linux-ext4 - Re: [PATCH] ext4: Fix performance regression in writeback of random writes

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Date:	Wed, 11 Sep 2013 11:45:03 +0200
From:	Bernd Schubert <bernd.schubert@...m.fraunhofer.de>
To:	Jan Kara <jack@...e.cz>
CC:	Ted Tso <tytso@....edu>, linux-ext4@...r.kernel.org,
	Yan Zheng <ukernel@...il.com>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] ext4: Fix performance regression in writeback of random
 writes

On 09/10/2013 09:40 PM, Jan Kara wrote:
> Linux Kernel Performance project guys have reported that commit 4e7ea81db5
> introduces a performance regression for the following fio workload:
> [global]
> direct=0
> ioengine=mmap
> size=1500M
> bs=4k
> pre_read=1
> numjobs=1
> overwrite=1
> loops=5
> runtime=300
> group_reporting
> invalidate=0
> directory=/mnt/
> file_service_type=random:36
> file_service_type=random:36
>
> [job0]
> startdelay=0
> rw=randrw
> filename=data0/f1:data0/f2
>
> [job1]
> startdelay=0
> rw=randrw
> filename=data0/f2:data0/f1
> ...
>
> [job7]
> startdelay=0
> rw=randrw
> filename=data0/f2:data0/f1
>
> The culprit of the problem is that after the commit ext4_writepages()
> are more aggressive in writing back pages. Thus we have less consecutive
> dirty pages resulting in more seeking.
>
> This increased aggressivity is caused by a bug in the condition
> terminating ext4_writepages(). We start writing from the beginning of
> the file even if we should have terminated ext4_writepages() because
> wbc->nr_to_write <= 0.
>
> After fixing the condition the throughput of the fio workload is about 20%
> better than before writeback reorganization.
>
> Reported-by: "Yan, Zheng" <zheng.z.yan@...el.com>
> Signed-off-by: Jan Kara <jack@...e.cz>
> ---
>   fs/ext4/inode.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index c79fd7d..7914c05 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -2563,7 +2563,7 @@ retry:
>   			break;
>   	}
>   	blk_finish_plug(&plug);
> -	if (!ret && !cycled) {
> +	if (!ret && !cycled && wbc->nr_to_write > 0) {
>   		cycled = 1;
>   		mpd.last_page = writeback_index - 1;
>   		mpd.first_page = 0;
>

Interesting, doesn't that mean generic_writepages (sub-sequent 
write_cache_pages() ) and all other file systems implementing their own 
->writepages()  should be updated?



Thanks,
Bernd

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