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

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

Hi Yunlei,

> -----Original Message-----
> From: He YunLei [mailto:heyunlei@huawei.com]
> Sent: Thursday, September 17, 2015 8:40 AM
> To: Chao Yu
> Cc: 'Jaegeuk Kim'; linux-kernel@vger.kernel.org; linux-f2fs-devel@lists.sourceforge.net
> Subject: Re: [f2fs-dev] [PATCH 5/7] f2fs: enhance multithread dio write performance
> 
> On 2015/9/16 18:15, 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.
> >
> > 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?
> 
> Does share writepages mutex lock have an effect on cache write? Here is AndroBench result on
> my phone:
> 
> Before patch:
> 				1R1W               8R8W               16R16W
> Sequential Write               161.31             163.85              154.67
> Random  Write                   9.48               17.66               18.09
> 
> 
> After patch:
> 				1R1W               8R8W               16R16W
> Sequential Write               159.61             157.24              160.11
> Random  Write                   9.17               8.51                8.8

Thanks for your report, I will do the investigation.

Thanks,

> 
> Unit:Mb/s, File size: 64M, Buffer size: 4k
> 
> >
> > 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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