Bug #62425 improve CPU cache performance for trx_t
Submitted: 13 Sep 2011 19:30 Modified: 15 Nov 2011 0:02
Reporter: Mark Callaghan Email Updates:
Status: No Feedback Impact on me:
None 
Category:MySQL Server: InnoDB Plugin storage engine Severity:S5 (Performance)
Version:5.1.52 OS:Any
Assigned to: CPU Architecture:Any

[13 Sep 2011 19:30] Mark Callaghan 
Description:
We get 50% better throughput on a benchmark that does nothing but starts innodb transactions by reordering a few fields in the trx_t struct to reduce CPU cache misses and by pre-allocating memory for trx_t.

See:
http://bazaar.launchpad.net/~mysqlatfacebook/mysqlatfacebook/5.1/revision/3692
http://bazaar.launchpad.net/~mysqlatfacebook/mysqlatfacebook/5.1/revision/3687
http://bazaar.launchpad.net/~mysqlatfacebook/mysqlatfacebook/5.1/revision/3667
http://bazaar.launchpad.net/~mysqlatfacebook/mysqlatfacebook/5.1/revision/3639

How to repeat:
NA

Suggested fix:
NA
[13 Sep 2011 21:33] Sunny Bains 
What is the difference without pre-allocation? ie. only rearranging the fields in trx_t.
[13 Sep 2011 22:54] Mark Callaghan 
I don't know. Most of the cache benefit comes from reordering the fields. Pre-allocating avoids mem allocation over while holding kernel_mutex (I think).
[13 Sep 2011 23:23] Sunny Bains 
Yes, that much I figured out but I was keen to see the numbers around the cache benefits. I didn't see
any difference in my tests when I rearranged the fields in 5.6. If the numbers are compelling then I
would like to make this change in 5.6 to start with. The trx_t layout is a little different in 5.6 compared to previous versions. In 5.6 the trx_t allocation is done outside of any mutex.
[14 Sep 2011 4:09] Valeriy Kravchuk 
Looks like InnoDB team is waiting for numbers with re-ordering alone to make final decision...
[14 Oct 2011 23:00] Bugs System 
No feedback was provided for this bug for over a month, so it is
being suspended automatically. If you are able to provide the
information that was originally requested, please do so and change
the status of the bug back to "Open".
[15 Nov 2011 7:00] Bugs System 
No feedback was provided for this bug for over a month, so it is
being suspended automatically. If you are able to provide the
information that was originally requested, please do so and change
the status of the bug back to "Open".