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

[Jaegeuk Kim]

On Wed, Sep 07, 2016 at 10:12:17PM +0800, Chao Yu wrote:
> 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...

Except cgroup, since we do not support it yet. :P

Anyway, I think we'd better verify the effect of this for a while.
For example, I'm able to write a simple program to measure fsync latency while
a bunch of buffered writes.
Meanwhile, I'll put it back to the end of dev-test repo. :)

Thanks,

> 
> What's your opinion? :)
> 
> thanks,
> 
> > 
> > Thanks,
> > 
> >>
> >> Thanks,
> >>
> >>>
> >>> Signed-off-by: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
> >>> ---
> >>>  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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