Messages in this thread | | | Date | Thu, 13 Apr 2023 13:13:21 +0200 | From | "Arnd Bergmann" <> | 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
| |