Threadpool MPM:
This is an experimental variant of the standard worker MPM.
Rather than queuing connections like the worker MPM, the threadpool
MPM queues idle worker threads and hands each accepted connection
to the next available worker.

The threadpool MPM can't match the performance of the worker MPM
in benchmark testing.  As of 2.0.39, some of the key load-throtting
concepts from the threadpool MPM have been incorporated into the
worker MPM.  The threadpool code is useful primarily as a research
platform; for general-purpose use, and for any production environments,
use worker instead.