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

[Chao Yu]

Hi Jaegeuk,

> -----Original Message-----
> From: Jaegeuk Kim [mailto:jaegeuk@xxxxxxxxxx]
> Sent: Wednesday, September 16, 2015 5:21 AM
> To: Chao Yu
> Cc: linux-f2fs-devel@xxxxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> 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@xxxxxxxxxxx>
> > ---
> >  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.

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

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