Re: [PATCH] cksum: Use ARMv8 SIMD extensions

[Pádraig Brady]

On 28/11/2024 19:59, Sam Russell wrote:
> I've ported the PCLMUL to for ARMv8 support, looks to be an 80% time
> reduction over CPU on an EC2 T4g instance:
>
> $ lscpu
> Architecture:             aarch64
>    CPU op-mode(s):         32-bit, 64-bit
>    Byte Order:             Little Endian
> CPU(s):                   2
>    On-line CPU(s) list:    0,1
> Vendor ID:                ARM
>    Model name:             Neoverse-N1
>      Model:                1
>      Thread(s) per core:   1
>      Core(s) per socket:   2
>      Socket(s):            1
>      Stepping:             r3p1
>      BogoMIPS:             243.75
>      Flags:                fp asimd evtstrm aes pmull sha1 sha2 crc32
> atomics fphp asimdhp cpuid asimdrdm lrcpc dcpop asimddp ssbs
>
> # ubuntu 24.04 package
> $ time cksum ubuntu.iso
> 914429447 2773874688 ubuntu.iso
>
> real    0m20.136s
> user    0m2.044s
> sys     0m1.691s
>
> # built from head
> $ time ./cksum_old ubuntu.iso
> 914429447 2773874688 ubuntu.iso
>
> real    0m20.217s
> user    0m2.022s
> sys     0m1.770s
>
> # this patch using only pmull opcodes
> $ time ./cksum_neon ubuntu.iso
> 914429447 2773874688 ubuntu.iso
>
> real    0m20.135s
> user    0m0.353s
> sys     0m1.819s
>
> # this patch using pmull and pmull2 opcodes
> $ time ./cksum_neon2 ubuntu.iso
> 914429447 2773874688 ubuntu.iso
>
> real    0m20.136s
> user    0m0.346s
> sys     0m1.819s
>
> Benchmark scripts (I used the crc_sum_stream() function so the hash output
> is different, but have verified against the pclmul script functions locally)
>
> $ time ./cksum_bench_old 65536 400000
> Hash: 8984ED89, length: 65536
>
> real    0m19.300s
> user    0m19.299s
> sys     0m0.001s
>
> $ time ./cksum_bench_neon2 65536 400000
> Hash: 828F9BAC, length: 65536
>
> real    0m5.001s
> user    0m4.997s
> sys     0m0.003s
>
> For hash validation
>
> $ time ./cksum_bench_neon2 1048576 40000
> Hash: EFA0B24F, length: 1048576
>
> real    0m7.540s
> user    0m7.538s
> sys     0m0.001s
>
> $ time ./cksum_bench_pclmul 1048576 10000
> Hash: EFA0B24F, length: 1048576
>
> real    0m3.018s
> user    0m3.018s
> sys     0m0.000s
>
> -O3 does most of the optimisation work for us, there may be more savings
> but this is still a good improvement.
>
> Some questions
> - There's no direct equivalent of "__builtin_cpu_supports" for ARM, but the
> hwcaps interface seems to be the way to test this [1] [2]
> - ARM is a much more diverse system than x86_64, it's possible that some
> platforms (e.g. phones) would see a slowdown, is this something we want to
> give maintainers a flag to disable?
> - ARMv8 also has a CRC32() opcode, a quick test showed it wasn't super
> efficient but it's possible that interleaving this against the folding
> approach might add extra speedups. This is an exercise for the reader.

Cool. I'll try this out on some of the arm64 machines at:
https://portal.cfarm.net/machines/list/

Note builders can disable this already with:
./configure utils_cv_vmull_intrinsic_exists=no

thanks!
Pádraig