Philippe M. Chiasson > mod_perl-1.29 > Apache::SizeLimit



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Module Version: 0.03   Source   Latest Release: mod_perl-1.31


Apache::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
    use Apache::SizeLimit;
    # sizes are in KB
    $Apache::SizeLimit::MAX_PROCESS_SIZE  = 10000; # 10MB
    $Apache::SizeLimit::MIN_SHARE_SIZE    = 1000;  # 1MB
    $Apache::SizeLimit::MAX_UNSHARED_SIZE = 12000; # 12MB

    # in your httpd.conf:
    PerlCleanupHandler Apache::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 scripts:

    # in your CGI:
    use Apache::SizeLimit;
    &Apache::SizeLimit::setmax(10000);          # Max size in KB
    &Apache::SizeLimit::setmin(1000);           # Min share in KB
    &Apache::SizeLimit::setmax_unshared(12000); # Max unshared size in KB

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:

    $Apache::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.

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, your process just does stuff 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 Apache::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 for some folks, using both in combination does the job. Personally, I just use the technique shown in this module and set my MaxRequestsPerChild value to 6000.


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 Guide 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.

Currently supported OSes:


For linux we read the process size out of /proc/self/status. This is a little slow, but usually not too bad. If you are worried about performance, try only setting up the the exit handler inside CGIs (with the setmax function), and see if the CHECK_EVERY_N_REQUESTS option is of benefit.

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?


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

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


Possibly provide a perl make/install so that the is created at build time with only the code you need on your platform.

If Apache was started in non-forking mode, should hitting the size limit cause the process to exit?


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

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