Bug #119076 new ut_delay

• Submitted: 26 Sep 9:11
• Modified: 30 Sep 16:41
• Reporter: Alex Zimnitski
• Status: Verified
• Category: MySQL Server: InnoDB storage engine
• Severity: S4 (Feature request)
• Version: 5.7.43, 8.0.40, 8.4
• OS: Any
• CPU Architecture: x86

[26 Sep 9:11] Alex Zimnitski

Description:
There's no point in leaving code 
j += i;
in place just to avoid the compiler's optimizer.

Current code on a CPU with a small number of cores and a large number of running threads overloads CPU registers.

I recommend new code for the function. The default multiplier is 50, and almost no one changes it. You can fix it to a multiple of 10.

How to repeat:
Current code

ulint ut_delay(ulint delay) {
  ulint i, j;
  /* We don't expect overflow here, as ut::spin_wait_pause_multiplier is limited
  to 100, and values of delay are not larger than @@innodb_spin_wait_delay
  which is limited by 1 000. Anyway, in case an overflow happened, the program
  would still work (as iterations is unsigned). */
  const ulint iterations = delay * ut::spin_wait_pause_multiplier;
  UT_LOW_PRIORITY_CPU();

  j = 0;

  for (i = 0; i < iterations; i++) {
    j += i;
    UT_RELAX_CPU();
  }

  UT_RESUME_PRIORITY_CPU();

  return (j);
}

Suggested fix:
New code

ut0ut.cc
#pragma GCC push_options
#pragma GCC optimize ("O0")
void __attribute__((optimize("O0"))) ut_delay(ulint delay) {
  volatile ulint i;
  const ulint iterations = delay * ut::spin_wait_pause_multiplier;

  UT_LOW_PRIORITY_CPU();

  for (i = 0; i < iterations; i+=10) {
    UT_RELAX_CPU();
    UT_RELAX_CPU();
    UT_RELAX_CPU();
    UT_RELAX_CPU();
    UT_RELAX_CPU();
    UT_RELAX_CPU();
    UT_RELAX_CPU();
    UT_RELAX_CPU();
    UT_RELAX_CPU();
    UT_RELAX_CPU();
  }

  UT_RESUME_PRIORITY_CPU();
}
#pragma GCC pop_options

[29 Sep 13:04] MySQL Verification Team

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