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Re: buffered writeback torture program

To: Chris Mason <chris.mason@xxxxxxxxxx>
Subject: Re: buffered writeback torture program
From: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Date: Thu, 21 Apr 2011 04:32:58 -0400
Cc: linux-fsdevel <linux-fsdevel@xxxxxxxxxxxxxxx>, linux-ext4 <linux-ext4@xxxxxxxxxxxxxxx>, xfs@xxxxxxxxxxx, jack@xxxxxxx, axboe@xxxxxxxxx, dchinner@xxxxxxxxxx
In-reply-to: <1303322378-sup-1722@think>
References: <1303322378-sup-1722@think>
User-agent: Mutt/1.5.21 (2010-09-15)
On Wed, Apr 20, 2011 at 02:23:29PM -0400, Chris Mason wrote:
> # fsync-tester
> setting up random write file
> done setting up random write file
> starting fsync run
> starting random io!
> write time 0.0009s fsync time: 2.0142s
> write time 128.9305s fsync time: 2.6046s
> run done 2 fsyncs total, killing random writer
> 
> In this case the 128s spent in write was on a single 4K overwrite on a
> 4K file.

I can't really reproduce this locally on XFS:

setting up random write file
done setting up random write file
starting fsync run
starting random io!
write time: 0.0023s fsync time: 0.5949s
write time: 0.0605s fsync time: 0.2339s
write time: 0.0018s fsync time: 0.0179s
write time: 0.0020s fsync time: 0.0201s
write time: 0.0019s fsync time: 0.0176s
write time: 0.0018s fsync time: 0.0209s
write time: 0.0025s fsync time: 0.0197s
write time: 0.0013s fsync time: 0.0183s
write time: 0.0013s fsync time: 0.0217s
write time: 0.0016s fsync time: 0.0158s
write time: 0.0022s fsync time: 0.0240s
write time: 0.0024s fsync time: 0.0190s
write time: 0.0017s fsync time: 0.0205s
write time: 0.0030s fsync time: 0.0688s
write time: 0.0045s fsync time: 0.0193s
write time: 0.0022s fsync time: 0.0356s

But given that you are able to reproduce it, does the following patch
help your latencies?  Currently XFS actually does stop I/O when
nr_to_write reaches zero, but only for non-blocking I/O. This behaviour
was introduced in commit efceab1d563153a2b1a6e7d35376241a48126989

        "xfs: handle negative wbc->nr_to_write during sync writeback"

and works around issues in the generic writeback code.


Index: linux-2.6/fs/xfs/linux-2.6/xfs_aops.c
===================================================================
--- linux-2.6.orig/fs/xfs/linux-2.6/xfs_aops.c  2011-04-21 10:20:48.303550404 
+0200
+++ linux-2.6/fs/xfs/linux-2.6/xfs_aops.c       2011-04-21 10:20:58.203496773 
+0200
@@ -765,8 +765,7 @@ xfs_convert_page(
                SetPageUptodate(page);
 
        if (count) {
-               if (--wbc->nr_to_write <= 0 &&
-                   wbc->sync_mode == WB_SYNC_NONE)
+               if (--wbc->nr_to_write <= 0)
                        done = 1;
        }
        xfs_start_page_writeback(page, !page_dirty, count);

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