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