linux-kernel - Re: [PATCH 0/3] ARM ZSTD boot compression

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Message-Id: <ecf9f34c-1f1f-47af-a470-0a6ae7773724@app.fastmail.com>
Date:   Thu, 13 Apr 2023 13:13:21 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     Jonathan Neuschäfer <j.neuschaefer@....net>,
        linux-arm-kernel@...ts.infradead.org
Cc:     "Russell King" <linux@...linux.org.uk>,
        "Nick Terrell" <terrelln@...com>,
        "Tony Lindgren" <tony@...mide.com>,
        "Geert Uytterhoeven" <geert+renesas@...der.be>,
        "Linus Walleij" <linus.walleij@...aro.org>,
        "Sebastian Reichel" <sebastian.reichel@...labora.com>,
        "Hawkins, Nick" <nick.hawkins@....com>,
        "Christophe Leroy" <christophe.leroy@...roup.eu>,
        "Florian Fainelli" <f.fainelli@...il.com>,
        "Nick Desaulniers" <ndesaulniers@...gle.com>,
        "Xin Li" <xin3.li@...el.com>,
        "Seung-Woo Kim" <sw0312.kim@...sung.com>,
        "Paul Bolle" <pebolle@...cali.nl>,
        "Bart Van Assche" <bvanassche@....org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] ARM ZSTD boot compression

On Wed, Apr 12, 2023, at 23:33, Arnd Bergmann wrote:
> On Wed, Apr 12, 2023, at 23:21, Jonathan Neuschäfer wrote:
>> This patchset enables ZSTD kernel (de)compression on 32-bit ARM.
>> Unfortunately, it is much slower than I hoped (tested on ARM926EJ-S):
>>
>>  - LZO:  7.2 MiB,  6 seconds
>>  - ZSTD: 5.6 MiB, 60 seconds
>
> That seems unexpected, as the usual numbers say it's about 25%
> slower than LZO. Do  you have an idea why it is so much slower
> here? How long does it take to decompress the
> generated arch/arm/boot/Image file in user space on the same
> hardware using lzop and zstd?

I looked through this a bit more and found two interesting points:

- zstd uses a lot more unaligned loads and stores while
  decompressing. On armv5 those turn into individual byte
  accesses, while the others can likely use word-aligned
  accesses. This could make a huge difference if caches are
  disabled during the decompression.

- The sliding window on zstd is much larger, with the kernel
  using an 8MB window (zstd=23), compared to the normal 32kb
  for deflate (couldn't find the default for lzo), so on
  machines with no L2 cache, it is much likely to thrash a
  small L1 dcache that are used on most arm9.

      Arnd