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

From: "Jonathan Neuschäfer" <j.neuschaefer@gmx.net>
To: Arnd Bergmann <arnd@arndb.de>
Cc: "Jonathan Neuschäfer" <j.neuschaefer@gmx.net>,
	linux-arm-kernel@lists.infradead.org,
	"Russell King" <linux@armlinux.org.uk>,
	"Nick Terrell" <terrelln@fb.com>,
	"Tony Lindgren" <tony@atomide.com>,
	"Geert Uytterhoeven" <geert+renesas@glider.be>,
	"Linus Walleij" <linus.walleij@linaro.org>,
	"Sebastian Reichel" <sebastian.reichel@collabora.com>,
	"Hawkins, Nick" <nick.hawkins@hpe.com>,
	"Christophe Leroy" <christophe.leroy@csgroup.eu>,
	"Florian Fainelli" <f.fainelli@gmail.com>,
	"Nick Desaulniers" <ndesaulniers@google.com>,
	"Xin Li" <xin3.li@intel.com>,
	"Seung-Woo Kim" <sw0312.kim@samsung.com>,
	"Paul Bolle" <pebolle@tiscali.nl>,
	"Bart Van Assche" <bvanassche@acm.org>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/3] ARM ZSTD boot compression
Date: Sat, 15 Apr 2023 04:00:33 +0200	[thread overview]
Message-ID: <ZDoFQUYMZykLdTbX@probook> (raw)
In-Reply-To: <ecf9f34c-1f1f-47af-a470-0a6ae7773724@app.fastmail.com>

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On Thu, Apr 13, 2023 at 01:13:21PM +0200, Arnd Bergmann wrote:
> 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

Make sense.

For ZSTD as used in kernel decompression (the zstd22 configuration), the
window is even bigger, 128 MiB. (AFAIU)

Thanks

Jonathan

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