linux-kernel - Re: Possible performance regression with CONFIG_SQUASHFS_DECOMP

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Date:   Mon, 4 Apr 2022 22:59:23 +0200
From:   Ahmad Fatoum <a.fatoum@...gutronix.de>
To:     Phillip Lougher <phillip@...ashfs.org.uk>
Cc:     kernel@...gutronix.de, linux-kernel@...r.kernel.org,
        squashfs-devel@...ts.sourceforge.net, phillip.lougher@...il.com
Subject: Re: Possible performance regression with
 CONFIG_SQUASHFS_DECOMP_SINGLE

Hello Phillip,

On 16.03.22 06:34, Phillip Lougher wrote:
> Ahmad Fatoum <a.fatoum@...gutronix.de> wrote:
> 
>> Hello,
>>
>> This an issue we had with v5.15 that we have since successfully worked around.
>> I am reporting it here as a pointer in case someone else runs into this and as
>> a heads up that there seems to be an underlying performance regression, so
>> here it goes:
>>
> 
> [snip]
> 
>>
>> Looking through the original kernel configuration we found that it has
>>  CONFIG_SQUASHFS_DECOMP_SINGLE=y and CONFIG_SQUASHFS_FILE_CACHE=y.
>>
>> Once changed to CONFIG_SQUASHFS_FILE_DIRECT=y and
>> CONFIG_SQUASHFS_DECOMP_MULTI_PERCPU=y, we were below 40s as we want.
> 
> This sounds like the slow-down that was introduced by commit c1f6925e1091
> "mm: put readahead pages in cache earlier" (Linux V5.8)
> 
> This commit prevents Squashfs from doing it's own readahead, which
> causes a slow-down in performance.  The slow-down is noticable when
> using a single decompressor (CONFIG_SQUASHFS_DECOMP_SINGLE=y), and
> can be solved by moving to a multi-decompressor configuration, because
> it removes contention on a single buffer in the single decompressor case.
> 
> This has been already been fixed by commit 9eec1d897139
> "squashfs: provide backing_dev_info in order to disable read-ahead"
> which is in Linux 5.17-rc1.

I just updated to v5.17.1 and I can confirm that this commit fixes the
performance regression. Single decompressor case is now nearly as fast
as multi decompressor. Reverting the fix increased boot time from
30s~ to 2min30s.

Thanks for clearing this up!

Cheers,
Ahmad

> 
> Phillip
> 


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