If you are unsure which MPM is built into Apache, it can be determined by running the Apache web server executable with the '-V' option. The output from running the web server executable with this option will be information about how it was configured when built.
Server version: Apache/2.2.1
Server built: Mar 4 2007 20:48:15
Server's Module Magic Number: 20051115:1
Server loaded: APR 1.2.6, APR-Util 1.2.6
Compiled using: APR 1.2.6, APR-Util 1.2.6
Architecture: 32-bit
Server MPM: Worker
threaded: yes (fixed thread count)
forked: yes (variable process count)
Server compiled with....
-D APACHE_MPM_DIR="server/mpm/worker"
-D APR_HAS_MMAP
-D APR_HAVE_IPV6 (IPv4-mapped addresses enabled)
-D APR_USE_SYSVSEM_SERIALIZE
-D APR_USE_PTHREAD_SERIALIZE
-D SINGLE_LISTEN_UNSERIALIZED_ACCEPT
-D APR_HAS_OTHER_CHILD
-D AP_HAVE_RELIABLE_PIPED_LOGS
-D DYNAMIC_MODULE_LIMIT=128
-D HTTPD_ROOT="/usr/local/apache-2.2"
-D SUEXEC_BIN="/usr/local/apache-2.2/bin/suexec"
-D DEFAULT_SCOREBOARD="logs/apache_runtime_status"
-D DEFAULT_ERRORLOG="logs/error_log"
-D AP_TYPES_CONFIG_FILE="conf/mime.types"
-D SERVER_CONFIG_FILE="conf/httpd.conf"
Which MPM is being used can be determined from the 'Server MPM' field.
On the Windows platform the only available MPM is 'winnt'.
The UNIX 'prefork' MPM¶
This MPM is the most commonly used. It was the only mode of operation available in Apache 1.3 and is still the default mode on UNIX systems in later versions of Apache. In this configuration, the main Apache process will at startup create multiple child processes. When a request is received by the parent process, it will be processed by which ever of the child processes is ready.
Each child process will only handle one request at a time. If another request arrives at the same time, it will be handled by the next available child process. When it is detected that the number of available processes is running out, additional child processes will be created as necessary. If a limit is specified as to the number of child processes which may be created and the limit is reached, plus there are sufficient requests arriving to fill up the listener socket queue, the client may instead receive an error resulting from not being able to establish a connection with the web server.
Where additional child processes have to be created due to a peak in the number of current requests arriving and where the number of requests has subsequently dropped off, the excess child processes may be shutdown and killed off. Child processes may also be shutdown and killed off after they have handled some set number of requests.
Although threads are not used to service individual requests, this does not preclude an application from creating separate threads to perform some specific task.
For the typical 'prefork' configuration where multiple processes are used, the WSGI environment key/value pairs indicating how processes and threads are being used will be as follows.
| wsgi.multithread | False |
| wsgi.multiprocess | True |
Because multiple processes are being used, a WSGI middleware component such as the interactive browser based debugger described would not be able to be used. If during development and testing of a WSGI application, use of such a debugger was required, the only option which would exist would be to limit the number of processes being used. This could be achieved using the Apache configuration:
StartServers 1
ServerLimit 1
With this configuration, only one process will be started, with no additional processes ever being created. The WSGI environment key/value pairs indicating how processes and threads are being used will for this configuration be as follows.
| wsgi.multithread | False |
| wsgi.multiprocess | False |
In effect, this configuration has the result of serialising all requests through a single process. This will allow an interactive browser based debugger to be used, but may prevent more complex WSGI applications which make use of AJAX techniques from working. This could occur where a web page initiates a sequence of AJAX requests and expects later requests to be able to complete while a response for an initial request is still pending. In other words, problems may occur where requests overlap, as subsequent requests will not be able to be executed until the initial request has completed.
The UNIX 'worker' MPM¶
The 'worker' MPM is similar to 'prefork' mode except that within each child process there will exist a number of worker threads. Instead of a request only being able to be processed by the next available idle child process and with the handling of the request being the only thing the child process is then doing, the request may be processed by a worker thread within a child process which already has other worker threads handling other requests at the same time.
It is possible that a WSGI application could be executed at the same time from multiple worker threads within the one child process. This means that multiple worker threads may want to access common shared data at the same time. As a consequence, such common shared data must be protected in a way that will allow access and modification in a thread safe manner. Normally this would necessitate the use of some form of synchronisation mechanism to ensure that only one thread at a time accesses and or modifies the common shared data.
If all worker threads within a child process were busy when a new request arrives the request would be processed by an idle worker thread in another child process. Apache may still create new child processes on demand if necessary. Apache may also still shutdown and kill off excess child processes, or child processes that have handled more than a set number of requests.
Overall, use of 'worker' MPM will result in less child processes needing to be created, but resource usage of individual child processes will be greater. On modern computer systems, the 'worker' MPM would in general be the prefered MPM to use and should if possible be used in preference to the 'prefork' MPM.
Although contention for the global interpreter lock (GIL) in Python can causes issues for pure Python programs, it is not generally as big an issue when using Python within Apache. This is because all the underlying infrastructure for accepting requests and mapping the URL to a WSGI application, as well as the handling of requests against static files are all performed by Apache in C code. While this code is being executed the thread will not be holding the Python GIL, thus allowing a greater level of overlapping execution where a system has multiple CPUs or CPUs with multiple cores.
This ability to make good use of more than processor, even when using multithreading, is further enchanced by the fact that Apache uses multiple processes for handling requests and not just a single process. Thus, even when there is some contention for the GIL within a specific process, it doesn't stop other processes from being able to run as the GIL is only local to a process and does not extend across processes.
For the typical 'worker' configuration where multiple processes and multiple threads are used, the WSGI environment key/value pairs indicating how processes and threads are being used will be as follows.
| wsgi.multithread | True |
| wsgi.multiprocess | True |
Similar to the 'prefork' MPM, the number of processes can be restricted to just one if required using the configuration:
StartServers 1
ServerLimit 1
With this configuration, only one process will be started, with no additional processes ever being created, but that one process would still make use of multiple threads.
The WSGI environment key/value pairs indicating how processes and threads are being used will for this configuration be as follows.
| wsgi.multithread | True |
| wsgi.multiprocess | False |
Because multiple threads are being used, there would be no problem with overlapping requests generated by an AJAX based web page.
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