Under certain Linux setups, malloc( ) uses mmap( ) instead of brk( ). This is done to conserve virtual memory—that is, when a program malloc( )s a large block of memory, the block is not actually returned to the program until it is initialized. The old-style brk( )system call obeyed resource limits on data segment sizes as set in setrlimit( ). mmap( ) does not.

Apache::Resource's defaults put limits on data size and stack size. Linux's current memory-allocation scheme does not honor these limits, so if we just do:

PerlSetEnv PERL_RLIMIT_DEFAULTS On
PerlModule Apache::Resource
PerlChildInitHandler Apache::Resource

our Apache processes are still free to use as much memory as they like.

However, BSD::Resource also has a limit called RLIMIT_AS (Address Space), which limits the total number of bytes of virtual memory assigned to a process. Fortunately, Linux's memory manager does honor this limit.

Therefore, we can limit memory usage under Linux with Apache::Resource. Simply add a line to httpd.conf:

PerlSetEnv PERL_RLIMIT_AS  67108864

This example sets hard and soft limits of 64 MB of total address space.

Refer to the Apache::Resource and setrlimit(2) manpages for more information.