On Tue, Jul 28, 2020 at 08:34:53AM -0700, Darrick J. Wong wrote: > On Tue, Jul 28, 2020 at 07:34:39PM +0800, Zhengyuan Liu wrote: > > Hi all, > > > > When doing random buffer write testing I found the bandwidth on EXT4 is much > > better than XFS under the same environment. > > The test case ,test result and test environment is as follows: > > Test case: > > fio --ioengine=sync --rw=randwrite --iodepth=64 --size=4G --name=test > > --filename=/mnt/testfile --bs=4k > > Before doing fio, use dd (if=/dev/zero of=/mnt/testfile bs=1M > > count=4096) to warm-up the file in the page cache. > > > > Test result (bandwidth): > > ext4 xfs > > ~300MB/s ~120MB/s > > > > Test environment: > > Platform: arm64 > > Kernel: v5.7 > > PAGESIZE: 64K > > Memtotal: 16G > > Storage: sata ssd(Max bandwidth about 350MB/s) > > FS block size: 4K > > > > The fio "Test result" shows that EXT4 has more than 2x bandwidth compared to > > XFS, but iostat shows the transfer speed of XFS to SSD is about 300MB/s too. > > So I debt XFS writing back many non-dirty blocks to SSD while writing back > > dirty pages. I tried to read the core writeback code of both > > filesystem and found > > XFS will write back blocks which is uptodate (seeing iomap_writepage_map()), > > Ahhh, right, because iomap tracks uptodate separately for each block in > the page, but only tracks dirty status for the whole page. Hence if you > dirty one byte in the 64k page, xfs will write all 64k even though we > could get away writing 4k like ext4 does. > > Hey Christoph & Matthew: If you're already thinking about changing > struct iomap_page, should we add the ability to track per-block dirty > state to reduce the write amplification that Zhengyuan is asking about? > > I'm guessing that between willy's THP series, Dave's iomap chunks > series, and whatever Christoph may or may not be writing, at least one > of you might have already implemented this? :) Well, this is good timing! I was wondering whether something along these lines was an important use-case. I propose we do away with the 'uptodate' bit-array and replace it with an 'writeback' bit-array. We set the page uptodate bit whenever the reads to fill the page have completed rather than checking the 'writeback' array. In page_mkwrite, we fill the writeback bit-array on the grounds that we have no way to track a block's non-dirtiness and we don't want to scan each block at writeback time to see if it's been written to. I'll do this now before the THP series gets reposted.
