Re: [PATCH] raid6: use normal stores in the x86 x4 xor_syndrome functions

[Miroslav Cimerman <mc@raidlab.org>]

Anton Gavriliuk tested the patch on Zen 5 (thread "xor_gen() md-raid6
performance improvement"):

HPE DL345 Gen11, AMD EPYC 9455, 6x PCIe 5.0 NVMe (CM7-V 3.2 TB),
Fedora 45 config, v7.4-rc2 with and without the patch, both kernels
select avx512x4. RAID-6 4+2, chunk 512K, no bitmap, rmw_level=2,
fio 4k randwrite, O_DIRECT, io_uring, 60 s, 3 runs per kernel:

  md defaults, 1 job x QD64:
    without  127k / 122k / 125k IOPS  (mean 124.7k)
    with     132k / 139k / 137k IOPS  (mean 136.0k)    +9.1%
    (a repeat run a day later: 127k without, 135k with)

  group_thread_cnt=8, stripe_cache_size=32768, 16 jobs x QD64:
    without  467k / 468k / 470k IOPS  (mean 468.3k)
    with     473k / 471k / 471k IOPS  (mean 471.7k)    +0.7%

With md's defaults one md thread does the stripe work for all writes
and is busy most of the time (about 80% in top), so a cheaper
xor_syndrome() shows up in IOPS. With 8 workers per node the limit is
lock contention in md instead (on my machine most of the time goes to
spinning on device_lock), so the saving hardly shows.

Miroslav