mpm_common - Apache HTTP Server

Apache HTTP Server Version 2.2

Apache > HTTP Server > Documentation > Version 2.2 > 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

## AcceptMutex Directive

Description: | Method that Apache uses to serialize multiple children accepting requests on network sockets
Syntax: | `AcceptMutex Default|method`
Default: | `AcceptMutex Default`
Context: | server config
Status: | MPM
Module: | `prefork`, `worker`

The `AcceptMutex` directives sets the method that Apache uses to serialize multiple children accepting requests on network sockets. Prior to Apache 2.0, the method was selectable only at compile time. The optimal method to use is highly architecture and platform dependent. For further details, see the performance tuning documentation.

If this directive is set to `Default`, then the compile-time selected default will be used. Other possible methods are listed below. Note that not all methods are available on all platforms. If a method is specified which is not available, a message will be written to the error log listing the available methods.

`flock`
  uses the `flock(2)` system call to lock the file defined by the `LockFile` directive.
`fcntl`
  uses the `fcntl(2)` system call to lock the file defined by the `LockFile` directive.
`posixsem`
  uses POSIX compatible semaphores to implement the mutex.
`pthread`
  uses POSIX mutexes as implemented by the POSIX Threads (PThreads) specification.
`sysvsem`
  uses SySV-style semaphores to implement the mutex.

If you want to find out the compile time chosen default for your system, you may set your `LogLevel` to `debug`. Then the default `AcceptMutex` will be written into the `ErrorLog`.

### Warning

On most systems, when the `pthread` option is selected, if a child process terminates abnormally while holding the `AcceptCntl` mutex the server will stop responding to requests. When this occurs, the server will require a manual restart to recover.

Solaris is a notable exception as it provides a mechanism, used by Apache, which usually allows the mutex to be recovered after a child process terminates abnormally while holding a mutex.

If your system implements the `pthread_mutexattr_setrobust_np()` function, you may be able to use the `pthread` option safely.

## CoreDumpDirectory Directive

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

This controls the directory to which Apache attempts to switch before dumping core. The default is in the `ServerRoot` directory, however since this should not be writable by the user the server runs as, core dumps won't normally get written. If you want a core dump for debugging, you can use this directive to place it in a different location.

### Core Dumps on Linux

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

## 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: | `prefork`, `worker`
Compatibility: | Available in version 2.0.49 and later

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

Description: | Specify a timeout after which a gracefully shutdown server will exit.
Syntax: | `GracefulShutDownTimeout seconds`
Default: | `GracefulShutDownTimeout 0`
Context: | server config
Status: | MPM
Module: | `prefork`, `worker`, `event`
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.

## Group Directive

Description: | Group under which the server will answer requests
Syntax: | `Group unix-group`
Default: | `Group #-1`
Context: | server config
Status: | MPM
Module: | `beos`, `mpmt_os2`, `prefork`, `worker`
Compatibility: | Only valid in global server config since Apache 2.0

The `Group` directive sets the group under which the server will answer requests. In order to use this directive, the server must be run initially as `root`. If you start the server as a non-root user, it will fail to change to the specified group, and will instead continue to run as the group of the original user. Unix-group is one of:

A group name
  Refers to the given group by name.
`#` followed by a group number.
  Refers to a group by its number.

### Example

`Group www-group`

It is recommended that you set up a new group specifically for running the server. Some admins use user `nobody`, but this is not always possible or desirable.

### Security

Don't set `Group` (or `User`) to `root` unless you know exactly what you are doing, and what the dangers are.

Special note: Use of this directive in `<VirtualHost>` is no longer supported. To configure your server for `suexec` use `SuexecUserGroup`.

### Note

Although the `Group` directive is present in the `beos` and `mpmt_os2` MPMs, it is actually a no-op there and only exists for compatibility reasons.

## Listen Directive

Description: | IP addresses and ports that the server listens to
Syntax: | `Listen [IP-address:]portnumber [protocol]`
Context: | server config
Status: | MPM
Module: | `beos`, `mpm_netware`, `mpm_winnt`, `mpmt_os2`, `prefork`, `worker`, `event`
Compatibility: | Required directive since Apache 2.0 The protocol argument was added in 2.1.5

The `Listen` directive instructs Apache 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.

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

- DNS Issues
- Setting which addresses and ports Apache uses

## ListenBackLog Directive

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

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.

## LockFile Directive

Description: | Location of the accept serialization lock file
Syntax: | `LockFile filename`
Default: | `LockFile logs/accept.lock`
Context: | server config
Status: | MPM
Module: | `prefork`, `worker`

The `LockFile` directive sets the path to the lockfile used when Apache is used with an `AcceptMutex` value of either `fcntl` or `flock`. This directive should normally be left at its default value. The main reason for changing it is if the `logs` directory is NFS mounted, since the lockfile must be stored on a local disk. The PID of the main server process is automatically appended to the filename.

### Security

It is best to avoid putting this file in a world writable directory such as `/var/tmp` because someone could create a denial of service attack and prevent the server from starting by creating a lockfile with the same name as the one the server will try to create.

### See also

- `AcceptMutex`

## MaxClients Directive

Description: | Maximum number of child processes that will be created to serve requests
Syntax: | `MaxClients number`
Default: | `See usage for details`
Context: | server config
Status: | MPM
Module: | `beos`, `prefork`, `worker`

The `MaxClients` directive sets the limit on the number of simultaneous requests that will be served. Any connection attempts over the `MaxClients` 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`), `MaxClients` 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. `beos` or `worker`) `MaxClients` restricts the total number of threads that will be available to serve clients. The default value for `beos` is `50`. For hybrid MPMs the default value is `16` (`ServerLimit`) multiplied by the value of `25` (`ThreadsPerChild`). Therefore, to increase `MaxClients` to a value that requires more than 16 processes, you must also raise `ServerLimit`.

## MaxMemFree Directive

Description: | Maximum amount of memory that the main allocator is allowed to hold without calling `free()`
Syntax: | `MaxMemFree KBytes`
Default: | `MaxMemFree 0`
Context: | server config
Status: | MPM
Module: | `beos`, `mpm_netware`, `prefork`, `worker`, `mpm_winnt`

The `MaxMemFree` directive sets the maximum number of free Kbytes that the main allocator is allowed to hold without calling `free()`. When not set, or when set to zero, the threshold will be set to unlimited.

## MaxRequestsPerChild Directive

Description: | Limit on the number of requests that an individual child server will handle during its life
Syntax: | `MaxRequestsPerChild number`
Default: | `MaxRequestsPerChild 10000`
Context: | server config
Status: | MPM
Module: | `mpm_netware`, `mpm_winnt`, `mpmt_os2`, `prefork`, `worker`

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

### Different default values

The default value for `mpm_netware` and `mpm_winnt` is `0`.

Setting `MaxRequestsPerChild` to a non-zero limit has two beneficial effects:

- it limits the amount of memory that process can consume by (accidental) memory leakage;
- by giving processes a finite lifetime, it helps reduce the number of processes when the server load reduces.

### Note

For `KeepAlive` requests, only the first request is counted towards this limit. In effect, it changes the behavior to limit the number of connections per child.

## MaxSpareThreads Directive

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

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

For `worker`, 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.

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

`beos` and `mpmt_os2` work similar to `mpm_netware`. The default for `beos` is `MaxSpareThreads 50`. For `mpmt_os2` the default value is `10`.

### Restrictions

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

- `mpm_netware` wants the value to be greater than `MinSpareThreads`.
- For `worker` the value must be greater or equal than the sum of `MinSpareThreads` and `ThreadsPerChild`.

### See also

- `MinSpareThreads`
- `StartServers`

## 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: | `beos`, `mpm_netware`, `mpmt_os2`, `worker`

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

`worker` uses 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.

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

`beos` and `mpmt_os2` work similar to `mpm_netware`. The default for `beos` is `MinSpareThreads 1`. For `mpmt_os2` the default value is `5`.

### See also

- `MaxSpareThreads`
- `StartServers`

## 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: | `beos`, `mpm_winnt`, `mpmt_os2`, `prefork`, `worker`

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 2 it is recommended to use only the `apachectl` script for (re-)starting or stopping the server.

## ReceiveBufferSize Directive

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

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.

## ScoreBoardFile Directive

Description: | Location of the file used to store coordination data for the child processes
Syntax: | `ScoreBoardFile file-path`
Default: | `ScoreBoardFile logs/apache_status`
Context: | server config
Status: | MPM
Module: | `beos`, `mpm_winnt`, `prefork`, `worker`

Apache 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 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 to always create the file on the disk.

### Example

`ScoreBoardFile /var/run/apache_status`

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

If you use a `ScoreBoardFile` 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 and security.

### See also

- Stopping and Restarting Apache

## SendBufferSize Directive

Description: | TCP buffer size
Syntax: | `SendBufferSize bytes`
Default: | `SendBufferSize 0`
Context: | server config
Status: | MPM
Module: | `beos`, `mpm_netware`, `mpm_winnt`, `mpmt_os2`, `prefork`, `worker`

The server will set the TCP send buffer size to the number of bytes specified. Very useful to increase past standard OS defaults on high speed high latency (i.e., 100ms or so, such as transcontinental fast pipes).

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

## ServerLimit Directive

Description: | Upper limit on configurable number of processes
Syntax: | `ServerLimit number`
Default: | `See usage for details`
Context: | server config
Status: | MPM
Module: | `prefork`, `worker`

For the `prefork` MPM, this directive sets the maximum configured value for `MaxClients` for the lifetime of the Apache process. For the `worker` MPM, this directive in combination with `ThreadLimit` sets the maximum configured value for `MaxClients` for the lifetime of the Apache process. Any attempts to change this directive during a restart will be ignored, but `MaxClients` can be modified during a restart.

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

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

With `worker` use this directive only if your `MaxClients` and `ThreadsPerChild` 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 for `MaxClients` and `ThreadsPerChild`.

### Note

There is a hard limit of `ServerLimit 20000` compiled into the server (for the `prefork` MPM 200000). This is intended to avoid nasty effects caused by typos.

### See also

- Stopping and Restarting Apache

## StartServers Directive

Description: | Number of child server processes created at startup
Syntax: | `StartServers number`
Default: | `See usage for details`
Context: | server config
Status: | MPM
Module: | `mpmt_os2`, `prefork`, `worker`

The `StartServers` directive sets the number of child server processes created on startup. As the number of processes is dynamically controlled depending on the load, there is usually little reason to adjust this parameter.

The default value differs from MPM to MPM. For `worker` the default is `StartServers 3`. For `prefork` defaults to `5` and for `mpmt_os2` to `2`.

## StartThreads Directive

Description: | Number of threads created on startup
Syntax: | `StartThreads number`
Default: | `See usage for details`
Context: | server config
Status: | MPM
Module: | `beos`, `mpm_netware`

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

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

For `beos` the default is `StartThreads 10`. It also reflects the total number of threads created at startup to serve requests.

## ThreadLimit Directive

Description: | Sets the upper limit on the configurable number of threads per child process
Syntax: | `ThreadLimit number`
Default: | `See usage for details`
Context: | server config
Status: | MPM
Module: | `mpm_winnt`, `worker`
Compatibility: | Available for `mpm_winnt` in Apache 2.0.41 and later

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

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

The default value for `ThreadLimit` is `1920` when used with `mpm_winnt` and `64` when used with the others.

### Note

There is a hard limit of `ThreadLimit 20000` (or `ThreadLimit 15000` with `mpm_winnt`) compiled into the server. This is intended to avoid nasty effects caused by typos.

## ThreadsPerChild Directive

Description: | Number of threads created by each child process
Syntax: | `ThreadsPerChild number`
Default: | `See usage for details`
Context: | server config
Status: | MPM
Module: | `mpm_winnt`, `worker`

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 like `mpm_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 like `worker`, where there are multiple child processes, the total number of threads should be high enough to handle the common load on the server.

The default value for `ThreadsPerChild` is `64` when used with `mpm_winnt` and `25` when used with the others.

## ThreadStackSize Directive

Description: | The size in bytes of the stack used by threads handling client connections
Syntax: | `ThreadStackSize size`
Default: | `65536 on NetWare; varies on other operating systems`
Context: | server config
Status: | MPM
Module: | `mpm_netware`, `mpm_winnt`, `worker`
Compatibility: | Available in Apache 2.1 and later

The `ThreadStackSize` 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 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 setting `ThreadStackSize` 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 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 if `ThreadStackSize` 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 can be exercised, as there may be infrequent requests which require more stack to process. A change in the web server configuration can invalidate the current `ThreadStackSize` setting.

## User Directive

Description: | The userid under which the server will answer requests
Syntax: | `User unix-userid`
Default: | `User #-1`
Context: | server config
Status: | MPM
Module: | `prefork`, `worker`
Compatibility: | Only valid in global server config since Apache 2.0

The `User` directive sets the user ID as which the server will answer requests. In order to use this directive, the server must be run initially as `root`. If you start the server as a non-root user, it will fail to change to the lesser privileged user, and will instead continue to run as that original user. If you do start the server as `root`, then it is normal for the parent process to remain running as root. Unix-userid is one of:

A username
  Refers to the given user by name.
# followed by a user number.
  Refers to a user by its number.

The user should have no privileges that result in it being able to access files that are not intended to be visible to the outside world, and similarly, the user should not be able to execute code that is not meant for HTTP requests. It is recommended that you set up a new user and group specifically for running the server. Some admins use user `nobody`, but this is not always desirable, since the `nobody` user can have other uses on the system.

### Security

Don't set `User` (or `Group`) to `root` unless you know exactly what you are doing, and what the dangers are.

Special note: Use of this directive in `<VirtualHost>` is no longer supported. To configure your server for `suexec` use `SuexecUserGroup`.

### Note

Although the `User` directive is present in the `beos` and `mpmt_os2` MPMs, it is actually a no-op there and only exists for compatibility reasons.

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