From: Chris Mason <chris.mason@oracle.com>
To: Jan Kara <jack@suse.cz>
Cc: Fengguang Wu <fengguang.wu@intel.com>,
Andrew Morton <akpm@linux-foundation.org>,
Jeff Moyer <jmoyer@redhat.com>, Jens Axboe <axboe@kernel.dk>,
linux-fsdevel@vger.kernel.org,
LKML <linux-kernel@vger.kernel.org>,
Dave Chinner <david@fromorbit.com>,
Christoph Hellwig <hch@infradead.org>,
Shaohua Li <shli@fusionio.com>
Subject: Re: [PATCH] btrfs: lower metadata writeback threshold on low dirty threshold
Date: Thu, 3 May 2012 08:31:45 -0400 [thread overview]
Message-ID: <20120503123145.GO25477@shiny> (raw)
In-Reply-To: <20120503092528.GA1104@quack.suse.cz>
On Thu, May 03, 2012 at 11:25:28AM +0200, Jan Kara wrote:
> On Thu 03-05-12 11:43:11, Wu Fengguang wrote:
> > This helps write performance when setting the dirty threshold to tiny numbers.
> >
> > 3.4.0-rc2 3.4.0-rc2-btrfs4+
> > ------------ ------------------------
> > 96.92 -0.4% 96.54 bay/thresh=1000M/btrfs-100dd-1-3.4.0-rc2
> > 98.47 +0.0% 98.50 bay/thresh=1000M/btrfs-10dd-1-3.4.0-rc2
> > 99.38 -0.3% 99.06 bay/thresh=1000M/btrfs-1dd-1-3.4.0-rc2
> > 98.04 -0.0% 98.02 bay/thresh=100M/btrfs-100dd-1-3.4.0-rc2
> > 98.68 +0.3% 98.98 bay/thresh=100M/btrfs-10dd-1-3.4.0-rc2
> > 99.34 -0.0% 99.31 bay/thresh=100M/btrfs-1dd-1-3.4.0-rc2
> > ==> 88.98 +9.6% 97.53 bay/thresh=10M/btrfs-10dd-1-3.4.0-rc2
> > ==> 86.99 +13.1% 98.39 bay/thresh=10M/btrfs-1dd-1-3.4.0-rc2
> > ==> 2.75 +2442.4% 69.88 bay/thresh=1M/btrfs-10dd-1-3.4.0-rc2
> > ==> 3.31 +2634.1% 90.54 bay/thresh=1M/btrfs-1dd-1-3.4.0-rc2
> >
> > Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
> > ---
> > fs/btrfs/disk-io.c | 3 ++-
> > 1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > --- linux-next.orig/fs/btrfs/disk-io.c 2012-05-02 14:04:00.989262395 +0800
> > +++ linux-next/fs/btrfs/disk-io.c 2012-05-02 14:04:01.773262414 +0800
> > @@ -930,7 +930,8 @@ static int btree_writepages(struct addre
> >
> > /* this is a bit racy, but that's ok */
> > num_dirty = root->fs_info->dirty_metadata_bytes;
> > - if (num_dirty < thresh)
> > + if (num_dirty < min(thresh,
> > + global_dirty_limit << (PAGE_CACHE_SHIFT-2)))
> > return 0;
> > }
> > return btree_write_cache_pages(mapping, wbc);
> Frankly, that whole condition on WB_SYNC_NONE in btree_writepages() looks
> like a hack. I think we also had problems with this condition when we tried
> to change b_more_io list handling. I found rather terse commit message
> explaining the code:
> Btrfs: Limit btree writeback to prevent seeks
It is definitely a hack ;) The basic point is that once we write a
metadata block, we have to cow it for any future changes. So writing
the metadata has a pretty big impact on performance, and I'd rather
write everything else that is dirty first. When that code was added I
was finding the metadata going to disk very soon under memory pressure.
I'm open to any ideas on this one.
-chris
next prev parent reply other threads:[~2012-05-03 12:31 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-04-08 1:06 [RESEND][PATCH v2] block: remove plugging at buffered write time Wu Fengguang
2012-04-09 14:34 ` Jeff Moyer
2012-04-11 23:13 ` Andrew Morton
2012-04-12 1:32 ` Fengguang Wu
2012-04-12 2:20 ` Fengguang Wu
2012-04-12 14:26 ` Jan Kara
2012-04-13 1:40 ` Fengguang Wu
2012-05-03 3:43 ` [PATCH] btrfs: lower metadata writeback threshold on low dirty threshold Fengguang Wu
2012-05-03 3:53 ` [PATCH] writeback: initialize global_dirty_limit Fengguang Wu
2012-05-03 9:25 ` [PATCH] btrfs: lower metadata writeback threshold on low dirty threshold Jan Kara
2012-05-03 10:02 ` Fengguang Wu
2012-05-03 12:31 ` Chris Mason [this message]
2012-05-03 13:30 ` Josef Bacik
2012-05-03 14:08 ` Fengguang Wu
2012-05-06 6:01 ` [RESEND][PATCH v2] block: remove plugging at buffered write time Fengguang Wu
2012-05-06 9:58 ` Fengguang Wu
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