MySQL Bugs: #77577: setup_timers initialization assumes CYCLE timer is always available

Bug #77577	setup_timers initialization assumes CYCLE timer is always available
Submitted:	1 Jul 2015 10:51	Modified:	14 Jul 2015 14:00
Reporter:	Alexey Kopytov	Email Updates:
Status:	Closed	Impact on me:	None
Category:	MySQL Server: Performance Schema	Severity:	S3 (Non-critical)
Version:	5.5, 5.6, 5.7	OS:	Any
Assigned to:		CPU Architecture:	Any

Description:
Performance Schema uses the CYCLE timer for 'wait' events by default:

mysql> select * from setup_timers;
+-----------+-------------+
| NAME      | TIMER_NAME  |
+-----------+-------------+
| idle      | MICROSECOND |
| wait      | CYCLE       |
| stage     | NANOSECOND  |
| statement | NANOSECOND  |
+-----------+-------------+
4 rows in set (0.00 sec)

The initial value may be adjusted if the corresponding timer is unavailable, which is documented at https://dev.mysql.com/doc/refman/5.6/en/performance-schema-timing.html

"By default, the Performance Schema uses the best timer available for each instrument type, but you can select a different one. "

The above is true for stage, statement, idle and (in 5.7) transaction events, see code in init_timers() from pfs_timer.cc after the following comment:

  /*
    Depending on the platform and build options,
    some timers may not be available.
    Pick best replacements.
  */

However, wait events have been left behind for some reason: they use the CYCLE timer by default, and that default is not adjusted if the cycle timer is unavailable.

This is what I get on a platform where the cycle timer is not implemented (which will be reported separately):

mysql> select * from performance_timers;
+-------------+-----------------+------------------+----------------+
| TIMER_NAME  | TIMER_FREQUENCY | TIMER_RESOLUTION | TIMER_OVERHEAD |
+-------------+-----------------+------------------+----------------+
| CYCLE       |            NULL |             NULL |           NULL |
| NANOSECOND  |      1000000000 |                1 |             60 |
| MICROSECOND |         1000000 |                1 |             80 |
| MILLISECOND |            1000 |                1 |            130 |
| TICK        |             100 |                1 |            940 |
+-------------+-----------------+------------------+----------------+
5 rows in set (0.00 sec)

As a result, the wait events timing is broken with default settings.

How to repeat:
Examine code in init_timers() from pfs_timer.cc. Note that wait_timer is initialized to TIMER_NAME_CYCLE and unlike other events is not adjusted if that timer is not available. Note that code in my_timer_init() called from init_timers() does assume that the cycle timer may not be available, in which case the nanosecond timer is used instead.

Suggested fix:
Auto-adjust the default timer for wait events if the CYCLE timer is not available, similar to other events.

Contribution submitted via Github - Bug #77577: setup_timers initialization assumes CYCLE timer is always available (*) Contribution by Alexey Kopytov (Github akopytov, mysql-server/pull/20#issuecomment-117642002): I confirm the code being submitted is offered under the terms of the OCA, and that I am authorized to contribute it.

Contribution: git_patch_38972448.txt (text/plain), 3.71 KiB.

Hello Alexey Kopytov,
 
Thank you for the report and contribution.

Thanks,
Umesh

Noted in 5.5.46, 5.6.27, 5.7.9, 5.8.0 changelogs.

For wait events, the Performance Schema uses the CYCLE timer by
default, but failed to fall back to a different timer if CYCLE was
unavailable.