On 09/11/2013 12:11 PM, Jan Kara wrote:
On Wed 11-09-13 11:45:03, Bernd Schubert wrote:
On 09/10/2013 09:40 PM, Jan Kara wrote:
Linux Kernel Performance project guys have reported that commit 4e7ea81db5
introduces a performance regression for the following fio workload:
[global]
direct=0
ioengine=mmap
size=1500M
bs=4k
pre_read=1
numjobs=1
overwrite=1
loops=5
runtime=300
group_reporting
invalidate=0
directory=/mnt/
file_service_type=random:36
file_service_type=random:36
[job0]
startdelay=0
rw=randrw
filename=data0/f1:data0/f2
[job1]
startdelay=0
rw=randrw
filename=data0/f2:data0/f1
...
[job7]
startdelay=0
rw=randrw
filename=data0/f2:data0/f1
The culprit of the problem is that after the commit ext4_writepages()
are more aggressive in writing back pages. Thus we have less consecutive
dirty pages resulting in more seeking.
This increased aggressivity is caused by a bug in the condition
terminating ext4_writepages(). We start writing from the beginning of
the file even if we should have terminated ext4_writepages() because
wbc->nr_to_write <= 0.
After fixing the condition the throughput of the fio workload is about 20%
better than before writeback reorganization.
Reported-by: "Yan, Zheng" <zheng.z.yan@xxxxxxxxx>
Signed-off-by: Jan Kara <jack@xxxxxxx>
---
fs/ext4/inode.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index c79fd7d..7914c05 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -2563,7 +2563,7 @@ retry:
break;
}
blk_finish_plug(&plug);
- if (!ret && !cycled) {
+ if (!ret && !cycled && wbc->nr_to_write > 0) {
cycled = 1;
mpd.last_page = writeback_index - 1;
mpd.first_page = 0;
Interesting, doesn't that mean generic_writepages (sub-sequent
write_cache_pages() ) and all other file systems implementing their
own ->writepages() should be updated?
No. write_cache_pages() has the condition like:
if (!cycled && !done) {
and 'done' is set when wbc->nr_to_write drops to zero. So that function
is OK. We cannot use 'done' in ext4_writepages() because the functions are
structured a bit differently and 'done' gets set also when reach end of
file.
Ah right, I missed that. If pagevec_lookup_tag() returns 0 there is
still a way to avoid setting done=1, but I guess wbc->nr_to_write also
wouldn't be zero then.
Btrfs' extent_write_cache_pages is another candidate and in combination
with the additional blk plug ext4 and generic_writepages are doing, it
might explain why I noticed extensive btrfs-raid6-rmw writes some time
ago. I'm going to check that and further discuss on that list.
Thanks,
Bernd
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