This article describes the old thread pool in MariaDB 5.1 - 5.3.
If you are using MariaDB 5.5, which has a new improved thread pool, then you should read [[thread-pool-in-mariadb-55|this]].
== About Pool of Threads
This is an extended version of the pool-of-threads code from MySQL 6.0. This
allows you to use a limited set of threads to handle all queries, instead of
the old 'one-thread-per-connection' style. In recent times, its also been referred to as "thread pool" or "thread pooling" as this feature (in a different implementation) is available in Enterprise editions of MySQL (not in the Community edition).
This can be a very big win if most of your queries are short running queries
and there are few table/row locks in your system.
== Instructions
To enable pool-of-threads you must first run configure with the
<<fixed>>--with-libevent<</fixed>> option. (This is automatically done if you
use any 'max' scripts in the BUILD directory):
<<code lang=bash inline=false>>
./configure --with-libevent
<</code>>
When starting mysqld with the pool of threads code you should use
<<code lang=bash inline=false>>
mysqld --thread-handling=pool-of-threads --thread-pool-size=20
<</code>>
Default values are:
<<code>>
thread-handling= one-thread-per-connection
thread-pool-size= 20
<</code>>
One issue with pool-of-threads is that if all worker threads are doing
work (like running long queries) or are locked by a row/table lock no
new connections can be established and you can't login and find out
what's wrong or login and kill queries.
To help this, we have introduced two new options for mysqld:
<<code>>
--extra-port=# (Default 0)
--extra-max-connections=# (Default 1)
<</code>>
If extra-port is <> 0 then you can connect max_connections number of
normal threads + 1 extra SUPER user through the 'extra-port' TCP/IP
port. These connections use the old one-thread-per-connection method.
To connect with through the extra port, use:
<<code lang=bash inline=false>>
mysql --port='number-of-extra-port' --protocol=tcp
<</code>>
This allows you to freely use, on connection bases, the optimal
connection/thread model.
Some useful links:
* [[http://dev.mysql.com/doc/refman/5.6/en/server-system-variables.html#sysvar_thread_handling|Thread-handling and thread-pool-size variables]]
* [[http://dev.mysql.com/doc/refman/5.6/en/connection-threads.html|Connection options]]
