linux-ext4 - Re: [Regression] ext4: changes to mb_optimize

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Date:   Wed, 17 Aug 2022 10:54:47 +0530
From:   Ojaswin Mujoo <ojaswin@...ux.ibm.com>
To:     Stefan Wahren <stefan.wahren@...e.com>
Cc:     Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org,
        Harshad Shirwadkar <harshadshirwadkar@...il.com>,
        "Theodore Ts'o" <tytso@....edu>,
        Ritesh Harjani <riteshh@...ux.ibm.com>,
        linux-fsdevel@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Geetika.Moolchandani1@....com, regressions@...ts.linux.dev,
        Florian Fainelli <f.fainelli@...il.com>
Subject: Re: [Regression] ext4: changes to mb_optimize_scan cause issues on
 Raspberry Pi

On Tue, Aug 16, 2022 at 10:45:48PM +0200, Stefan Wahren wrote:
> Hi Jan,
> 
> Am 16.08.22 um 11:34 schrieb Jan Kara:
> > Hi Stefan!
> > So this is interesting. We can see the card is 100% busy. The IO submitted
> > to the card is formed by small requests - 18-38 KB per request - and each
> > request takes 0.3-0.5s to complete. So the resulting throughput is horrible
> > - only tens of KB/s. Also we can see there are many IOs queued for the
> > device in parallel (aqu-sz columnt). This does not look like load I would
> > expect to be generated by download of a large file from the web.
> > 
> > You have mentioned in previous emails that with dd(1) you can do couple
> > MB/s writing to this card which is far more than these tens of KB/s. So the
> > file download must be doing something which really destroys the IO pattern
> > (and with mb_optimize_scan=0 ext4 happened to be better dealing with it and
> > generating better IO pattern). Can you perhaps strace the process doing the
> > download (or perhaps strace -f the whole rpi-update process) so that we can
> > see how does the load generated on the filesystem look like? Thanks!
> 
> i didn't create the strace yet, but i looked at the source of rpi-update. At
> the end the download phase is a curl call to download a tar archive and pipe
> it directly to tar.
> 
> You can find the content list of the tar file here:
> 
> https://raw.githubusercontent.com/lategoodbye/mb_optimize_scan_regress/main/rpi-firmware-tar-content-list.txt
> 
> Best regards
> 
> > 
> > 								Honza
Hi Jan and Stefan,

I did some analysis of this on my Raspberry Pi 3B+. Not sure of the root
cause yet but here is what I observed:

1. So I noticed that the download itself is not causing any issues in my
case, but the download with a pipe to tar is what causes the degradation.
With the pipe to tar, mb_optimize_scan=1 takes around 7mins whereas
mb_optimize_scan=0 takes 1 min

2. I tried to replicate this performance degradation by running untar
on an x86 machine but I not able to get the degradation. It is
reproducible pretty consistently on my Raspberry Pi though (w/ an 8GB
memory card).

3. I did analyse the resulting mb_optimize_scan=0 vs mb_optmize_scan=1
filesystem and seems like the allocated blocks are more spread out in
mb_optmize_scan=1 case but not yet sure if that is the issue.

Will update here if I notice anything else.

Regards,
Ojaswin