This Multi-Processing Module (MPM) implements a hybrid multi-process multi-threaded server. By using threads to serve requests, it is able to serve a large number of requests with fewer system resources than a process-based server. However, it retains much of the stability of a process-based server by keeping multiple processes available, each with many threads.
The most important directives used to control this MPM are ThreadsPerChild, which controls the number of threads deployed by each child process and MaxClients, which controls the maximum total number of threads that may be launched.
Maximum number of connections that will be processed simultaneously
MaxClients 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 MaxClients to a value that requires more than 16 processes, you must also raise ServerLimit.
Limit on the number of requests that an individual child server will handle during its life
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.
Maximum number of idle threads
For worker, the default is MaxSpareThreads 250 (75 in Gentoo). 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.
The value must be greater or equal than the sum of MinSpareThreads and ThreadsPerChild.
See also MinSpareThreads and StartServers.
Minimum number of idle threads available to handle request spikes
Minimum number of idle threads to handle request spikes.
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.
See also MaxSpareThreads and StartServers
Upper limit on configurable number of processes
For the worker MPM, this directive in combination with ThreadLimit sets the maximum configured value for MaxClients for the lifetime of the Apache process.
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.
Number of child server processes created at startup
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.
Number of threads created on startup
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.
Sets the upper limit on the configurable number of threads per child process
This directive sets the maximum configured value for ThreadsPerChild for the lifetime of the Apache process.
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.
Number of threads created by each child process
This directive sets the number of threads created by each child process. The child creates these threads at startup and never creates more.
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.