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

[J. Neuschäfer]

On Thu, Oct 12, 2023 at 10:33:23PM +0000, Nick Terrell wrote:
> > On Apr 14, 2023, at 10:00 PM, Jonathan Neuschäfer <j.neuschaefer@xxxxxxx> wrote:
> > 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
[...]
> > For ZSTD as used in kernel decompression (the zstd22 configuration), the
> > window is even bigger, 128 MiB. (AFAIU)
> 
> Sorry, I’m a bit late to the party, I wasn’t getting LKML email for some time...
> 
> But this is totally configurable. You can switch compression configurations
> at any time. If you believe that the window size is the issue causing speed
> regressions, you could use a zstd compression to use a e.g. 256KB window
> size like this:
> 
>   zstd -19 --zstd=wlog=18
> 
> This will keep the same algorithm search strength, but limit the decoder memory
> usage.

Noted.

> I will also try to get this patchset working on my machine, and try to debug.
> The 10x slower speed difference is not expected, and we see much better speed
> in userspace ARM. I suspect it has something to do with the preboot environment.
> E.g. when implementing x86-64 zstd kernel decompression, I noticed that
> memcpy(dst, src, 16) wasn’t getting inlined properly, causing a massive performance
> penalty.

In the meantime I've seen 8s for ZSTD vs. 2s for other algorithms, on
only mildly less ancient hardware (Hi3518A, another ARM9 SoC), so I
think the main culprit here was particularly bad luck in my choice of
test hardware.

The inlining issues are a good point, noted for the next time I work on this.


Thanks,
Jonathan

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