RE: [PATCH 5/7] f2fs: enhance multithread dio write performance

[Chao Yu <chao2.yu@samsung.com>]

Hi Jaegeuk,

> -----Original Message-----
> From: Jaegeuk Kim [mailto:jaegeuk@kernel.org]
> Sent: Thursday, September 17, 2015 2:13 AM
> To: Chao Yu
> Cc: linux-f2fs-devel@lists.sourceforge.net; linux-kernel@vger.kernel.org
> Subject: Re: [PATCH 5/7] f2fs: enhance multithread dio write performance
> 
> Hi Chao,
> 
> On Wed, Sep 16, 2015 at 06:15:55PM +0800, Chao Yu wrote:
> > Hi Jaegeuk,
> >
> > > -----Original Message-----
> > > From: Jaegeuk Kim [mailto:jaegeuk@kernel.org]
> > > Sent: Wednesday, September 16, 2015 5:21 AM
> > > To: Chao Yu
> > > Cc: linux-f2fs-devel@lists.sourceforge.net; linux-kernel@vger.kernel.org
> > > Subject: Re: [PATCH 5/7] f2fs: enhance multithread dio write performance
> > >
> > > Hi Chao,
> > >
> > > On Fri, Sep 11, 2015 at 02:41:53PM +0800, Chao Yu wrote:
> > > > When dio writes perform concurrently, our performace will be low because of
> > > > Thread A's allocation of multi continuous blocks will be break by Thread B,
> > > > there are two cases as below:
> > > >  - In Thread B, we may change current segment to a new segment for LFS
> > > >    allocation if we dio write in the beginning of the file.
> > > >  - In Thread B, we may allocate blocks in the middle of Thread A's
> > > >    allocation, which make blocks which allocated in Thread A being
> > > >    discontinuous.
> > > >
> > > > This patch adds writepages mutex lock to make block allocation in dio write
> > > > atomic to avoid above issues.
> > > >
> > > > Test environment:
> > > > ubuntu os with linux kernel 4.2+, intel i7-3770, 16g memory,
> > > > 32g kingston sd card.
> > > >
> > > > fio --name seqw --ioengine=sync --invalidate=1 --rw=write --directory=/mnt/f2fs
> > > --filesize=256m --size=16m --bs=2m --direct=1
> > > > --numjobs=10
> > > >
> > > > before:
> > > >   WRITE: io=163840KB, aggrb=3145KB/s, minb=314KB/s, maxb=411KB/s, mint=39836msec,
> > > maxt=52083msec
> > > >
> > > > patched:
> > > >   WRITE: io=163840KB, aggrb=10033KB/s, minb=1003KB/s, maxb=1124KB/s, mint=14565msec,
> > > maxt=16329msec
> > > >
> > > > Signed-off-by: Chao Yu <chao2.yu@samsung.com>
> > > > ---
> > > >  fs/f2fs/data.c | 13 ++++++++++---
> > > >  1 file changed, 10 insertions(+), 3 deletions(-)
> > > >
> > > > diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> > > > index a737ca5..a0a5849 100644
> > > > --- a/fs/f2fs/data.c
> > > > +++ b/fs/f2fs/data.c
> > > > @@ -1536,7 +1536,9 @@ static ssize_t f2fs_direct_IO(struct kiocb *iocb, struct iov_iter
> *iter,
> > > >  	struct file *file = iocb->ki_filp;
> > > >  	struct address_space *mapping = file->f_mapping;
> > > >  	struct inode *inode = mapping->host;
> > > > +	struct f2fs_sb_info *sbi = F2FS_I_SB(inode);
> > > >  	size_t count = iov_iter_count(iter);
> > > > +	int rw = iov_iter_rw(iter);
> > > >  	int err;
> > > >
> > > >  	/* we don't need to use inline_data strictly */
> > > > @@ -1555,12 +1557,17 @@ static ssize_t f2fs_direct_IO(struct kiocb *iocb, struct iov_iter
> > > *iter,
> > > >
> > > >  	trace_f2fs_direct_IO_enter(inode, offset, count, iov_iter_rw(iter));
> > > >
> > > > -	if (iov_iter_rw(iter) == WRITE)
> > > > +	if (rw == WRITE) {
> > > > +		mutex_lock(&sbi->writepages);
> > >
> > > Why do we have to share sbi->writepages?
> >
> > The root cause of this issue is that: in f2fs, we have no suitable
> > dispatcher which can do the following things as an atomic operation:
> > a) allocate position(s) in flash device for current block(s);
> > b) submit user data in allocated position(s) in block layer.
> >
> > Without the dispatcher, we will suffer performance issue in following
> > scenario:
> > Thread A		Thread B		Thread C
> > allocate pos+1
> > 			allocate pos+2
> > 						allocate pos+3
> > submit pos+1
> > 						submit pos+3
> > 			submit pos+2
> >
> > Our final submitting series will: pos+1, pos+3, pos+2, this makes f2fs
> > running into non-LFS mode, therefore resulting in bad performance.
> >
> > writepages mutex lock supply us with a good solution for above issue.
> > It not only make the allocating and submitting pair executing atomically,
> > but also reduce the fragmentation for one file since we submit blocks
> > belong to single inode as continuous as possible.
> >
> > So here I choose to use writepages mutex lock to fix the performance
> > issue caused by both dio write vs dio write and dio write vs buffered
> > write.
> 
> Understood, but the concern was the multi-thread performance as you mentioned.
> If one thread throws a big dio request, anybody cannot write at all?

Buffered write will not be stopped, but actually my way completely stops the
concurrency of multithreads which are doing dio writes, for aspect of improving
concurrency, moving mutex_unlock below __allocate_data_blocks is a good solution
so far.

> How about adding some limits likewise f2fs_write_data_pages whieh is for example
> nr_pages_to_write?

Could you share more details about your idea?

As Yunlei reported, there is performance regression issue, so how about
holding this patch and let me do some investigation?

Thanks,

> 
> Thanks,
> 
> >
> > If I'm missing something, please correct me.
> >
> > >
> > > >  		__allocate_data_blocks(inode, offset, count);
> > >
> > > If the problem lies on the misaligned blocks, how about calling mutex_unlock
> > > here?
> >
> > When changing to unlock here, I got regression when testing with following command:
> > fio --name seqw --ioengine=sync --invalidate=1 --rw=write --directory=/mnt/f2fs
> --filesize=256m --size=4m --bs=64k --direct=1
> > --numjobs=20
> >
> > unlock here:
> >   WRITE: io=81920KB, aggrb=5802KB/s, minb=290KB/s, maxb=292KB/s, mint=14010msec,
> maxt=14119msec
> > unlock after dio finished:
> >   WRITE: io=81920KB, aggrb=6088KB/s, minb=304KB/s, maxb=1081KB/s, mint=3786msec,
> maxt=13454msec
> >
> > So how about keep it in original place in this patch?
> >
> > Thanks,
> > >
> > > Thanks,
> > >
> > > > +	}
> > > >
> > > >  	err = blockdev_direct_IO(iocb, inode, iter, offset, get_data_block_dio);
> > > > -	if (err < 0 && iov_iter_rw(iter) == WRITE)
> > > > -		f2fs_write_failed(mapping, offset + count);
> > > > +	if (rw == WRITE) {
> > > > +		mutex_unlock(&sbi->writepages);
> > > > +		if (err)
> > > > +			f2fs_write_failed(mapping, offset + count);
> > > > +	}
> > > >
> > > >  	trace_f2fs_direct_IO_exit(inode, offset, count, iov_iter_rw(iter), err);
> > > >
> > > > --
> > > > 2.4.2