RE: quota: dqio_mutex design

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Hello Jan,

   We have tested your patches, in generally, it helped in our case. Noticed,
our test case is only one user with many process create/remove file.


	4.13.0-rc3 without any patches					
	no Quota		-O quota'		-O quota, project'	
	File Creation	File Unlink	File Creation	File Unlink	File Creation	File Unlink
0	93,068 	         296,028 	            86,860 	285,131 	           85,199 	        189,653 
1	79,501 	         280,921 	            91,079 	277,349 	          186,279 	170,982 
2	79,932 	         299,750 	            90,246 	274,457 	           133,922 	191,677 
3	80,146 	         297,525 	            86,416 	272,160 	           192,354 	198,869 

	4.13.0-rc3/w Jan Kara patch					
	no Quota		-O quota'		-O quota, project'	
	File Creation	File Unlink	File Creation	File Unlink	File Creation	File Unlink  
0	73,057 	        311,217 	         74,898 	          286,120 	 81,217 	        288,138  ops/per second
1	78,872             312,471 	         76,470 	          277,033 	 77,014 	        288,057 
2	79,170             291,440 	         76,174              283,525 	 73,686            283,526 
3	79,941             309,168            78,493              277,331          78,751            281,377 

	4.13.0-rc3/with https://patchwork.ozlabs.org/patch/799014/				
	no Quota		-O quota'		-O quota, project'	
	File Creation	File Unlink	File Creation	File Unlink	File Creation	File Unlink
0	100,319 	        322,746 	         87,480 	         302,579 	         84,569 	        218,969 
1	728,424 	        299,808            312,766           293,471           219,198          199,389 
2	729,410           300,930            315,590           289,664           218,283          197,871 
3	727,555           298,797 	         316,837 	         289,108           213,095          213,458 
						
	4.13.0-rc3/w https://patchwork.ozlabs.org/patch/799014/ + Jan Kara patch					
	no Quota		-O quota'		-O quota, project'	
	File Creation	File Unlink	File Creation	File Unlink	File Creation	File Unlink
0	100,312 	         324,871 	         87,076 	         267,303 	          86,258 	        288,137 
1	707,524 	         298,892           361,963           252,493            421,919 	282,492 
2	707,792            298,162           363,450           264,923            397,723 	283,675 
3	707,420            302,552 	         354,013 	         266,638 	          421,537 	281,763 


In conclusion, your patches helped a lot for our testing, noticed, please ignored test0 running
for creation, the first time testing will loaded inode cache in memory, we used test1-3 to compare.

With extra patch applied, your patches improved File creation(quota+project) 2X, File unlink
1.5X.

Thanks,
Shilong

________________________________________
From: Jan Kara [jack@xxxxxxx]
Sent: Wednesday, August 09, 2017 0:06
To: Wang Shilong
Cc: Jan Kara; Andrew Perepechko; Shuichi Ihara; Wang Shilong; Li Xi; Ext4 Developers List; linux-fsdevel@xxxxxxxxxxxxxxx
Subject: Re: quota: dqio_mutex design

Hi,

On Thu 03-08-17 22:39:51, Wang Shilong wrote:
> Please send me patches, we could test and response you!

So I finally have something which isn't obviously wrong (it survives basic
testing and gives me improvements for some workloads). I have pushed out
the patches to:

git://git.kernel.org/pub/scm/linux/kernel/git/jack/linux-fs.git quota_scaling

I'd be happy if you can share your results with my patches. I have not yet
figured out a safe way to reduce the contention on dq_lock during update of
on-disk structure when lot of processes bang single dquot. I have
experimental patch but it didn't bring any benefit in my testing - I'll
rebase it on top of other patches I have send it to you for some testing.

                                                                Honza

> On Thu, Aug 3, 2017 at 10:36 PM, Jan Kara <jack@xxxxxxx> wrote:
> > Hello!
> >
> > On Thu 03-08-17 19:31:04, Wang Shilong wrote:
> >> We DDN is investigating the same issue!
> >>
> >> Some comments comes:
> >>
> >> On Thu, Aug 3, 2017 at 1:52 AM, Andrew Perepechko <anserper@xxxxxxxxx> wrote:
> >> >> On Tue 01-08-17 15:02:42, Jan Kara wrote:
> >> >> > Hi Andrew,
> >> >> >
> >> >> I've been experimenting with this today but this idea didn't bring any
> >> >> benefit in my testing. Was your setup with multiple users or a single user?
> >> >> Could you give some testing to my patches to see whether they bring some
> >> >> benefit to you?
> >> >>
> >> >>                                                               Honza
> >> >
> >> > Hi Jan!
> >> >
> >> > My setup was with a single user. Unfortunately, it may take some time before
> >> > I can try a patched kernel other than RHEL6 or RHEL7 with the same test,
> >> > we have a lot of dependencies on these kernels.
> >> >
> >> > The actual test we ran was mdtest.
> >> >
> >> > By the way, we had 15+% performance improvement in creates from the
> >> > change that was discussed earlier in this thread:
> >> >
> >> >            EXT4_SB(dquot->dq_sb)->s_qf_names[GRPQUOTA]) {
> >> > +              if (test_bit(DQ_MOD_B, &dquot->dq_flags))
> >> > +                       return 0;
> >>
> >> I don't think this is right, as far as i understand, journal quota need go
> >> together with quota space change update inside same transaction, this will
> >> break consistency if power off or RO happen.
> >>
> >> Here is some ideas that i have thought:
> >>
> >> 1) switch dqio_mutex to a read/write lock, especially, i think most of
> >> time journal quota updates is in-place update, that means we don't need
> >> change quota tree in memory, firstly try read lock, retry with write lock if
> >> there is real tree change.
> >>
> >> 2)another is similar idea of Andrew's walkaround, but we need make correct
> >> fix, maintain dirty list for per transaction, and gurantee quota updates are
> >> flushed when commit transaction, this might be complex, i am not very
> >> familiar with JBD2 codes.
> >>
> >> It will be really nice if we could fix this regression, as we see 20% performace
> >> regression.
> >
> > So I have couple of patches:
> >
> > 1) I convert dqio_mutex do rw semaphore and use it in exclusive mode only
> > when quota tree is going to change. We also use dq_lock to serialize writes
> > of dquot - you cannot have two writes happening in parallel as that could
> > result in stale data being on disk. This patch brings benefit when there
> > are multiple users - now they don't contend on common lock. It shows
> > advantage in my testing so I plan to merge these patches. When the
> > contention is on a structure for single user this change however doesn't
> > bring much (the performance change is in statistical noise in my testing).
> >
> > 2) I have patches to remove some contention on dq_list_lock by not using
> > dirty list for tracking dquots in ext4 (and thus avoid dq_list_lock
> > completely in quota modification path). This does not bring measurable
> > benefit in my testing even on ramdisk but lockstat data for dq_list_lock
> > looks much better after this - it seems lock contention just shifted to
> > dq_data_lock - I'll try to address that as well and see whether I'll be
> > able to measure some advantage.
> >
> > 3) I have patches to convert dquot dirty bit to sequence counter so that
> > in commit_dqblk() we can check whether dquot state we wanted to write is
> > already on disk. Note that this is different from Andrew's approach in that
> > we do wait for dquot to be actually written before returning. We just don't
> > repeat the write unnecessarily. However this didn't bring any measurable
> > benefit in my testing so unless I'll be able to confirm it benefits some
> > workloads I won't merge this change.
> >
> > If you can experiment with your workloads, I can send you patches. I'd be
> > keen on having some performance data from real setups...
> >
> >                                                                 Honza
> >
> >>
> >> Thanks,
> >> Shilong
> >>
> >> >                dquot_mark_dquot_dirty(dquot);
> >> >                return ext4_write_dquot(dquot);
> >> >
> >> > The idea was that if we know that some thread is somewhere between
> >> > mark_dirty and clear_dirty, then we can avoid blocking on dqio_mutex,
> >> > since that thread will update the ondisk dquot for us.
> >> >
> > --
> > Jan Kara <jack@xxxxxxxx>
> > SUSE Labs, CR
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR



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