Thread Pool in MariaDB 5.1 - 5.3

This page describes the old thread pool implementation in MariaDB up to version 5.3.

It's left here because some compatibility pages in the release notes refer to it.

For the current implementation, refer to the Thread Pool in MariaDB page.

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--with-libevent option. (This is automatically done if you use any 'max' scripts in the BUILD directory):

./configure --with-libevent

When starting mysqld with the pool of threads code you should use:

mysqld --thread-handling=pool-of-threads --thread-pool-size=20

Default values are:

thread-handling=  one-thread-per-connection
thread-pool-size= 20

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; extra_port and extra_max_connections:

--extra-port=#             (Default 0)
--extra-max-connections=#  (Default 1)

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:

mysql --port='number-of-extra-port' --protocol=tcp

This allows you to freely use, on connection bases, the optimal connection/thread model.

See also

• Thread-handling and thread-pool-size variables

• How MySQL Uses Threads for Client Connections

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