Apache MPM Common Directives

CoreDumpDirectory Directive

This controls the directory to which Apache httpd attempts to switch before dumping core. If your operating system is configured to create core files in the working directory of the crashing process, CoreDumpDirectory is necessary to change working directory from the default ServerRoot directory, which should not be writable by the user the server runs as.

If you want a core dump for debugging, you can use this directive to place it in a different location. This directive has no effect if your operating system is not configured to write core files to the working directory of the crashing processes.

EnableExceptionHook Directive

For safety reasons this directive is only available if the server was configured with the --enable-exception-hook option. It enables a hook that allows external modules to plug in and do something after a child crashed.

There are already two modules, mod_whatkilledus and mod_backtrace that make use of this hook. Please have a look at Jeff Trawick's EnableExceptionHook site for more information about these.

GracefulShutdownTimeout Directive

The GracefulShutdownTimeout specifies how many seconds after receiving a "graceful-stop" signal, a server should continue to run, handling the existing connections.

Setting this value to zero means that the server will wait indefinitely until all remaining requests have been fully served.

Listen Directive

The Listen directive instructs Apache httpd to listen to only specific IP addresses or ports; by default it responds to requests on all IP interfaces. Listen is now a required directive. If it is not in the config file, the server will fail to start. This is a change from previous versions of Apache httpd.

The Listen directive tells the server to accept incoming requests on the specified port or address-and-port combination. If only a port number is specified, the server listens to the given port on all interfaces. If an IP address is given as well as a port, the server will listen on the given port and interface.

Multiple Listen directives may be used to specify a number of addresses and ports to listen to. The server will respond to requests from any of the listed addresses and ports.

For example, to make the server accept connections on both port 80 and port 8000, use:

Listen 80
Listen 8000

To make the server accept connections on two specified interfaces and port numbers, use

Listen 192.170.2.1:80
Listen 192.170.2.5:8000

IPv6 addresses must be surrounded in square brackets, as in the following example:

Listen [2001:db8::a00:20ff:fea7:ccea]:80

The optional protocol argument is not required for most configurations. If not specified, https is the default for port 443 and http the default for all other ports. The protocol is used to determine which module should handle a request, and to apply protocol specific optimizations with the AcceptFilter directive.

You only need to set the protocol if you are running on non-standard ports. For example, running an https site on port 8443:

Listen 192.170.2.1:8443 https

See also

• DNS Issues
• Setting which addresses and ports Apache HTTP Server uses
• Further discussion of the Address already in use error message, including other causes.

ListenBackLog Directive

The maximum length of the queue of pending connections. Generally no tuning is needed or desired; however on some systems, it is desirable to increase this when under a TCP SYN flood attack. See the backlog parameter to the listen(2) system call.

This will often be limited to a smaller number by the operating system. This varies from OS to OS. Also note that many OSes do not use exactly what is specified as the backlog, but use a number based on (but normally larger than) what is set.

ListenCoresBucketsRatio Directive

A ratio between the number of (online) CPU cores and the number of listeners' buckets can be used to make Apache HTTP Server create num_cpu_cores / ratio listening buckets, each containing its own Listen-ing socket(s) on the same port(s), and then make each child handle a single bucket (with round-robin distribution of the buckets at children creation time).

ListenCoresBucketsRatio can improve the scalability when accepting new connections is/becomes the bottleneck. On systems with a large number of CPU cores, enabling this feature has been tested to show significant performances improvement and shorter responses time.

There must be at least twice the number of CPU cores than the configured ratio for this to be active. The recommended ratio is 8, hence at least 16 cores should be available at runtime when this value is used. The right ratio to obtain maximum performance needs to be calculated for each target system, testing multiple values and observing the variations in your key performance metrics.

This directive influences the calculation of the MinSpareThreads and MaxSpareThreads lower bound values. The number of children processes needs to be a multiple of the number of buckets to optimally accept connections.

MaxConnectionsPerChild Directive

The MaxConnectionsPerChild directive sets the limit on the number of connections that an individual child server process will handle. After MaxConnectionsPerChild connections, the child process will die. If MaxConnectionsPerChild is 0, then the process will never expire.

Setting MaxConnectionsPerChild to a non-zero value limits the amount of memory that a process can consume by (accidental) memory leakage.

MaxMemFree