question is: how did you test this? Can you share the resul=
ts?

Meh, sorry about that, they were in the series introduction the last
time and I forgot to copy them over.

I did single-threaded, linear writing to an USB stick as the effect is
most pronounced with slow backing devices.

[ The write deferring on ext4 because of delalloc is so extreme that I
  could trigger it even with simple linear writers on a mediocre
  rotating disk, though.  I can not access the logfiles right now, but
  the nr_vmscan_writes went practically away here as well and runtime
  was unaffected with the patched kernel. ]

			Test results

15M DMA + 3246M DMA32 + 504M Normal =3D 3765M memory
40% dirty ratio, 10% background ratio
16G USB thumb drive
10 runs of dd if=3D/dev/zero of=3Ddisk/zeroes bs=3D32k count=3D$((10 << 1=
5))

		seconds			nr_vmscan_write
		        (stddev)	       min|     median|        max
xfs
vanilla:	 549.747( 3.492)	     0.000|      0.000|      0.000
patched:	 550.996( 3.802)	     0.000|      0.000|      0.000

fuse-ntfs
vanilla:	1183.094(53.178)	 54349.000|  59341.000|  65163.000
patched:	 558.049(17.914)	     0.000|      0.000|     43.000

btrfs
vanilla:	 573.679(14.015)	156657.000| 460178.000| 606926.000
patched:	 563.365(11.368)	     0.000|      0.000|   1362.000

ext4
vanilla:	 561.197(15.782)	     0.000|2725438.000|4143837.000
patched:	 568.806(17.496)	     0.000|      0.000|      0.000

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