Follow up to r1789220.
Document the implicit behaviour of ListenCoresBucketsRatio when multiple
Listen-ers are configured on the same IP:port.
@@ -300,6 +300,27 @@ in *BSDs.</compatibility>
<directive module="mpm_common">MaxSpareThreads</directive> lower bound values.
The number of children processes needs to be a multiple of the number
of buckets to optimally accept connections.</p>
+
+ <note>
+ <title>Multiple <directive>Listen</directive>ers or Apache HTTP servers on
+ the same IP address and port</title>
+ <p>Setting the <code>SO_REUSEPORT</code> option on the listening socket(s)
+ consequently allows multiple processes (sharing the same <code>EUID</code>,
+ e.g. <code>root</code>) to bind to the the same IP address and port,
+ without the binding error raised by the system in the usual case.</p>
+ <p>This also means that multiple instances of Apache httpd configured on a
+ same <code>IP:port</code> and with a positive <directive>ListenCoresBucketsRatio</directive>
+ would start without an error too, and then run with incoming connections
+ evenly distributed accross both instances (this is NOT a recommendation or
+ a sensible usage in any case, but just a notice that it would prevent such
+ possible issues to be detected).</p>
+ <p>Within the same instance, Apache httpd will check and fail to start if
+ multiple <directive>Listen</directive> directives on the exact same IP (or
+ hostname) and port are configured, thus avoiding the creation of some
+ duplicated buckets which would be useless and kill performances. However
+ it can't (and won't try harder to) catch all the possible overlapping cases
+ (like a hostname resolving to an IP used elsewhere).</p>
+ </note>
</usage>
</directivesynopsis>
