This article describes the old thread pool in MariaDB 5.1 - 5.3.
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.
--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.