Re: quota: dqio_mutex design

Wang Shilong
2017-08-14T03:28:37Z

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On Mon, Aug 14, 2017 at 11:24 AM, Wang Shilong <wshilong@ddn.com> wrote:
> 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@suse.cz]
> 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@vger.kernel.org
> 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@suse.cz> 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@yandex.ru> 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@suse.com>
>> > SUSE Labs, CR
> --
> Jan Kara <jack@suse.com>
> SUSE Labs, CR

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