CoreDumpDirectory Directive

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.

EnableExceptionHook Directive

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

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

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

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

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

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).

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

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 a process can consume
by (accidental) memory leakage.

MaxMemFree Directive

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

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

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.

See also

• MinSpareThreads
• StartServers
• MaxSpareServers

MinSpareThreads Directive

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 basis.

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

See also

• MaxSpareThreads
• StartServers
• MinSpareServers

PidFile Directive

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.

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.

ReceiveBufferSize Directive

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

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 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

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.

ServerLimit Directive

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).

See also

• Stopping and Restarting Apache HTTP Server

StartServers Directive

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

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

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.

ThreadsPerChild Directive

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.

The value of ThreadsPerChild can not exceed the
value of ThreadLimit. 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 in ThreadLimit.

ThreadStackSize Directive

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 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 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.