Re: [PATCH 0/5] jbd2: Avoid unnecessary locking when buffer is already journaled

[Dmitry Monakhov <dmonlist@gmail.com>]

Jan Kara <jack@suse.cz> writes:

>   Hello,
>
>   this patch set improves do_get_write_access(), jbd2_journal_get_undo_access(),
> and jbd2_journal_dirty_metadata() to be completely lockless in case buffer
> is already part of an appropriate journalling list. First three patches
> are independent small cleanups so they can go in right away I think.
>
> The other two patches *should* improve the situation for frequent bitmap
> or inode table block updates. But frankly, I haven't been able to come up
> with a load where I'd see significant contention on update of a single buffer
> (or it's hidden by a larger lock). Similarly we could see improvements when
> do_get_write_access() would be waiting for buffer lock because buffer is
> being written out by checkpointing code. But again I wasn't able to hit this
> reliably.
One of most annoying performance issues was unpredictable latency of aio submission
This is typical workload on chunk server (object storage, cloud storage,
ceph) where one some tasks performs  aio/dio submission and other
performs fsync(). Some times we got this io_submit->touch_mtime()->
do_get_write_access() observes that jh->b_jlist == BJ_Shadow and wait
for transaction commit. So aio-dio submission can block (even if file
was previously allocated) for a long time(1-5sec) on ext3/4
But this was fixed by 'lazytime' option

#Simplified testcase
#BAD workload which provoke endless fsync->commit_transaction
while true; do 
      xfs_io -c "pwrite -b 1M 1M 32M" \
	-f t{1,2,3,5,6,7,8,9,10};
      xfs_io -c "pwrite -b 1M 1M 1M" -c \
	"fsync" -f -d t11

# Measure aio-dio latency
[root@alice Z]# uname -a
Linux alice.qa.sw.ru 4.0.0-rc7+ #13 SMP Sun Apr 12 00:34:51 MSK 2015
x86_64 x86_64 x86_64 GNU/Linux
[root@alice Z]# ioping -A  -C -D  -WWW t
4.0 KiB from t (ext4 /dev/sdb1): request=1 time=441 us
...
4.0 KiB from t (ext4 /dev/sdb1): request=12 time=393 us
4.0 KiB from t (ext4 /dev/sdb1): request=13 time=2.7 s <---- too long
4.0 KiB from t (ext4 /dev/sdb1): request=14 time=397 us
4.0 KiB from t (ext4 /dev/sdb1): request=15 time=398 us
^C
--- t (ext4 /dev/sdb1) ioping statistics ---
15 requests completed in 17.2 s, 5 iops, 22.0 KiB/s
min/avg/max/mdev = 384 us / 182.2 ms / 2.7 s / 679.1 ms
>
> Ted, you mentioned at Vault you had a setup where frequent
> do_get_write_access() calls were contending in the revoke code. What was the
> load exactly? These patches should improve that as well...
>
> 								Honza
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