Backporting mod_event's doc updates from trunk

        thread serving the response content can flush the first bytes until <code>EWOULDBLOCK</code>

        or <code>EAGAIN</code> is returned, delegating the rest to the listener. This one in turn

        waits for an event on the socket, and delegates the work to flush the rest of the content

        to the first idle worker thread. Meanwhile in the latter example (FCGI/CGI/proxed content)

        to the first idle worker thread. Meanwhile in the latter example (FCGI/CGI/proxied content)

        the MPM can't predict the end of the response and a worker thread has to finish its work

        before returning the control to the listener. The only alternative is to buffer the 

        response in memory, but it wouldn't be the safest option for the sake of the

    no worker thread is available to handle new work on established async

    connections.</p>

    <p>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

    <p>To mitigate this problem, the event MPM does two things:</p>

    <ul>

        <li>it limits the number of connections accepted per process, depending on the

            number of idle request workers;</li>

        <li>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.</p>

            different process which may still have worker threads available.</li>

    </ul>

    <p>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

    limit. A <strong>process</strong> will only accept new connections if the current number of

    connections (not counting connections in the "closing" state) is lower

    than:</p>

        <var>number of idle workers</var>)

    </strong></p>

    <p>This means the absolute maximum numbers of concurrent connections is:</p>

    <p>An estimation of the maximum concurrent connections across all the processes given

        an average value of idle worker threads can be calculated with:

    </p>

    <p class="indent"><strong>

        (<directive module="mpm_common">ThreadsPerChild</directive> +

        (<directive>AsyncRequestWorkerFactor</directive> *

        <var>number of idle workers</var>)) * 

        <directive module="mpm_common">ServerLimit</directive>

    </strong></p>

    <note><title>Example</title>

    <highlight language="config">

ThreadsPerChild = 10

ServerLimit = 4

AsyncRequestWorkerFactor = 2

MaxRequestWorkers = 40

idle_workers = 4 (average for all the processes to keep it simple)

max_connections = (ThreadsPerChild + (AsyncRequestWorkerFactor * idle_workers)) * ServerLimit 

                = (10 + (2 * 4)) * 4 = 72

    </highlight>

    </note>

    <p>When all the worker threads are idle, then absolute maximum numbers of concurrent 

        connections can be calculared in a simpler way:</p>

    <p class="indent"><strong>

        (<directive>AsyncRequestWorkerFactor</directive> + 1) *

        <directive module="mpm_common">MaxRequestWorkers</directive>

    </strong></p>

    <note><title>Example</title>

    <highlight language="config">

ThreadsPerChild = 10 

ServerLimit = 4

MaxRequestWorkers = 40

AsyncRequestWorkerFactor = 2 

    </highlight>

    <p>If all the processes have all threads idle then: </p>

    <highlight language="config">idle_workers = 10</highlight>

    <p>We can calculate the absolute maximum numbers of concurrent connections in two ways:</p>

    <highlight language="config">

max_connections = (ThreadsPerChild + (AsyncRequestWorkerFactor * idle_workers)) * ServerLimit 

                = (10 + (2 * 10)) * 4 = 120

max_connections = (AsyncRequestWorkerFactor + 1) * MaxRequestWorkers 

                = (2 + 1) * 40 = 120

    </highlight>

    </note>

    <p>Tuning <directive>AsyncRequestWorkerFactor</directive> requires knowledge about the traffic handled by httpd in each specific use case, so changing the default value requires extensive testing and data gathering from <module>mod_status</module>.</p>

    <p><directive module="mpm_common">MaxRequestWorkers</directive> was called

    <directive>MaxClients</directive> prior to version 2.3.13. The above value

    shows that the old name did not accurately describe its meaning for the event MPM.</p>