I've been thinking about strategies for building a multiple-connection-per-thread MPM for 2.0. It's conceptually easy to do this:
* Start with worker.
* Keep the model of one worker thread per request, so that blocking or CPU-intensive modules don't need to be rewritten as state machines.
* In the core output filter, instead of doing actual socket writes, hand off the output brigades to a "writer thread."
* As soon as the worker thread has sent an EOS to the writer thread, let the worker thread move on to the next request.
* In the writer thread, use a big event loop (with /dev/poll or RT signals or kqueue, depending on platform) to do nonblocking writes for all open connections.
This would allow us to use a much smaller number of worker threads for the same amount amount of traffic (at least for typical workloads in which the network write time constitutes the majority of each requests's duration).
The problem, though, is that passing brigades between threads is unsafe:
* The bucket allocator alloc/free code isn't thread-safe, so bad things will happen if the writer thread tries to free a bucket (that's just been written to the client) at the same time that a worker thread is allocating a new bucket for a subsequent request on the same connection.
* If we delete the request pool when the worker thread finishes its work on the request, the pool cleanup will close the underlying objects for the request's file/pipe/mmap/etc buckets. When the writer thread tries to output these buckets, the writes will fail.
There are other ways to structure an async MPM, but in almost all cases we'll face the same problem: buckets that get created by one thread must be delivered and then freed by a different thread, and the current memory management design can't handle that.
The cleanest solution I've thought of so far is:
* Modify the bucket allocator code to allow thread-safe alloc/free of buckets. For the common cases, it should be possible to do this without mutexes by using apr_atomic_cas() based spin loops. (There will be at most two threads contending for the same allocator-- one worker thread and the writer thread--so the amount of spinning should be minimal.)
* Don't delete the request pool at the end of a request. Instead, delay its deletion until the last bucket from that request is sent. One way to do this is to create a new metadata bucket type that stores the pointer to the request pool. The worker thread can append this metadata bucket to the output brigade, right before the EOS. The writer thread then reads the metadata bucket and deletes (or clears and recycles) the referenced pool after sending the response. This would mean, however, that the request pool couldn't be a subpool of the connection pool. The writer thread would have to be careful to clean up the request pool(s) upon connection abort.
I'm eager to hear comments from others who have looked at the async design issues.
Thanks, Brian
