We have executed a number of benchmarks using the thread pool to
see how it operates in various workloads. A thorough study on this
can be found in Dimitri's blog here.
Optimal number of active connections is the number of active connections
needed to achieve the best throughput for the MySQL Server. For an InnoDB
workload this is usually around 32-128 active connections.
From all our benchmarks we've seen that the performance of the thread pool
when operated with less than the optimal number of active connections is
about 1-3% slower than without thread pool since the behaviour is the same
and the thread pool adds a little bit more overhead. More or less all of
this overhead is to handle KILL query correctly.
When operated in the region of the optimal number of active connections
the performance is very similar. We have seen though that the thread pool
benefits very much from locking the MySQL Server to a number of CPUs
equal to the setting of the thread_pool_size configuration parameter.
When not locked to CPUs the performance is similar, when locked to CPUs
the thread pool gives 10-15% higher performance when using the optimal
number of active connections. The MySQL Server operated without thread
pool and locked to CPUs have no significant change of throughput compared
to not locking to CPUs.
When operating above optimal number of connections the thread pool
provides a great benefit, we've seen numbers all the way up to 100x
better performance when operating with a few thousand concurrently