Re: [PATCH 0/2] ext4: Fix performance regression with mballoc

Jan Kara
2022-08-26T09:07:27Z

On Thu 25-08-22 23:19:48, Ojaswin Mujoo wrote:
> On Wed, Aug 24, 2022 at 04:13:38PM +0200, Jan Kara wrote:
> > On Wed 24-08-22 12:40:10, Jan Kara wrote:
> > > Hi Stefan!
> > >
> > > On Wed 24-08-22 12:17:14, Stefan Wahren wrote:
> > > > Am 23.08.22 um 22:15 schrieb Jan Kara:
> > > > > Hello,
> > > > >
> > > > > So I have implemented mballoc improvements to avoid spreading allocations
> > > > > even with mb_optimize_scan=1. It fixes the performance regression I was able
> > > > > to reproduce with reaim on my test machine:
> > > > >
> > > > > mb_optimize_scan=0 mb_optimize_scan=1 patched
> > > > > Hmean disk-1 2076.12 ( 0.00%) 2099.37 ( 1.12%) 2032.52 ( -2.10%)
> > > > > Hmean disk-41 92481.20 ( 0.00%) 83787.47 * -9.40%* 90308.37 ( -2.35%)
> > > > > Hmean disk-81 155073.39 ( 0.00%) 135527.05 * -12.60%* 154285.71 ( -0.51%)
> > > > > Hmean disk-121 185109.64 ( 0.00%) 166284.93 * -10.17%* 185298.62 ( 0.10%)
> > > > > Hmean disk-161 229890.53 ( 0.00%) 207563.39 * -9.71%* 232883.32 * 1.30%*
> > > > > Hmean disk-201 223333.33 ( 0.00%) 203235.59 * -9.00%* 221446.93 ( -0.84%)
> > > > > Hmean disk-241 235735.25 ( 0.00%) 217705.51 * -7.65%* 239483.27 * 1.59%*
> > > > > Hmean disk-281 266772.15 ( 0.00%) 241132.72 * -9.61%* 263108.62 ( -1.37%)
> > > > > Hmean disk-321 265435.50 ( 0.00%) 245412.84 * -7.54%* 267277.27 ( 0.69%)
> > > > >
> > > > > Stefan, can you please test whether these patches fix the problem for you as
> > > > > well? Comments & review welcome.
> > > >
> > > > i tested the whole series against 5.19 and 6.0.0-rc2. In both cases the
> > > > update process succeed which is a improvement, but the download + unpack
> > > > duration ( ~ 7 minutes ) is not as good as with mb_optimize_scan=0 ( ~ 1
> > > > minute ).
> > >
> > > OK, thanks for testing! I'll try to check specifically untar whether I can
> > > still see some differences in the IO pattern on my test machine.
> >
> > I have created the same tar archive as you've referenced (files with same
> > number of blocks) and looked at where blocks get allocated with
> > mb_optimize_scan=0 and with mb_optimize_scan=1 + my patches. And the
> > resulting IO pattern looks practically the same on my test machine. In
> > particular in both cases files get allocated only in 6 groups, if I look
> > at the number of erase blocks that are expected to be touched by file data
> > (for various erase block sizes from 512k to 4MB) I get practically same
> > numbers for both cases.
> >
> > Ojaswin, I think you've also mentioned you were able to reproduce the issue
> > in your setup? Are you still able to reproduce it with the patched kernel?
> > Can you help debugging while Stefan is away?
> >
> > Honza
> Hi Jan,
>
> So I ran some more tests on v6.0-rc2 kernel with and without your patches and
> here are the details:
>
> Workload:-
> time tar -xf rpi-firmware.tar -C ./test
> time sync
>
> System details:
> - Rpi 3b+ w/ 8G memory card (~4G free)
> - tar is ~120MB compressed

Hum, maybe the difference is that I've tried with somewhat larger (20G) and
otherwise empty filesystem...

> And here is the output of time command for various tests. Since some of them
> take some time to complete, I ran them only 2 3 times each so the numbers might
> vary but they are indicative of the issue.
>
> v6.0-rc2 (Without patches)
>
> mb_optimize_scan = 0
>
> **tar**
> real 1m39.574s
> user 0m10.311s
> sys 0m2.761s
>
> **sync**
> real 0m22.269s
> user 0m0.001s
> sys 0m0.005s
>
> mb_optimize_scan = 1
>
> **tar**
> real 21m25.288s
> user 0m9.607
> sys 0m3.026
>
> **sync**
> real 1m23.402s
> user 0m0.005s
> sys 0m0.000s
>
> v6.0-rc2 (With patches)
>
> mb_optimize_scan = 0
>
> * similar to unpatched (~1 to 2mins) *
>
> mb_optimize_scan = 1
>
> **tar**
> real 5m7.858s
> user 0m11.008s
> sys 0m2.739s
>
> **sync**
> real 6m7.308s
> user 0m0.003s
> sys 0m0.001s
>
> At this point, I'm pretty confident that it is the untar operation that is
> having most of the regression and no other download/delete operations in
> rpi-update are behind the delay. Further, it does seem like your patches
> improve the performance but, from my tests, we are still not close to the
> mb_optimize_scan=0 numbers.

Yes, thanks for the tests!

> I'm going to spend some more time trying to collect the perfs and which block
> group the allocations are happening during the untar to see if we can get a better
> idea from that data. Let me know if you'd want me to collect anything else.
>
> PS: One question, to find the blocks groups being used I'm planning to take
> the dumpe2fs output before and after untar and then see the groups where free blocks
> changed (since there is nothing much running on Pi i assume this should give us
> a rough idea of allocation pattern of untar), just wanted to check if there's a
> better way to get this data.

I have used 'find <target-dir> -exec filefrag -v {} \;' to get block
numbers of files. That gets you better insight than plain dumpe2fs
numbers...

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR