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From:   Phillip Lougher <phillip@squashfs.org.uk>
To:     a.fatoum@pengutronix.de
Cc:     kernel@pengutronix.de, linux-kernel@vger.kernel.org,
        squashfs-devel@lists.sourceforge.net, phillip.lougher@gmail.com
Subject: Re: Possible performance regression with CONFIG_SQUASHFS_DECOMP_SINGLE
Date:   Wed, 16 Mar 2022 05:34:28 +0000
Message-Id: <20220316053428.5758-1-phillip@squashfs.org.uk>
In-Reply-To: <c22a0f1c-883d-5122-ef88-0d7c57ab4e66@pengutronix.de>
References: <c22a0f1c-883d-5122-ef88-0d7c57ab4e66@pengutronix.de>
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Ahmad Fatoum <a.fatoum@pengutronix.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.

Phillip