Another experimental MPM derived from worker:

Another experimental MPM derived from worker:

The threadpool MPM implements Aaron Bannert's "time-space tradeoff"
design managing idle workers.  Rather than putting accepted connections
into a queue, the threadpool MPM keeps idle worker threads in a stack.
Its dedicated listener thread retrieves an idle worker from the stack
before accepting a connection.  If there are no idle workers, the
listener blocks until a worker becomes available before doing an accept.

In many ways, threadpool is also a variant of leader/follower.  They
both maintain a stack of idle threads.  The difference is that threadpool
has a dedicated listener thread, and leader/follower rotates the listening
responsibility among its worker threads.  In my initial testing, the
leader/follower MPM performs very well on multiprocessor Solaris 8 when
listening on a single port, but poorly when listening on multiple ports.
(I don't know why this is happening.  What I've found so far is that
when you add a poll on the listen socket(s) before the accept in the
leader/follower MPM, all the socket-related syscalls in the httpd get
slower.  My hypothesis is that the thread scheduler is making an optimal
decision about where (on what CPU) to run the newly awakened thread if
its first syscall is an accept, and a nonoptimal decision if its first
syscall is a poll.)  The threadpool MPM performs better with multiple
listener ports, and in my testing so far it looks competitive with
leader/follower when running with a single listener.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@94672 13f79535-47bb-0310-9956-ffa450edef68