Apache2::SizeLimit - Because size does matter.


This module allows you to kill off Apache httpd processes if they grow too large. You can choose to set up the process size limiter to check the process size on every request:

  # in your, or a <Perl> section:
  use Apache2::SizeLimit;
  # sizes are in KB
  $Apache2::SizeLimit::MAX_PROCESS_SIZE  = 12000; # 12MB
  $Apache2::SizeLimit::MIN_SHARE_SIZE    = 6000;  # 6MB
  $Apache2::SizeLimit::MAX_UNSHARED_SIZE = 5000;  # 5MB
  # in your httpd.conf:
  PerlCleanupHandler Apache2::SizeLimit

Or you can just check those requests that are likely to get big, such as CGI requests. This way of checking is also easier for those who are mostly just running CGI scripts under ModPerl::Registry|docs::2.0::api::ModPerl::Registry:

  # in your script:
  use Apache2::SizeLimit;
  # sizes are in KB

This will work in places where you are using SetHandler perl-script|docs::2.0::user::config::config/C_perl_script_> or anywhere you enable PerlOptions +GlobalRequest|docs::2.0::user::config::config/C_GlobalRequest_>. If you want to avoid turning on GlobalRequest, you can pass an Apache2::RequestRec|docs::2.0::api::Apache2::RequestRec object as the second argument in these subs:

  my $r = shift; # if you don't have $r already
  Apache2::SizeLimit::setmax(12000, $r);
  Apache2::SizeLimit::setmin(6000, $r);
  Apache2::SizeLimit::setmax_unshared(5000, $r);

Since checking the process size can take a few system calls on some platforms (e.g. linux), you may want to only check the process size every N times. To do so, put this in your or CGI:

  $Apache2::SizeLimit::CHECK_EVERY_N_REQUESTS = 2;

This will only check the process size every other time the process size checker is called.


This module is highly platform dependent, please read the Caveats section. It also does not work under threaded MPMs.

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, it could have a bug that causes the process to increase in size dramatically, until your system starts swapping. Second, it may just do things that requires a lot of memory, and the more different kinds of requests your server handles, the larger the httpd processes grow over time.

This module will not really help you with the first problem. For that you should probably look into Apache2::Resource|docs::2.0::api::Apache2::Resource or some other means of setting a limit on the data size of your program. BSD-ish systems have setrlimit() which will croak 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. The idea is to check the memory usage after every request, and if it exceeds a threshold, exit gracefully.

By using this module, you should be able to discontinue using the Apache configuration directive MaxRequestsPerChild, although you can use both if you are feeling paranoid. Most users use the technique shown in this module and set their MaxRequestsPerChild value to 0.

Shared Memory Options

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 extensive documentation 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 platform-dependent, since finding the size of a process is pretty different from OS to 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, please do.

Supported OSes


For linux we read the process size out of /proc/self/statm. This seems to be fast enough on modern systems. If you are worried about performance, try setting the CHECK_EVERY_N_REQUESTS option.

Since linux 2.6 /proc/self/statm does not report the amount of memory shared by the copy-on-write mechanism as shared memory. Hence decisions made on the basis of MAX_UNSHARED_SIZE or MIN_SHARE_SIZE are inherently wrong.

To correct the situation there is a patch to the linux kernel that adds a /proc/self/smaps entry for each process. At the time of this writing the patch is included in the mm-tree (linux-2.6.13-rc4-mm1) and is expected to make it into the vanilla kernel in the near future.

/proc/self/smaps reports various sizes for each memory segment of a process and allows to count the amount of shared memory correctly.

If Apache2::SizeLimit detects a kernel that supports /proc/self/smaps and if the Linux::Smaps module is installed it will use them instead of /proc/self/statm. You can prevent Apache2::SizeLimit from using /proc/self/smaps and turn on the old behaviour by setting $Apache2::SizeLimit::USE_SMAPS to 0 before the first check.

Apache2::SizeLimit also resets $Apache2::SizeLimit::USE_SMAPS to 0 if it somehow decides not to use /proc/self/smaps. Thus, you can check it to determine what is actually used.

NOTE: 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. Hence, you are encouraged to set the CHECK_EVERY_N_REQUESTS option.

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

    require Apache2::SizeLimit;
    package X;
    use strict;
    use Apache2::RequestRec ();
    use Apache2::RequestIO ();
    use Apache2::Const -compile=>qw(OK);
    my $x= "a" x (1024*1024);
    sub handler {
      my $r = shift;
      my ($size, $shared) = $Apache2::SizeLimit::HOW_BIG_IS_IT->();
      $x =~ tr/a/b/;
      my ($size2, $shared2) = $Apache2::SizeLimit::HOW_BIG_IS_IT->();
      $r->print("1: size=$size shared=$shared\n");
      $r->print("2: size=$size2 shared=$shared2\n");
      return Apache2::Const::OK;
  <Location /X>
    SetHandler modperl
    PerlResponseHandler X

The parent apache allocates a megabyte 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?


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


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


Under mod_perl 1, SizeLimit provided basic functionality by using Win32::API to access process memory information. This worked because there was only one mod_perl thread. With mod_perl 2, Win32 runs a true threaded MPM, which unfortunately means that we can't tell the size of each interpreter. Win32 support is disabled until a solution for this can be found.

If your platform is not supported, and if you can tell us how to check for the size of a process under your OS (in KB), then we will add it to the list. The more portable/efficient the solution, the better, of course.

Supported MPMs

At this time, Apache2::SizeLimit does not support use under threaded MPMs. This is because there is no efficient way to get the memory usage of a thread, or make a thread exit cleanly. Suggestions and patches are welcome on the mod_perl dev mailing list.


mod_perl 2.0 and its core modules are copyrighted under The Apache Software License, Version 2.0.


Doug Bagley <doug+modperl>, channeling Procrustes.

Brian Moseley <ix>: Solaris 2.6 support

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

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

Torsten Foertsch <torsten.foertsch>: Linux::Smaps support