Stas Bekman


Apache::GTopLimit - Limit Apache httpd processes


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

    # in your
    # ___________________

    use Apache::GTopLimit;

    # Control the life based on memory size
    # in KB, so this is ~10MB
    $Apache::GTopLimit::MAX_PROCESS_SIZE = 10000; 

    # Control the life based on Shared memory size
    # in KB, so this is ~4MB 
    $Apache::GTopLimit::MIN_PROCESS_SHARED_SIZE = 4000;

    # Control the life based on UnShared memory size
    # in KB, so this is ~6MB 
    $Apache::GTopLimit::MAX_PROCESS_UNSHARED_SIZE = 6000;

    # in your httpd.conf:
    # ___________________

    # debug mode must be set before the module is loaded
    PerlSetVar Apache::GTopLimit::DEBUG 1

    # register handler
    PerlFixupHandler Apache::GTopLimit
    # you can set this up as any Perl*Handler that handles 
    # part of the request, even the LogHandler will do.

Or you can just check those requests that are likely to get big or unshared. This way of checking is also easier for those who are mostly just running Apache::Registry scripts:

    # in your handler/CGI script
    use Apache::GTopLimit;
      # Max Process Size in KB


    use Apache::GTopLimit;
       # Min Shared process Size in KB


    use Apache::GTopLimit;
       # Min UnShared process Size in KB

Since accessing the process info might add a little overhead, you may want to only check the process size every N times. To do so, put this in your or CGI:

    $Apache::GTopLimit::CHECK_EVERY_N_REQUESTS = 2;

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

Note: The MAX_PROCESS_SIZE, MIN_PROCESS_SHARED_SIZE and MAX_PROCESS_UNSHARED_SIZE are independent, and each will be checked if only set. So if you set the first two -- the process can be killed if it grows beyond the limit or its shared memory goes below the limit. It's better not to mix MAX_PROCESS_UNSHARED_SIZE with the first two.


This module will run on platforms supported by a Perl interface to libgtop (which in turn needs libgtop : See ).

This module was written in response to questions on the mod_perl mailing list on how to tell the httpd process to exit if:

  • its memory size goes beyond a specified limit

  • its shared memory size goes below a specified limit

  • its unshared memory size goes beyond a specified limit

Limiting memory size

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 (or leaks 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.

Limiting shared memory size

We want the reverse the above limit for a shared memory limitation and kill the process when its hs too little of shared memory.

When the same memory page is being shared between many processes, you need less physical memory relative to the case where the each process will have its own copy of the memory page.

If your OS supports shared memory you will get a great benefit when you deploy this feature. With mod_perl you enable it by preloading the modules at the server startup. When you do that, each child uses the same memory page as the parent does, after it forks. The memory pages get unshared when a child modifies the page and it can no longer be shared, that's when the page is being copied to the child's domain and then modified as it pleased to. When this happens a child uses more real memory and less shared.

Because of Perl's nature, memory pages get unshared pretty fast, when the code is being executed and it's internal data is being modified. That's why as the child gets older the size of the shared memory goes down.

You can tune your server to kill off the child when its shared memory is too low, but it demands a constant retuning of the configuration directives if you do any heavy updates in the code the server executes. This module allows you to save up the time to make this tuning and retuning, by simply specifying the minimum size of the shared memory for each process. And when it crosses the line, to kill it off.

Finally instead of trying to tune the memory size and shared memory thresholds, it's much easier to only specify the amount of unshared memory that can be tolerated and kill the process which has too much of unshared memory.


Stas Bekman <>

An almost complete rewrite of Apache::SizeLimit toward using GTop module (based on crossplatfom glibtop). The moment glibtop will be ported on all the platforms Apache::SizeLimit runs at (I think only Solaris is missing) Apache::SizeLimit will become absolete.

Doug Bagley wrote the original Apache::SizeLimit


See external file 'Changes'.


The Apache::GTopLimit module is free software; you can redistribute it and/or modify it under the same terms as Perl itself.