MySQL Bugs: #99432: Improving memory barrier during rseg allocation

Bug #99432	Improving memory barrier during rseg allocation
Submitted:	4 May 2020 8:09	Modified:	4 May 2020 12:46
Reporter:	Krunal Bauskar	Email Updates:
Status:	Verified	Impact on me:	None
Category:	MySQL Server: InnoDB storage engine	Severity:	S5 (Performance)
Version:	8.0.20	OS:	Any
Assigned to:		CPU Architecture:	ARM
Tags:	arm, memory barrier, performance

Description:
- DML flow allocates rollback segment using a static rollback counter
  (rseg_counter, temp_rseg_counter). Existing code uses atomic operation
  to increment it but fails to specify explicit memory barrier given
  it was originally meant for x86.

- Now that MySQL officially support ARM and given ARM relaxed memory model
  simple increment (not meant for synchronization) can be done using
  relaxed memory barrier that aids in a marginal improvement in performance.

- Also, as part of the change, moved to use std::atomic (C++11).

How to repeat:
- Test any use-case/test-case that exercises the allocation of rollback-segment.
- I tried it using sysbench update-non-index and found that the use of a relaxed memory model improves performance on ARM by up to 2%.

Suggested fix:
Needed patch could be found here.

https://github.com/mysqlonarm/mysql-server/commit/fd1b93c2d11d3e337562c5d4d7369dfdca3226b8

Will also contribute it as part of normal patch contribution given I have signed OCA.

Hello Mr. Bauskar,

Thank you for your performance improvement report.

I have studied your patch and I think that it makes lots of sense.

Verified as reported.

Thank you, very much, for your contribution.

Thank you, Mr. Bauskar.

Your patch will be made available to our Development.