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

[Jonathan Neuschäfer]

On Wed, Apr 12, 2023 at 11:33:15PM +0200, 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?

No clear idea.

I guess it might be related to caching or unaligned memory accesses
somehow.

I suspected CONFIG_CPU_DCACHE_WRITETHROUGH, which was enabled, but
disabling it didn't improve performance.

> 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?


Unfortunately, the unzstd userspace tool requires a buffer of of 128 MiB
(the window size), which is too big for my usual devboard (which has about
100 MiB available). I'd have to test on a different board.


Jonathan

---
# uname -a
Linux buildroot 6.3.0-rc6-00020-g023058d50f2f #1212 PREEMPT Fri Apr 14 20:58:21 CEST 2023 armv5tejl GNU/Linux

# ls -lh
total 13M
-rw-r--r--    1 root     root        7.5M Jan  1 00:07 piggy.lzo
-rw-r--r--    1 root     root        5.8M Jan  1 00:07 piggy.zstd

# time lzop -d piggy.lzo -c > /dev/null
lzop: piggy.lzo: warning: ignoring trailing garbage in lzop file
Command exited with non-zero status 2
real    0m 3.38s
user    0m 3.20s
sys     0m 0.18s

# time unzstd piggy.zstd -c > /dev/null
[  858.270000] __vm_enough_memory: pid: 114, comm: unzstd, not enough memory for the allocation
piggy.zstd : Decoding error (36) : Allocation error : not enough memory
Command exited with non-zero status 1
real    0m 0.03s
user    0m 0.01s
sys     0m 0.03s

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