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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
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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 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.

Multiple Listeners or Apache HTTP servers on the same IP address and port

Setting the SO_REUSEPORT option on the listening socket(s) consequently allows multiple processes (sharing the same EUID, e.g. root) to bind to the the same IP address and port, without the binding error raised by the system in the usual case.

This also means that multiple instances of Apache httpd configured on a same IP:port and with a positive ListenCoresBucketsRatio 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).

Within the same instance, Apache httpd will check and fail to start if multiple Listen 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).

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MaxConnectionsPerChild Directive

Description:Limit on the number of connections that an individual child server will handle during its life
Syntax:MaxConnectionsPerChild number
Default:MaxConnectionsPerChild 0
Context:server config
Status:MPM
Module:event, worker, prefork, mpm_winnt, mpm_netware, mpmt_os2
Compatibility:Available Apache HTTP Server 2.3.9 and later. The old name MaxRequestsPerChild is still supported.

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 process can consume by (accidental) memory leakage.

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MaxMemFree Directive

Description:Maximum amount of memory that the main allocator is allowed to hold without calling free()
Syntax:MaxMemFree KBytes
Default:MaxMemFree 2048
Context:server config
Status:MPM
Module:event, worker, prefork, mpm_winnt, mpm_netware

The MaxMemFree directive sets the maximum number of free Kbytes that every allocator is allowed to hold without calling free(). In threaded MPMs, every thread has its own allocator. When set to zero, the threshold will be set to unlimited.

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MaxRequestWorkers Directive

Description:Maximum number of connections that will be processed simultaneously
Syntax:MaxRequestWorkers number
Default:See usage for details
Context:server config
Status:MPM
Module:event, worker, prefork

The MaxRequestWorkers directive sets the limit on the number of simultaneous requests that will be served. Any connection attempts over the MaxRequestWorkers limit will normally be queued, up to a number based on the ListenBacklog directive. Once a child process is freed at the end of a different request, the connection will then be serviced.

For non-threaded servers (i.e., prefork), MaxRequestWorkers translates into the maximum number of child processes that will be launched to serve requests. The default value is 256; to increase it, you must also raise ServerLimit.

For threaded and hybrid servers (e.g. event or worker) MaxRequestWorkers restricts the total number of threads that will be available to serve clients. For hybrid MPMs the default value is 16 (ServerLimit) multiplied by the value of 25 (ThreadsPerChild). Therefore, to increase MaxRequestWorkers to a value that requires more than 16 processes, you must also raise ServerLimit.

MaxRequestWorkers was called MaxClients before version 2.3.13. The old name is still supported.

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MaxSpareThreads Directive

Description:Maximum number of idle threads
Syntax:MaxSpareThreads number
Default:See usage for details
Context:server config
Status:MPM
Module:event, worker, mpm_netware, mpmt_os2

Maximum number of idle threads. Different MPMs deal with this directive differently.

For worker and event, the default is MaxSpareThreads 250. These MPMs deal with idle threads on a server-wide basis. If there are too many idle threads in the server then child processes are killed until the number of idle threads is less than this number. Additional processes/threads might be created if ListenCoresBucketsRatio is enabled.

For mpm_netware the default is MaxSpareThreads 100. Since this MPM runs a single-process, the spare thread count is also server-wide.

mpmt_os2 works similar to mpm_netware. For mpmt_os2 the default value is 10.

Restrictions

The range of the MaxSpareThreads value is restricted. Apache httpd will correct the given value automatically according to the following rules:

See also

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MinSpareThreads Directive

Description:Minimum number of idle threads available to handle request spikes
Syntax:MinSpareThreads number
Default:See usage for details
Context:server config
Status:MPM
Module:event, worker, mpm_netware, mpmt_os2

Minimum number of idle threads to handle request spikes. Different MPMs deal with this directive differently.

worker and event use a default of MinSpareThreads 75 and deal with idle threads on a server-wide basis. If there aren't enough idle threads in the server then child processes are created until the number of idle threads is greater than number. Additional processes/threads might be created if ListenCoresBucketsRatio is enabled.

mpm_netware uses a default of MinSpareThreads 10 and, since it is a single-process MPM, tracks this on a server-wide bases.

mpmt_os2 works similar to mpm_netware. For mpmt_os2 the default value is 5.

See also

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PidFile Directive

Description:File where the server records the process ID of the daemon
Syntax:PidFile filename
Default:PidFile logs/httpd.pid
Context:server config
Status:MPM
Module:event, worker, prefork, mpm_winnt, mpmt_os2

The PidFile directive sets the file to which the server records the process id of the daemon. If the filename is not absolute then it is assumed to be relative to the ServerRoot.

Example

PidFile /var/run/apache.pid

It is often useful to be able to send the server a signal, so that it closes and then re-opens its ErrorLog and TransferLog, and re-reads its configuration files. This is done by sending a SIGHUP (kill -1) signal to the process id listed in the PidFile.

The PidFile is subject to the same warnings about log file placement and security.

Note

As of Apache HTTP Server 2, we recommended that you only use the apache2ctl script, or the init script that your OS provides, for (re-)starting or stopping the server.

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ReceiveBufferSize Directive

Description:TCP receive buffer size
Syntax:ReceiveBufferSize bytes
Default:ReceiveBufferSize 0
Context:server config
Status:MPM
Module:event, worker, prefork, mpm_winnt, mpm_netware, mpmt_os2

The server will set the TCP receive buffer size to the number of bytes specified.

If set to the value of 0, the server will use the OS default.

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ScoreBoardFile Directive

Description:Location of the file used to store coordination data for the child processes
Syntax:ScoreBoardFile file-pa