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Apache HTTP Server Version 2.4



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

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

Bugfix checklisthttpd changelogKnown issuesReport a bugSee also

Comments


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.



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.

    ExamplePidFile /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.

    ExampleScoreBoardFile /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.




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