MySQL Bugs: #35074: max_used_connections is not correct

Bug #35074	max_used_connections is not correct
Submitted:	5 Mar 2008 11:47	Modified:	26 Mar 2008 18:04
Reporter:	Alexander Nozdrin	Email Updates:
Status:	Closed	Impact on me:	None
Category:	MySQL Server	Severity:	S2 (Serious)
Version:	5.1	OS:	Any
Assigned to:	Alexander Nozdrin	CPU Architecture:	Any

Description:
According to The Manual: Max_used_connections is the maximum number of connections
that have been in use simultaneously since the server started. This is however
not always true.

The problem is that max_used_connections is calculated as follows:
  max_used_connections= thread_count - delayed_insert_threads;

This expression does not take into account other server threads,
like Event Scheduler, Event Execution threads, NDB threads, ...
That leads to the wrong statistics.

This bug is quite close to Bug#33507.

How to repeat:
Start another internal server thread. For example, let's start Event Scheduler
with no event in the queue (the exactly one thread will be started).

$ ./mysqld # Start mysqld, event scheduler is disabled.

$ ./mysql -u root # Connect as root.
> select id, user from information_schema.processlist order by id;
+----+------+
| id | user |
+----+------+
|  1 | root |
+----+------+

> show status like 'max_used_connections';
+----------------------+-------+
| Variable_name        | Value |
+----------------------+-------+
| Max_used_connections | 1     |
+----------------------+-------+

> set global event_scheduler = on;

> select id, user from information_schema.processlist order by id;
+----+-----------------+
| id | user            |
+----+-----------------+
|  1 | root            |
|  2 | event_scheduler |
+----+-----------------+

> exit

Reconnect to update/recalculate max_used_connections.

$ ./mysql -u root

> show status like 'max_used_connections';
+----------------------+-------+
| Variable_name        | Value |
+----------------------+-------+
| Max_used_connections | 2     |
+----------------------+-------+

A patch for this bug has been committed. After review, it may
be pushed to the relevant source trees for release in the next
version. You can access the patch from:

  http://lists.mysql.com/commits/43902

ChangeSet@1.2562, 2008-03-13 12:02:12+03:00, anozdrin@quad. +3 -0
  Fix for Bug#35074: max_used_connections is not correct.
  
  The problem was that number of threads was used to calculate
  max_used_connections.
  
  The fix is to use number of active connections.

Pushed into 5.1-runtime.

Pushed into 5.1.24-rc

Noted in 5.1.24 changelog.

Non-connection threads were being counted in the value of the
Max_used_connections status variable.

Pushed into 6.0.5-alpha

Also noted fix in the 5.1.23-ndb-6.3.11 and 6.0.5 changelogs.