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

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Date:	Wed, 11 Sep 2013 13:13:50 +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 09/11/2013 12:11 PM, Jan Kara wrote:
> On Wed 11-09-13 11:45:03, Bernd Schubert wrote:
>> 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?
>    No. write_cache_pages() has the condition like:
> if (!cycled && !done) {
>
>    and 'done' is set when wbc->nr_to_write drops to zero. So that function
> is OK. We cannot use 'done' in ext4_writepages() because the functions are
> structured a bit differently and 'done' gets set also when reach end of
> file.

Ah right, I missed that. If pagevec_lookup_tag() returns 0 there is 
still a way to avoid setting done=1, but I guess wbc->nr_to_write also 
wouldn't be zero then.
Btrfs' extent_write_cache_pages is another candidate and in combination 
with the additional blk plug ext4 and generic_writepages are doing, it 
might explain why I noticed extensive btrfs-raid6-rmw writes some time 
ago. I'm going to check that and further discuss on that list.


Thanks,
Bernd

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