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Apache MPM event

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Description:A variant of the worker MPM with the goal of consuming threads only for connections with active processing
Module Identifier:mpm_event_module
Source File:event.c


The event Multi-Processing Module (MPM) is designed to allow more requests to be served simultaneously by passing off some processing work to supporting threads, freeing up the main threads to work on new requests. It is based on the worker MPM, which implements a hybrid multi-process multi-threaded server. Run-time configuration directives are identical to those provided by worker.

To use the event MPM, add --with-mpm=event to the configure script's arguments when building the httpd.



See also


How it Works

This MPM tries to fix the 'keep alive problem' in HTTP. After a client completes the first request, the client can keep the connection open, and send further requests using the same socket. This can save significant overhead in creating TCP connections. However, Apache HTTP Server traditionally keeps an entire child process/thread waiting for data from the client, which brings its own disadvantages. To solve this problem, this MPM uses a dedicated thread to handle both the Listening sockets, all sockets that are in a Keep Alive state, and sockets where the handler and protocol filters have done their work and the only remaining thing to do is send the data to the client. The status page of mod_status shows how many connections are in the mentioned states.

The improved connection handling may not work for certain connection filters that have declared themselves as incompatible with event. In these cases, this MPM will fall back to the behaviour of the worker MPM and reserve one worker thread per connection. All modules shipped with the server are compatible with the event MPM.

A similar restriction is currently present for requests involving an output filter that needs to read and/or modify the whole response body, like for example mod_ssl, mod_deflate, or mod_include. If the connection to the client blocks while the filter is processing the data, and the amount of data produced by the filter is too big to be buffered in memory, the thread used for the request is not freed while httpd waits until the pending data is sent to the client.

The MPM assumes that the underlying apr_pollset implementation is reasonably threadsafe. This enables the MPM to avoid excessive high level locking, or having to wake up the listener thread in order to send it a keep-alive socket. This is currently only compatible with KQueue and EPoll.



This MPM depends on APR's atomic compare-and-swap operations for thread synchronization. If you are compiling for an x86 target and you don't need to support 386s, or you are compiling for a SPARC and you don't need to run on pre-UltraSPARC chips, add --enable-nonportable-atomics=yes to the configure script's arguments. This will cause APR to implement atomic operations using efficient opcodes not available in older CPUs.

This MPM does not perform well on older platforms which lack good threading, but the requirement for EPoll or KQueue makes this moot.


AsyncRequestWorkerFactor Directive

Description:Limit concurrent connections per process
Syntax:AsyncRequestWorkerFactor factor
Context:server config
Compatibility:Available in version 2.3.13 and later

The event MPM handles some connections in an asynchronous way, where request worker threads are only allocated for short periods of time as needed, and other connections with one request worker thread reserved per connection. This can lead to situations where all workers are tied up and no worker thread is available to handle new work on established async connections.

To mitigate this problem, the event MPM does two things: Firstly, it limits the number of connections accepted per process, depending on the number of idle request workers. Secondly, if all workers are busy, it will close connections in keep-alive state even if the keep-alive timeout has not expired. This allows the respective clients to reconnect to a different process which may still have worker threads available.

This directive can be used to fine-tune the per-process connection limit. A process will only accept new connections if the current number of connections (not counting connections in the "closing" state) is lower than:

ThreadsPerChild + (AsyncRequestWorkerFactor * number of idle workers)

This means the absolute maximum numbers of concurrent connections is:

(AsyncRequestWorkerFactor + 1) * MaxRequestWorkers

MaxRequestWorkers was called MaxClients prior to version 2.3.13. The above value shows that the old name did not accurately describe its meaning for the event MPM.

AsyncRequestWorkerFactor can take non-integer arguments, e.g "1.5".

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