linux-kernel - Re: [f2fs-dev] [PATCH] f2fs: merge WRITE bio into previous WRITE

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Date:   Wed, 7 Sep 2016 22:12:17 +0800
From:   Chao Yu <chao@...nel.org>
To:     Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <yuchao0@...wei.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net
Subject: Re: [f2fs-dev] [PATCH] f2fs: merge WRITE bio into previous WRITE_SYNC

On 2016/9/3 2:36, Jaegeuk Kim wrote:
> On Fri, Sep 02, 2016 at 03:33:33PM +0800, Chao Yu wrote:
>> Hi Jaegeuk,
>>
>> On 2016/8/27 8:53, Jaegeuk Kim wrote:
>>> This can avoid bio splits due to different op_flags.
>>
>> I thought about this, but I think this is not a good idea to increase merging
>> ratio of pages in bio. It breaks the rule of SYNC/ASYNC IO defined by system
>> which indicate degree of IO emergency, finally, some/more non-emergent IO will
>> treated as emergent one by IO scheduler, it will interrupt SYNC IOs in block
>> layer, more seriously, it may make real SYNC IO starvation.
> 
> I understand your concern.
> Originally, I tried to avoid breaking a big WRITE_SYNC by a small number of

Hmm.. I'm worry about the opposite case: user triggers small WRITE_SYNC IO
periodically, meanwhile there are big number of WRITE, with our new approach,
actually we will increase the number of synchronous WRITE IO obviously because
we will mix ASYNC/SYNC WRITE into bio cache intensively more than before since
we drop writepages mutexlock. So I'm afread the result is that it will mislead
scheduling of block layer.

> WRITE. And, I thought new WRITE can be piggybacked into previous WRITE_SYNC.
> 
> IMO, this happens very occassionally since previous pending bio should be
> WRITE_SYNC while a new request is WRITE. Even if this happens, the piggybacked
> size would not exceed over bio's max pages.
> If lots of WRITE come, we won't change at all.

I thinks this is related to writeback / blocklayer / cgroup subsystem which use
this tag frequently, maybe we should Cc their's mailing list for more opinion...

What's your opinion? :)

thanks,

> 
> Thanks,
> 
>>
>> Thanks,
>>
>>>
>>> Signed-off-by: Jaegeuk Kim <jaegeuk@...nel.org>
>>> ---
>>>  fs/f2fs/data.c | 5 +++++
>>>  1 file changed, 5 insertions(+)
>>>
>>> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
>>> index 7c8e219..c7c2022 100644
>>> --- a/fs/f2fs/data.c
>>> +++ b/fs/f2fs/data.c
>>> @@ -267,6 +267,11 @@ void f2fs_submit_page_mbio(struct f2fs_io_info *fio)
>>>  
>>>  	down_write(&io->io_rwsem);
>>>  
>>> +	/* WRITE can be merged into previous WRITE_SYNC */
>>> +	if (io->bio && io->last_block_in_bio == fio->new_blkaddr - 1 &&
>>> +			io->fio.op == fio->op && io->fio.op_flags == WRITE_SYNC)
>>> +		fio->op_flags = WRITE_SYNC;
>>> +
>>>  	if (io->bio && (io->last_block_in_bio != fio->new_blkaddr - 1 ||
>>>  	    (io->fio.op != fio->op || io->fio.op_flags != fio->op_flags)))
>>>  		__submit_merged_bio(io);
>>>
> 
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