Apache HTTP Server Version 2.4

Apache > HTTP Server > Documentation > Version 2.4 > Modules

# Apache MPM Common Directives

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

### Directives

### Bugfix checklist

- httpd changelog
- Known issues
- Report a bug

## 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.

## 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.

## 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.

## 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

- 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

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.

## 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 `Listen`ers 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).

## 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.

## 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.

## 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.

## 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:

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

### See also

- `MinSpareThreads`
- `StartServers`
- `MaxSpareServers`

## 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

- `MaxSpareThreads`
- `StartServers`
- `MinSpareServers`

## 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 `apachectl` script, or the init script that your OS provides, 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: | `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.

## ScoreBoardFile Directive

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

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.

### Example

    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 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 HTTP Server

## SendBufferSize Directive

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

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 of `0`, 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.

On some operating systems, changes in TCP behavior resulting from a larger `SendBufferSize` may not be seen unless `EnableSendfile` is set to OFF. This interaction applies only to static files.

## ServerLimit Directive

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

For the `prefork` MPM, this directive sets the maximum configured value for `MaxRequestWorkers` for the lifetime of the Apache httpd process. For the `worker` and `event` MPMs, this directive in combination with `ThreadLimit` sets the maximum configured value for `MaxRequestWorkers` for the lifetime of the Apache httpd process. For the `event` 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, but `MaxRequestWorkers` 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 `MaxRequestWorkers` are set to values higher than the system can handle, Apache httpd may not start or the system may become unstable.

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

With `worker`, use this directive only if your `MaxRequestWorkers` 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 `MaxRequestWorkers` and `ThreadsPerChild`.

With `event`, increase this directive if the process number defined by your `MaxRequestWorkers` and `ThreadsPerChild` settings, plus the number of gracefully shutting down processes, is more than 16 server processes (default).

### 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. To increase it even further past this limit, you will need to modify the value of MAX_SERVER_LIMIT in the mpm source file and rebuild the server.

### See also

- Stopping and Restarting Apache HTTP Server

## 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: | `event`, `worker`, `prefork`, `mpmt_os2`

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, (see `MinSpareThreads`, `MaxSpareThreads`, `MinSpareServers`, `MaxSpareServers`) there is usually little reason to adjust this parameter.

The default value differs from MPM to MPM. `worker` and `event` default to `StartServers 3`; `prefork` defaults to `5`; `mpmt_os2` defaults 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: | `mpm_netware`

Number of threads created on startup. As the number of threads is dynamically controlled depending on the load, (see `MinSpareThreads`, `MaxSpareThreads`, `MinSpareServers`, `MaxSpareServers`) 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.

## 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: | `event`, `worker`, `mpm_winnt`

This directive sets the maximum configured value for `ThreadsPerChild` for the lifetime of the Apache httpd 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 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 of `ThreadsPerChild` for the current run of Apache httpd.

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 100000` with `event`, `ThreadLimit 15000` with `mpm_winnt`) compiled into the server. This is intended to avoid nasty effects caused by typos. To increase it even further past this limit, you will need to modify the value of MAX_THREAD_LIMIT in the mpm source file and rebuild the server.

## 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: | `event`, `worker`, `mpm_winnt`

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: | `event`, `worker`, `mpm_winnt`, `mpm_netware`, `mpmt_os2`
Compatibility: | Available in Apache HTTP Server 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 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 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 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 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. The minimum required stack size strongly depends on the modules used, but any change in the web server configuration can invalidate the current `ThreadStackSize` setting.
- On Linux, this directive can only be used to increase the default stack size, as the underlying system call uses the value as a minimum stack size. The (often large) soft limit for `ulimit -s` (8MB if unlimited) is used as the default stack size.

It is recommended to not reduce `ThreadStackSize` unless a high number of threads per child process is needed. On some platforms (including Linux), a setting of 128000 is already too low and causes crashes with some common modules.