[Bug 1988240] Re: Performance regression with memcpy on Intel CPU

[Shantanu Jain]

> different tunable values
With the good libc (libc6-dev==2.31-0ubuntu9.7):
I can trigger a performance regression by explicitly setting the tunable threshold to `1024*1024*3/4`. If I explicitly set to `1024*1024*16*3/4` I once again have good perf.

> or other distros
With Ubuntu 22.04.1 LTS and libc6 2.35-0ubuntu3.1 I see the performance regression. It seems marginally better than 2.31-0ubuntu9.9, e.g. 2.9x worse perf instead of 3.2x.

With 22.10 devel and libc6 2.35-0ubuntu3, I see performance regression
equal to that with 22.04.1.

With Debian bullseye and libc6 2.31-13+deb11u3, I see the performance
regression.

> double-check whether that patch is responsible
Unfortunately, I wasn't able to get glibc to make install :-( Ran into some scary looking "_dl_call_libc_early_init: Assertion `sym != NULL' failed!" errors. Let me know if you have advice on how to build glibc.

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https://bugs.launchpad.net/bugs/1988240

Title:
  Performance regression with memcpy on Intel CPU

Status in glibc package in Ubuntu:
  Incomplete

Bug description:
  # lsb_release -rd
  Description:	Ubuntu 20.04.4 LTS
  Release:	20.04

  Reporting a performance regression in libc6-dev==2.31-0ubuntu9.9 when
  upgrading from 9.7.

  Regression was observed on Intel Xeon(R) Gold 6248 CPU @ 2.50GHz
  (Cascade Lake)

  We're seeing a 3x slowdown on e.g. the following tiny program and similar slowdowns on important workloads:
  ```
  #include <stdio.h>
  #include <stdlib.h>
  #include <string.h>
  #include <time.h>

  int main(void) {
      size_t SIZE = (1 << 20);
      char *src = malloc(SIZE);
      char *dst = malloc(SIZE);

      for(int i = 0; i < (SIZE); ++i) {
          src[i] = rand() % 256;
          dst[i] = rand() % 256;
      }
      clock_t start = clock();
      for(int i = 0; i < 10000; ++i) {
          memcpy(dst, src, SIZE);
      }
      clock_t end = clock();
      printf("%f\n", (double) (end - start)/CLOCKS_PER_SEC);
  }
  ```

  Probably due to changes resulting from
  https://bugs.launchpad.net/ubuntu/+source/glibc/+bug/1928508

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