Thread Pool in MariaDB 5.1 - 5.3
This article describes the old thread pool in MariaDB 5.1 - 5.3.
MariaDB 5.5 and later use an improved thread pool - see Thread pool in MariaDB.
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.
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):
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.