Practical mod_perl

Chapter 10. Improving Performance with Shared Memory and Proper Forking

In this chapter we will talk about two issues that play an important role in optimizing server performance: sharing memory and forking.

Firstly, mod_perl Apache processes can become quite large, and it is therefore very important to make sure that the memory used by the Apache processes is shared between them as much as possible.

Secondly, if you need the Apache processes to fork new processes, it is important to perform the fork( ) calls in the proper way.

Sharing Memory

The sharing of memory is a very important factor. If your OS supports it (and most sane systems do), a lot of memory can be saved by sharing it between child processes. This is possible only when code is preloaded at server startup. However, during a child process’s life, its memory pages tend to become unshared. Here is why.

There is no way to make Perl allocate memory so that (dynamic) variables land on different memory pages from constants or the rest of your code (which is really just data to the Perl interpreter), so the copy-on-write effect (explained in a moment) will hit almost at random.

If many modules are preloaded, you can trade off the memory that stays shared against the time for an occasional fork of a new Apache child by tuning the MaxRequestsPerChild Apache directive. Each time a child reaches this upper limit and dies, it will release its unshared pages. The new child will have to be forked, but it will share its fresh pages ...