[Bug 2030515] Re: Terrible memcpy performance on Zen 3 when using rep movsb

[Florian Weimer]

Does your glibc already have ld.so --list-diagnostics? It would be good
to know the x86.cpu_features.shared_cache_size and
x86.cpu_features.non_temporal_threshold values.

I suspect you are on a machine with a large core count, so the heuristic
tunes things down to a low non-temporal threshold.

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Title:
  Terrible memcpy performance on Zen 3 when using rep movsb

Status in glibc package in Ubuntu:
  New

Bug description:
  On CPUs that advertise FSRM (fast short rep movsb), glibc 2.35 uses
  REP MOVSB for memcpy for sizes above 2112 (up to some threshold that
  depends on the cache size). Unfortunately, it seems that Zen 3 (at
  least in the microcode we're running) is extremely slow at REP MOVSB
  when the data are not well-aligned.

  I've found this using a memcpy benchmark at https://github.com/ska-
  sa/katgpucbf/blob/69752be58fb8ab0668ada806e0fd809e782cc58b/scratch/memcpy_loop.cpp
  (compiled with the adjacent Makefile). To demonstrate the issue, run

  ./memcpy_loop -b 2113 -p 1000000 -t mmap -S 0 -D 1 0

  This runs:
  - 2113-byte memory copies
  - 1,000,000 times per timing measurement
  - in memory allocated with mmap
  - with the source 0 bytes from the start of the page
  - with the destination 1 byte from the start of the page
  - on core 0.

  It reports about 3.2 GB/s. Change the -b argument to 2111 and it
  reports over 100 GB/s. So the REP MOVSB case is about 30× slower!

  This will most likely need to be reported and fixed upstream, but I'm
  reporting it to Ubuntu first since I don't know if Ubuntu has modified
  glibc in any way that would be significant.

  ProblemType: Bug
  DistroRelease: Ubuntu 22.04
  Package: libc6 2.35-0ubuntu3.1
  ProcVersionSignature: Ubuntu 5.19.0-46.47~22.04.1-generic 5.19.17
  Uname: Linux 5.19.0-46-generic x86_64
  NonfreeKernelModules: nvidia_modeset nvidia
  ApportVersion: 2.20.11-0ubuntu82.5
  Architecture: amd64
  CasperMD5CheckResult: unknown
  Date: Mon Aug  7 14:02:28 2023
  RebootRequiredPkgs: Error: path contained symlinks.
  SourcePackage: glibc
  UpgradeStatus: No upgrade log present (probably fresh install)

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