Assuming you have enough RAM to serve the requests, and you are using the prefork multi-processing module, you can use the 2 directives MaxClients and ServerLimit to set the total number of simultaneous requests that will be served;
To check for prefork run apachectl -V | grep MPM and look for the following output;
# apachectl -V | grep MPM
Server MPM: Prefork
-D APACHE_MPM_DIR="server/mpm/prefork"
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.
But the ServerLimit also puts a maximum limit on the number of processes that apache will spawn to serve requests.
For the prefork MPM, the ServerLimit directive sets the maximum configured value
for MaxClients for the lifetime of the Apache process..
hence I would go for something like;
ServerLimit 1000
MaxClients 1000
Alternatively for For the worker MPM, this directive in combination with ThreadLimit sets the maximum configured value for MaxClients for the lifetime of the Apache process. Any attempts to change this directive during a restart will be ignored workers ThreadLimit docs here
There are a number of MPM modules (Multi-Processing Modules), but by far the most widely used (at least on *nix platforms) are the three main ones: prefork, worker, and event. Essentially, they represent the evolution of the Apache web server, and the different ways that the server has been built to handle HTTP requests within the computing constraints of the time over its long (in software terms) history.
mpm_prefork is.. well.. it's compatible with everything. It spins off a number of child processes for serving requests, and the child processes only serve one request at a time. Because it's got the server process sitting there, ready for action, and not needing to deal with thread marshaling, it's actually faster than the more modern threaded MPMs when you're only dealing with a single request at a time - but concurrent requests suffer, since they're made to wait in line until a server process is free. Additionally, attempting to scale up in the count of prefork child processes, you'll easily suck down some serious RAM.
It's probably not advisable to use prefork unless you need a module that's not thread safe.
Use if: You need modules that break when threads are used, like mod_php. Even then, consider using FastCGI and php-fpm.
Don't use if: Your modules won't break in threading.
mpm_worker uses threading - which is a big help for concurrency. Worker spins off some child processes, which in turn spin off child threads; similar to prefork, some spare threads are kept ready if possible, to service incoming connections. This approach is much kinder on RAM, since the thread count doesn't have a direct bearing on memory use like the server count does in prefork. It also handles concurrency much more easily, since the connections just need to wait for a free thread (which is usually available) instead of a spare server in prefork.
Use if: You're on Apache 2.2, or 2.4 and you're running primarily SSL.
Don't use if: You really can't go wrong, unless you need prefork for compatibility.
However, note that the treads are attached to connections and not requests - which means that a keep-alive connection always keeps a hold of a thread until it's closed (which can be a long time, depending on your configuration). Which is why we have..
mpm_event is very similar to worker, structurally; it's just been moved from 'experimental' to 'stable' status in Apache 2.4. The big difference is that it uses a dedicated thread to deal with the kept-alive connections, and hands requests down to child threads only when a request has actually been made (allowing those threads to free back up immediately after the request is completed). This is great for concurrency of clients that aren't necessarily all active at a time, but make occasional requests, and when the clients might have a long keep-alive timeout.
The exception here is with SSL connections; in that case, it behaves identically to worker (gluing a given connection to a given thread until the connection closes).
Use if: You're on Apache 2.4 and like threads, but you don't like having threads waiting for idle connections. Everyone likes threads!
Don't use if: You're not on Apache 2.4, or you need prefork for compatibility.
In today's world of slowloris, AJAX, and browsers that like to multiplex 6 TCP connections (with keep-alive, of course) to your server, concurrency is an important factor in making your server scale and scale well. Apache's history has tied it down in this regard, and while it's really still not up to par with the likes of nginx or lighttpd in terms of resource usage or scale, it's clear that the development team is working toward building a web server that's still relevant in today's high-request-concurrency world.
Best Answer
MPMs are configured in to your apache server at compile time. So to use a different MPM you would have to recompile apache, or install a different pre compiled binary with the desired MPM (if it is available for your OS). However for most purposes the
preforkMPM is just fine, especially if you're going to run PHP. PHP is not considered thread safe, so you shouldn't useworker, andeventis experimental...So stay with
preforkand the only parameters of relevance to you are the one in the<IfModule mpm_prefork_module>section. What the best values are for you mainly depends on how many concurrent users you expect, and how that varies. I would start with just leaving the parameters as they are, and only start performance tuning if you really run in to performance issues. Use the status page to look at how many clients typically connect to your server. If you do end up serving more than 150 concurrent users increas MaxClients.