Apache::SizeLimit - Because size does matter.


     Apache::SizeLimit->set_max_process_size(150_000);   # Max size in KB
     Apache::SizeLimit->set_min_shared_size(10_000);     # Min share in KB
     Apache::SizeLimit->set_max_unshared_size(120_000);  # Max unshared size in KB
    PerlCleanupHandler Apache::SizeLimit


******************************** NOIICE *******************

    This version is only for httpd 1.x and mod_perl 1.x 
    Future versions of this module may support both.
    Currently, Apache2::SizeLimit is bundled with 
    mod_perl 2.x for that series.
******************************** NOTICE *******************

This module allows you to kill off Apache httpd processes if they grow too large. You can make the decision to kill a process based on its overall size, by setting a minimum limit on shared memory, or a maximum on unshared memory.

You can set limits for each of these sizes, and if any limit is exceeded, the process will be killed.

You can also limit the frequency that these sizes are checked so that this module only checks every N requests.

This module is highly platform dependent, please read the PER-PLATFORM BEHAVIOR section for details. It is possible that this module simply does not support your platform.


You can set set the size limits from a Perl module or script loaded by Apache by calling the appropriate class method on Apache::SizeLimit:

The two methods related to shared memory size are effectively a no-op if the module cannot determine the shared memory size for your platform. See PER-PLATFORM BEHAVIOR for more details.

Running the handler()

There are several ways to make this module actually run the code to kill a process.

The simplest is to make Apache::SizeLimit a PerlCleanupHandler in your Apache config:

    PerlCleanupHandler Apache::SizeLimit

This will ensure that Apache::SizeLimit->handler() is run for all requests.

If you want to combine this module with a cleanup handler of your own, make sure that Apache::SizeLimit is the last handler run:

    PerlCleanupHandler  Apache::SizeLimit My::CleanupHandler

Remember, mod_perl will run stacked handlers from right to left, as they're defined in your configuration.

If you have some cleanup code you need to run, but stacked handlers aren't appropriate for your setup, you can also explicitly call the Apache::SizeLimit->handler() function from your own cleanup handler:

    package My::CleanupHandler
    sub handler {
        my $r = shift;
        # Causes File::Temp to remove any temp dirs created during the
        # request
        return Apache::SizeLimit->handler($r);

Checking Every N Requests

Since checking the process size can take a few system calls on some platforms (e.g. linux), you may not want to check the process size for every request.


In addition to simply checking the total size of a process, this module can factor in how much of the memory used by the process is actually being shared by copy-on-write. If you don't understand how memory is shared in this way, take a look at the mod_perl docs at

You can take advantage of the shared memory information by setting a minimum shared size and/or a maximum unshared size. Experience on one heavily trafficked mod_perl site showed that setting maximum unshared size and leaving the others unset is the most effective policy. This is because it only kills off processes that are truly using too much physical RAM, allowing most processes to live longer and reducing the process churn rate.


This module is highly platform dependent, since finding the size of a process is different for each OS, and some platforms may not be supported. In particular, the limits on minimum shared memory and maximum shared memory are currently only supported on Linux and BSD. If you can contribute support for another OS, patches are very welcome.

Currently supported OSes:


For linux we read the process size out of /proc/self/statm. If you are worried about performance, you can consider using < Apache::SizeLimit-set_check_interval() >> to reduce how often this read happens.

As of linux 2.6, /proc/self/statm does not report the amount of memory shared by the copy-on-write mechanism as shared memory. This means that decisions made based on shared memory as reported by that interface are inherently wrong.

However, as of the 2.6.14 release of the kernel, there is /proc/self/smaps entry for each process. /proc/self/smaps reports various sizes for each memory segment of a process and allows us to count the amount of shared memory correctly.

If Apache::SizeLimit detects a kernel that supports /proc/self/smaps and the Linux::Smaps module is installed it will use that module instead of /proc/self/statm.

Reading /proc/self/smaps is expensive compared to /proc/self/statm. It must look at each page table entry of a process. Further, on multiprocessor systems the access is synchronized with spinlocks. Again, you might consider using < Apache::SizeLimit-set_check_interval() >>.

Copy-on-write and Shared Memory

The following example shows the effect of copy-on-write:

    require Apache::SizeLimit;
    package X;
    use strict;
    use Apache::Constants qw(OK);
    my $x = "a" x (1024*1024);
    sub handler {
      my $r = shift;
      my ($size, $shared) = $Apache::SizeLimit->_check_size();
      $x =~ tr/a/b/;
      my ($size2, $shared2) = $Apache::SizeLimit->_check_size();
      $r->print("1: size=$size shared=$shared\n");
      $r->print("2: size=$size2 shared=$shared2\n");
      return OK;
  <Location /X>
    SetHandler modperl
    PerlResponseHandler X

The parent Apache process allocates memory for the string in $x. The tr-command then overwrites all ``a'' with ``b'' if the handler is called with an argument. This write is done in place, thus, the process size doesn't change. Only $x is not shared anymore by means of copy-on-write between the parent and the child.

If /proc/self/smaps is available curl shows:

  r2@s93:~/work/mp2> curl http://localhost:8181/X
  1: size=13452 shared=7456
  2: size=13452 shared=6432

Shared memory has lost 1024 kB. The process' overall size remains unchanged.

Without /proc/self/smaps it says:

  r2@s93:~/work/mp2> curl http://localhost:8181/X
  1: size=13052 shared=3628
  2: size=13052 shared=3636

One can see the kernel lies about the shared memory. It simply doesn't count copy-on-write pages as shared.

solaris 2.6 and above

For solaris we simply retrieve the size of /proc/self/as, which contains the address-space image of the process, and convert to KB. Shared memory calculations are not supported.

NOTE: This is only known to work for solaris 2.6 and above. Evidently the /proc filesystem has changed between 2.5.1 and 2.6. Can anyone confirm or deny?

BSD (and OSX)

Uses BSD::Resource::getrusage() to determine process size. This is pretty efficient (a lot more efficient than reading it from the /proc fs anyway).

According to recent tests on OSX (July, 2006), BSD::Resource simply reports zero for process and shared size on that platform, so OSX is not supported by Apache::SizeLimit.


Uses BSD::Resource::getrusage() to determine process size. Not sure if the shared memory calculations will work or not. AIX users?


Uses Win32::API to access process memory information. Win32::API can be installed under ActiveState perl using the supplied ppm utility.

Everything Else

If your platform is not supported, then please send a patch to check the process size. The more portable/efficient/correct the solution the better, of course.


This module was written in response to questions on the mod_perl mailing list on how to tell the httpd process to exit if it gets too big.

Actually, there are two big reasons your httpd children will grow. First, your code could have a bug that causes the process to increase in size very quickly. Second, you could just be doing operations that require a lot of memory for each request. Since Perl does not give memory back to the system after using it, the process size can grow quite large.

This module will not really help you with the first problem. For that you should probably look into Apache::Resource or some other means of setting a limit on the data size of your program. BSD-ish systems have setrlimit(), which will kill your memory gobbling processes. However, it is a little violent, terminating your process in mid-request.

This module attempts to solve the second situation, where your process slowly grows over time. It checks memory usage after every request, and if it exceeds a threshold, exits gracefully.

By using this module, you should be able to discontinue using the Apache configuration directive MaxRequestsPerChild, although for some folks, using both in combination does the job.


Previous versions of this module documented three globals for defining memory size limits:

Direct use of these globals is deprecated, but will continue to work for the foreseeable future.

It also documented three functions for use from registry scripts:

Besides setting the appropriate limit, these functions also add a cleanup handler to the current request.


Doug Bagley <>, channeling Procrustes.

Brian Moseley <>: Solaris 2.6 support

Doug Steinwand and Perrin Harkins <>: added support for shared memory and additional diagnostic info

Matt Phillips <> and Mohamed Hendawi <>: Win32 support

Dave Rolsky <>, maintenance and fixes outside of mod_perl tree (0.9+).