From b8a27dd6720c5ca2a9e82afb8d427986f9f53d2c Mon Sep 17 00:00:00 2001
From: Luca Toscano <elukey@apache.org>
Date: Sun, 14 Feb 2016 08:50:05 +0000
Subject: [PATCH] Added examples to mod_event's AsyncRequestWorkerFactor
 section.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1730297 13f79535-47bb-0310-9956-ffa450edef68
---
 docs/manual/mod/event.xml | 87 +++++++++++++++++++++++++++++++++++----
 1 file changed, 79 insertions(+), 8 deletions(-)

diff --git a/docs/manual/mod/event.xml b/docs/manual/mod/event.xml
index 6b5670df68..9c0117111e 100644
--- a/docs/manual/mod/event.xml
+++ b/docs/manual/mod/event.xml
@@ -116,7 +116,7 @@ of the <directive>AsyncRequestWorkerFactor</directive>.</p>
         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
@@ -231,12 +231,15 @@ of the <directive>AsyncRequestWorkerFactor</directive>.</p>
     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
-    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>
+    <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.</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
@@ -249,13 +252,81 @@ of the <directive>AsyncRequestWorkerFactor</directive>.</p>
         <var>number of idle workers</var>)
     </strong></p>
 
-    <p>This means the absolute maximum numbers of concurrent connections is:</p>
+    <note><title>Idle connections handled by each process</title>
+    <highlight language="config">
+
+max_connections = ThreadsPerChild + (AsyncRequestWorkerFactor * idle_workers)
+
+ThreadsPerChild = idle_workers + busy_workers
+            
+max_connections = (idle_workers + busy_workers) + (AsyncRequestWorkerFactor * idle_workers)
+                = busy_workers + (AsyncRequestWorkerFactor + 1) * idle_workers 
+
+max_connections = max_idle_connections + busy_workers 
+
+max_idle_connections + busy_workers = 
+                busy_workers + (AsyncRequestWorkerFactor + 1) * idle_workers 
+
+max_idle_connections = (AsyncRequestWorkerFactor + 1) * idle_workers 
+
+    </highlight>
+    </note>
+
+    <p>The absolute maximum numbers of concurrent connections is:</p>
 
     <p class="indent"><strong>
         (<directive>AsyncRequestWorkerFactor</directive> + 1) *
         <directive module="mpm_common">MaxRequestWorkers</directive>
     </strong></p>
 
+
+    <note><title>Example 1</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>The above example is only related to a theoretical maximum, let's take a look to a more common use case:</p>
+
+    <note><title>Example 2</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>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>
-- 
2.40.0