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Apache > HTTP Server > Documentation > Version 2.4 > Modules

Apache MPM Common Directives

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Description:A collection of directives that are implemented by more than one multi-processing module (MPM)
Status:MPM

Directives

Bugfix checklist

See also

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CoreDumpDirectory Directive

Description:Directory where Apache HTTP Server attempts to switch before dumping core
Syntax:CoreDumpDirectory directory
Default:See usage for the default setting
Context:server config
Status:MPM
Module:event, worker, prefork

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.

Core Dumps on Linux

If Apache httpd starts as root and switches to another user, the Linux kernel disables core dumps even if the directory is writable for the process. Apache httpd (2.0.46 and later) reenables core dumps on Linux 2.4 and beyond, but only if you explicitly configure a CoreDumpDirectory.

Core Dumps on BSD

To enable core-dumping of suid-executables on BSD-systems (such as FreeBSD), set kern.sugid_coredump to 1.

Specific signals

CoreDumpDirectory processing only occurs for a select set of fatal signals: SIGFPE, SIGILL, SIGABORT, SIGSEGV, and SIGBUS.

On some operating systems, SIGQUIT also results in a core dump but does not go through CoreDumpDirectory or EnableExceptionHook processing, so the core location is dictated entirely by the operating system.

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EnableExceptionHook Directive

Description:Enables a hook that runs exception handlers after a crash
Syntax:EnableExceptionHook On|Off
Default:EnableExceptionHook Off
Context:server config
Status:MPM
Module:event, worker, prefork

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.

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GracefulShutdownTimeout Directive

Description:Specify a timeout after which a gracefully shutdown server will exit.
Syntax:GracefulShutdownTimeout seconds
Default:GracefulShutdownTimeout 0
Context:server config
Status:MPM
Module:event, worker, prefork
Compatibility:Available in version 2.2 and later

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.

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Listen Directive

Description:IP addresses and ports that the server listens to
Syntax:Listen [IP-address:]portnumber [protocol]
Context:server config
Status:MPM
Module:event, worker, prefork, mpm_winnt, mpm_netware, mpmt_os2
Compatibility:The protocol argument was added in 2.1.5

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

Error condition

Multiple Listen directives for the same ip address and port will result in an Address already in use error message.

See also

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ListenBackLog Directive

Description:Maximum length of the queue of pending connections
Syntax:ListenBacklog backlog
Default:ListenBacklog 511
Context:server config
Status:MPM
Module:event, worker, prefork, mpm_winnt, mpm_netware, mpmt_os2

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.

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ListenCoresBucketsRatio Directive

Description:Ratio between the number of CPU cores (online) and the number of listeners' buckets
Syntax:ListenCoresBucketsRatio ratio
Default:ListenCoresBucketsRatio 0 (disabled)
Context:server config
Status:MPM
Module:event, worker, prefork
Compatibility:Available in Apache HTTP Server 2.4.17, with a kernel supporting the socket option SO_REUSEPORT and distributing new connections evenly across listening processes' (or threads') sockets using it (eg. Linux 3.9 and later, but not the current implementations of SO_REUSEPORT in *BSDs.

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).

Meaning of "online" CPU core

On Linux (and also BSD) a CPU core can be turned on/off if Hotplug is configured, therefore ListenCoresBucketsRatio needs to take this parameter into account while calculating the number of buckets to create.

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