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Re: [PATCH 10/27] xfs: improve sync behaviour in the fact of aggressive

To: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Subject: Re: [PATCH 10/27] xfs: improve sync behaviour in the fact of aggressive dirtying
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Thu, 30 Jun 2011 12:52:11 +1000
Cc: xfs@xxxxxxxxxxx
In-reply-to: <20110629140338.467151398@xxxxxxxxxxxxxxxxxxxxxx>
References: <20110629140109.003209430@xxxxxxxxxxxxxxxxxxxxxx> <20110629140338.467151398@xxxxxxxxxxxxxxxxxxxxxx>
User-agent: Mutt/1.5.20 (2009-06-14)
On Wed, Jun 29, 2011 at 10:01:19AM -0400, Christoph Hellwig wrote:
> The following script from Wu Fengguang shows very bad behaviour in XFS
> when aggressively dirtying data during a sync on XFS, with sync times
> up to almost 10 times as long as ext4.
> 
> A large part of the issue is that XFS writes data out itself two times
> in the ->sync_fs method, overriding the lifelock protection in the core
> writeback code, and another issue is the lock-less xfs_ioend_wait call,
> which doesn't prevent new ioend from beeing queue up while waiting for
> the count to reach zero.
> 
> This patch removes the XFS-internal sync calls and relies on the VFS
> to do it's work just like all other filesystems do.  Note that the
> i_iocount wait which is rather suboptimal is simply removed here.
> We already do it in ->write_inode, which keeps the current supoptimal
> behaviour.  We'll eventually need to remove that as well, but that's
> material for a separate commit.
> 
> ------------------------------ snip ------------------------------
> #!/bin/sh
> 
> umount /dev/sda7
> mkfs.xfs -f /dev/sda7
> # mkfs.ext4 /dev/sda7
> # mkfs.btrfs /dev/sda7
> mount /dev/sda7 /fs
> 
> echo $((50<<20)) > /proc/sys/vm/dirty_bytes
> 
> pid=
> for i in `seq 10`
> do
>       dd if=/dev/zero of=/fs/zero-$i bs=1M count=1000 &
>       pid="$pid $!"
> done
> 
> sleep 1
> 
> tic=$(date +'%s')
> sync
> tac=$(date +'%s')
> 
> echo
> echo sync time: $((tac-tic))
> egrep '(Dirty|Writeback|NFS_Unstable)' /proc/meminfo
> 
> pidof dd > /dev/null && { kill -9 $pid; echo sync NOT livelocked; }
> ------------------------------ snip ------------------------------
> 
> Reported-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>

Reviewed-by: Dave Chinner <dchinner@xxxxxxxxxx>

-- 
Dave Chinner
david@xxxxxxxxxxxxx

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