MariaDB-5.5.21-beta is the first MariaDB release featuring the new thread pool. Oracle offers a commercial thread pool plugin for MySQL Enterprise, but now MariaDB brings a thread pool implementation to the community!

If you are not familiar with the term, please read the Knowledge Base article about it.

The main design goal of the thread pool is to increase the scalability of the MariaDB server with many concurrent connections. In order to test and demonstrate this, I have run the sysbench OLTP RO benchmark with up to 4096 threads to compare the new pool-of-threads and the traditional thread-per-connection scheduler:

Benchmark description:

• sysbench multi table OLTP, readonly
• 16 tables, totaling 40 mio …

In MySQL 6.0 a threadpool design was implemented based on
libevents and mutexes.

This design unfortunately had a number of deficiences:
1) The performance under high load was constrained due to a global
mutex protecting libevent (see BUG#42288).

2) The design had no flexibility to handle cases where threads were
blocked due to either locking or latches. E.g. a thread held up by a
table lock will be kept in the threadpool as an active thread until
the table lock is released. If all threads are blocked in this state,
it's easy to see that also any query that want to release the table
lock cannot be processed since all threads in the thread pool are
blocked waiting for the table lock (see BUG#34797).

3) The design is intended to support very many connections but
didn't use the most efficient methods to do this on Windows.
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