Apache MPM Common Directives

Directives

• CoreDumpDirectory
• EnableExceptionHook
• GracefulShutdownTimeout
• Listen
• ListenBackLog
• ListenCoresBucketsRatio
• MaxConnectionsPerChild
• MaxMemFree
• MaxRequestWorkers
• MaxSpareThreads
• MinSpareThreads
• PidFile
• ReceiveBufferSize
• ScoreBoardFile
• SendBufferSize
• ServerLimit
• StartServers
• StartThreads
• ThreadLimit
• ThreadsPerChild
• ThreadStackSize

Bugfix checklist

• httpd changelog
• Known issues
• Report a bug

See also

• Comments

CoreDumpDirectoryDirective

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 defaultServerRoot 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.

EnableExceptionHookDirective

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 andmod_backtrace that make use of this hook. Please have a look at Jeff Trawick'sEnableExceptionHook site for more information about these.

GracefulShutdownTimeoutDirective

TheGracefulShutdownTimeout 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.

ListenDirective

TheListen 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.

TheListen 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.

MultipleListen 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 80Listen 8000

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

Listen 192.170.2.1:80Listen 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 optionalprotocol argument is not required for most configurations. If not specified,https is the default for port 443 andhttp 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 theAcceptFilter directive.

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

Listen 192.170.2.1:8443 https

See also

• DNS Issues
• Setting which addresses and ports Apache HTTP Server uses
• Furtherdiscussion of theAddress already in use error message,including other causes.

ListenBackLogDirective

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 thelisten(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.

ListenCoresBucketsRatioDirective

Aratio between the number of (online) CPU cores and the number of listeners' buckets can be used to make Apache HTTP Server createnum_cpu_cores / ratio listening buckets, each containing its ownListen-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 configuredratio for this to be active. The recommendedratio is8, hence at least16 cores should be available at runtime when this value is used. The rightratio 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 theMinSpareThreads andMaxSpareThreads lower bound values. The number of children processes needs to be a multiple of the number of buckets to optimally accept connections.

MaxConnectionsPerChildDirective

TheMaxConnectionsPerChild directive sets the limit on the number of connections that an individual child server process will handle. AfterMaxConnectionsPerChild connections, the child process will die. IfMaxConnectionsPerChild is0, then the process will never expire.

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

MaxMemFreeDirective

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

MaxRequestWorkersDirective

TheMaxRequestWorkers directive sets the limit on the number of simultaneous requests that will be served. Any connection attempts over theMaxRequestWorkers limit will normally be queued, up to a number based on theListenBacklog 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 is256; to increase it, you must also raiseServerLimit.

For threaded and hybrid servers (e.g.event orworker),MaxRequestWorkers restricts the total number of threads that will be available to serve clients. For hybrid MPMs, the default value is16 (ServerLimit) multiplied by the value of25 (ThreadsPerChild). Therefore, to increaseMaxRequestWorkers to a value that requires more than 16 processes, you must also raiseServerLimit.

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

MaxSpareThreadsDirective

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

Forworker andevent, the default isMaxSpareThreads 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 ifListenCoresBucketsRatio is enabled.

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

mpmt_os2 works similar tompm_netware. Formpmt_os2 the default value is10.

See also

• MinSpareThreads
• StartServers
• MaxSpareServers

MinSpareThreadsDirective

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

worker andevent use a default ofMinSpareThreads 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 thannumber. Additional processes/threads might be created ifListenCoresBucketsRatio is enabled.

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

mpmt_os2 works similar tompm_netware. Formpmt_os2 the default value is5.

See also

• MaxSpareThreads
• StartServers
• MinSpareServers

PidFileDirective

ThePidFile 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 theServerRoot.

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 itsErrorLog andTransferLog, and re-reads its configuration files. This is done by sending a SIGHUP (kill -1) signal to the process id listed in thePidFile.

ThePidFile is subject to the same warnings about log file placement andsecurity.

ReceiveBufferSizeDirective

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

If set to the value of0, the server will use the OS default.

ScoreBoardFileDirective

Apache HTTP Server uses a scoreboard to communicate between its parent and child processes. Some architectures require a file to facilitate this communication. If the file is left unspecified, Apache httpd first attempts to create the scoreboard entirely in memory (using anonymous shared memory) and, failing that, will attempt to create the file on disk (using file-based shared memory). Specifying this directive causes Apache httpd to always create the file on the disk.

ScoreBoardFile /var/run/apache_runtime_status

File-based shared memory is useful for third-party applications that require direct access to the scoreboard.

If you use aScoreBoardFile, then you may see improved speed by placing it on a RAM disk. But be careful that you heed the same warnings about log file placement andsecurity.

See also

• Stopping and RestartingApache HTTP Server

SendBufferSizeDirective

Sets the server's TCP send buffer size to the number of bytes specified. It is often useful to set this past the OS's standard default value on high speed, high latency connections (i.e., 100ms or so, such as transcontinental fast pipes).

If set to the value of0, the server will use the default value provided by your OS.

Further configuration of your operating system may be required to elicit better performance on high speed, high latency connections.

ServerLimitDirective

For theprefork MPM, this directive sets the maximum configured value forMaxRequestWorkers for the lifetime of the Apache httpd process. For theworker andevent MPMs, this directive in combination withThreadLimit sets the maximum configured value forMaxRequestWorkers for the lifetime of the Apache httpd process. For theevent MPM, this directive also defines how many old server processes may keep running and finish processing open connections. Any attempts to change this directive during a restart will be ignored, butMaxRequestWorkers can be modified during a restart.

Special care must be taken when using this directive. IfServerLimit is set to a value much higher than necessary, extra, unused shared memory will be allocated. If bothServerLimit andMaxRequestWorkers are set to values higher than the system can handle, Apache httpd may not start or the system may become unstable.

With theprefork MPM, use this directive only if you need to setMaxRequestWorkers higher than 256 (default). Do not set the value of this directive any higher than what you might want to setMaxRequestWorkers to.

Withworker, use this directive only if yourMaxRequestWorkers andThreadsPerChild settings require more than 16 server processes (default). Do not set the value of this directive any higher than the number of server processes required by what you may want forMaxRequestWorkers andThreadsPerChild.

Withevent, increase this directive if the process number defined by yourMaxRequestWorkers andThreadsPerChild settings, plus the number of gracefully shutting down processes, is more than 16 server processes (default).

See also

• Stopping and Restarting Apache HTTP Server

StartServersDirective

TheStartServers directive sets the number of child server processes created on startup. As the number of processes is dynamically controlled depending on the load, (seeMinSpareThreads,MaxSpareThreads,MinSpareServers,MaxSpareServers) there is usually little reason to adjust this parameter.

The default value differs from MPM to MPM.worker andevent default toStartServers 3; prefork defaults to5;mpmt_os2 defaults to2.

StartThreadsDirective

Number of threads created on startup. As the number of threads is dynamically controlled depending on the load, (seeMinSpareThreads,MaxSpareThreads,MinSpareServers,MaxSpareServers) there is usually little reason to adjust this parameter.

Formpm_netware the default isStartThreads 50 and, since there is only a single process, this is the total number of threads created at startup to serve requests.

ThreadLimitDirective

This directive sets the maximum configured value forThreadsPerChild for the lifetime of the Apache httpd process. Any attempts to change this directive during a restart will be ignored, butThreadsPerChild can be modified during a restart up to the value of this directive.

Special care must be taken when using this directive. IfThreadLimit is set to a value much higher thanThreadsPerChild, extra unused shared memory will be allocated. If bothThreadLimit andThreadsPerChild are set to values higher than the system can handle, Apache httpd may not start or the system may become unstable. Do not set the value of this directive any higher than your greatest predicted setting ofThreadsPerChild for the current run of Apache httpd.

The default value forThreadLimit is1920 when used withmpm_winnt and64 when used with the others.

ThreadsPerChildDirective

This directive sets the number of threads created by each child process. The child creates these threads at startup and never creates more. If using an MPM likempm_winnt, where there is only one child process, this number should be high enough to handle the entire load of the server. If using an MPM likeworker, where there are multiple child processes, thetotal number of threads should be high enough to handle the common load on the server.

The default value forThreadsPerChild is64 when used withmpm_winnt and25 when used with the others.

The value ofThreadsPerChild can not exceed the value ofThreadLimit. If a higher value is configured, it will be automatically reduced at start-up and a warning will be logged. The relationship between these 2 directives is explained inThreadLimit.

ThreadStackSizeDirective

TheThreadStackSize directive sets the size of the stack (for autodata) of threads which handle client connections and call modules to help process those connections. In most cases the operating system default for stack size is reasonable, but there are some conditions where it may need to be adjusted:

• On platforms with a relatively small default thread stack size (e.g., HP-UX), Apache httpd may crash when using some third-party modules which use a relatively large amount of autodata storage. Those same modules may have worked fine on other platforms where the default thread stack size is larger. This type of crash is resolved by settingThreadStackSize to a value higher than the operating system default. This type of adjustment is necessary only if the provider of the third-party module specifies that it is required, or if diagnosis of an Apache httpd crash indicates that the thread stack size was too small.
• On platforms where the default thread stack size is significantly larger than necessary for the web server configuration, a higher number of threads per child process will be achievable ifThreadStackSize is set to a value lower than the operating system default. This type of adjustment should only be made in a test environment which allows the full set of web server processing to be exercised, as there may be infrequent requests which require more stack to process. The minimum required stack size strongly depends on the modules used, but any change in the web server configuration can invalidate the currentThreadStackSize setting.
• On Linux, this directive can only be used to increase the default stack size, as the underlying system call uses the value as aminimum stack size. The (often large) soft limit forulimit -s (8MB if unlimited) is used as the default stack size.