Apache 2.x is a web server, designed to provide a balance of flexibility, portability, and performance, However there are compile-time and run-time configuration choices that can significantly affect performance. The server administrator can configure to tune the performance of an Apache 2.x installation. Some of these configuration options enable the httpd to better take advantage of the capabilities of the hardware and OS, while others allow the administrator to trade functionality for speed.
The single biggest hardware issue affecting web server performance is RAM. A web server should never ever have to swap, as swapping increases the latency of each request beyond a point that users consider “fast enough”. This causes users to hit stop and reload, further increasing the load. You can, and should, control the MaxClients setting so that your server does not spawn so many children it starts swapping.
Prior to Apache 1.3, HostnameLookups defaulted to On. This adds latency to every request because it requires a DNS lookup to complete before the request is finished. In Apache 1.3 this setting defaults to Off. If you need to have addresses in your log files resolved to hostnames, use the logresolve program that comes with Apache, or one of the numerous log reporting packages which are available. It is recommended you keep it off
ex. HostnameLookups Off
The MinSpareServers directive sets the desired minimum number of idle child server processes. An idle process is one which is not handling a request. If there are fewer than MinSpareServers idle, then the parent process creates new children at a maximum rate of 1 per second. Tuning of this parameter should only be necessary on very busy sites. Setting this parameter to a large number is almost always a bad idea.
ex. MinSpareServers 5
The MaxSpareServers directive sets the desired maximum number of idle child server processes. An idle process is one which is not handling a request. If there are more than MaxSpareServers idle, then the parent process will kill off the excess processes. Tuning of this parameter should only be necessary on very busy sites. Setting this parameter to a large number is almost always a bad idea. If you are trying to set the value lower than MinSpareServers, Apache will automatically adjust it to MinSpareServers + 1.
ex. MaxSpareServers 10
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
ex. StartServers 3
The MaxClients directive sets the limit on the number of simultaneous requests that will be served. Any connection attempts over the MaxClients 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), MaxClients 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. Worker) MaxClients restricts the total number of threads that will be available to serve clients. The default value for beos is 50. 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.
ex. Maxclients 250
This directive sets the maximum configured value for MaxClients for the lifetime of the Apache process, 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 MaxClients are set to values higher than the system can handle, Apache may not start or the system may become unstable.
Use this directive only if you need to set MaxClients higher than 256 (default). Do not set the value of this directive any higher than what you might want to set MaxClients to.
For ex. ServerLimit 500 i.e MaxClients should be similar or lower then the serverlimit value.
A good example of Apache optimization on a 4GB multithreaded server. Add the following parameters in the httpd.conf file.
Apache 2.x supports pluggable concurrency models, called Multi-Processing Modules (MPMs). When building Apache, you must choose an MPM to use. There are platform-specific MPMs for some platforms: beos, mpm_netware, mpmt_os2, and mpm_winnt. For general Unix-type systems, there are several MPMs from which to choose. The choice of MPM can affect the speed and scalability of the httpd:
The worker MPM uses multiple child processes with many threads each. Each thread handles one connection at a time. Worker generally is a good choice for high-traffic servers because it has a smaller memory footprint than the prefork MPM.
The prefork MPM uses multiple child processes with one thread each. Each process handles one connection at a time. On many systems, prefork is comparable in speed to worker, but it uses more memory. Prefork’s threadless design has advantages over worker in some situations: it can be used with non-thread-safe third-party modules, and it is easier to debug on platforms with poor thread debugging support.