Bug #53045 LOCK_thread_count locked when per connection threads created

• Submitted: 22 Apr 2010 4:58
• Modified: 22 Apr 2010 5:34
• Reporter: Mark Callaghan
• Status: Verified
• Category: MySQL Server: Locking
• Severity: S5 (Performance)
• Version: 5.1.45, 5.5
• OS: Any
• CPU Architecture: Any
• Tags: create, LOCK_thread_count, thread, thread_cache_size

[22 Apr 2010 4:58] Mark Callaghan

Description:
LOCK_thread_count is locked when per-connection threads are created. I just learned this by reading the slides from Yoshinori -- http://www.slideshare.net/matsunobu/linux-performance-tuning-stabilization-tips-mysqlconf2...

LOCK_thread_count is already too hot. Holding it while calling pthread_create() is not a good idea.

How to repeat:
Read the code. Search for the add_connection() callback that calls create_thread_to_handle_connection. create_new_thread() locks LOCK_thread_count and then calls add_connection() which can be a reference to create_thread_to_handle_connection which can call pthread_create() before unlocking LOCK_thread_count.

[22 Apr 2010 5:34] MySQL Verification Team

create_new_thread locks LOCK_thread_count.
create_thread_to_handle_connection unlocks it after calling pthread_create
in the case a cached thread couldn't be found.

[22 Apr 2010 6:12] James Day

The workaround is to ensure that thread_cache_size is large enough so that it is not common to have to allocate a new thread from the operating system. This is the expected normal way of tuning this variable so it should be a pretty good workaround.

This still needs a fix.

[22 Apr 2010 6:26] James Day

I've asked our documentation team to add this tuning guidance for thread_cache_size, which reflects common practice: "If your server sees hundreds of connections per second you should normally set thread_cache_size so that most new connections use cached threads".

There are reasons not to do it, like it being suspected as a cause of a bug or suspected of having a RAM leak in the associated memory allocations. Those aren't common but sometimes it is a suspect.