Handling Growing FastCGI Processes

You've finished writing your Catalyst application! You tested it thoroughly yourself, had some others help you with QA, and even wrote some unit tests. You just launched it into production, and you couldn't be more proud.

But wait... after a few hours of production level traffic, something looks wrong. Those FCGI processes that started out around 40MB just keep getting bigger. Everything was fine at first, but then they grew to 100MB each. Now they're over 200MB each and have thrown the machine into swap! Help! What to do?

If you're running on a Linux system, this article will give you some answers. If you're running on a different UNIX-like system, much of it should be easily adaptable. If you're on Windows... well, at least some of the concepts should apply. I think.

Buy Some Time

First, buy yourself some time. Since the processes start out small and grow over time, you can temporarily calm the chaos by restarting all of your FCGI child processes. Sound drastic? It depends on how desperate your situation already is. If your processes are growing without bound, your server(s) are thrashing in swap, and your web site response rate is grinding to a halt, you need some immediate relief. A simple restart will give you that.

Assuming you are running the standard myapp_fastcgi.pl, you can gracefully restart all of the child processes by sending a HUP signal to the manager (parent) process. Hopefully you started your app with the -p switch and have a pidfile handy. If so, all you'll need to do is this:

 kill -HUP `cat myapp.pid`

Now check your process list again. Things should be back at the nice, sane, small size for another few hours.

The Root of the Evil

So what is causing the memory bloat? Do you have a memory leak? While it's certainly possible, it could be something far less sinister. Before you spend the next several days chipping away at your sanity while hunting for leaks, you should consider the possibility that you are seeing normal copy-on-write behavior. There is an excellent description of this effect in the book Practical mod_perl, and you can read the relevant portions online:


In a nutshell, when the parent FCGI process forks a child process, much of the memory is shared. As the child process continues to run, it writes its own data into memory, which causes more and more of that shared memory to become unshared. Over time, this really starts to add up.

Of course, you could actually have a memory leak as well. The root cause could be a combination of copy-on-write and leaky plumbing. Luckily, the solution for copy-on-write management happens to also buy you time for hunting down a memory leak.

Process Management

Those of you with mod_perl experience may wonder where the FCGI equivalent to MaxRequestsPerChild is. This configuration directive sets a maximum number of requests that each Apache instance should handle before it dies, releasing its memory, and allowing a fresh new Apache instance to be spawned in its place. Using this setting is a common and recommended practice when running mod_perl applications. It would certainly solve our FCGI problem, if we only had such a setting.

You could simply set an upper memory limit with ulimit or the daemontools softlimit command. That way, if one of the children reached the predetermined upper limit, it would die; the FCGI manager would spawn a new child; all would be well. There is only one problem with this approach.

When the child process hits the upper memory limit, it dies a horrible death. The unfortunate web user that process was serving will be cut off in mid-request, left with a broken page, and even worse -- a half-finished action on the backend. At best, this means a bad user experience. At worst, if you aren't using a fully transactional database system, it could mean data corruption.

Faced with this dilemma and growing tired of restarting my application by hand every few hours, I reached for the tools I knew. I wrote my own process reaping cron that would gracefully kill any child processes that had grown too large. Thankfully, FCGI::ProcManager gives us a relatively easy way to do this. If you send a HUP signal to an individual child FCGI process, it will finish handling its current request and then end. The FCGI manager will then spawn a new child to replace it. That is exactly what we want.

The Script

This script will check all of the child FCGI processes to see if any have grown beyond a specified memory size. If they have, it will send them a HUP signal, asking them to end after finishing their current request.

I currently run this every 30 minutes via cron, just to be safe. You never know exactly when a process may cross that line:

 */30 * * * * /path/to/kidreaper `cat /tmp/myapp.pid` 104857600

The script is fairly simple and relies on a ps command that supports the --ppid switch:


 use strict;

 my ($ppid, $bytes) = @ARGV;

 die "Usage: kidreaper PPID ram_limit_in_bytes\n" unless $ppid && $bytes;

 my $kids;

 if (open($kids, "/bin/ps -o pid=,vsz= --ppid $ppid|")) {
    my @goners;

    while (<$kids>) {
       my ($pid, $mem) = split;

       # ps shows KB.  we want bytes.
       $mem *= 1024;

       if ($mem >= $bytes) {
          push @goners, $pid;


    if (@goners) {
       # kill them slowly, so that all connection serving
       # children don't suddenly die at once.

       foreach my $victim (@goners) {
          kill 'HUP', $victim;
          sleep 10;
 else {
    die "Can't get process list: $!\n";

No, the script is not bullet-proof. It could stand to have better error checking, improved option handling, etc. It's a functional starting point, though.

A Better Solution?

Is there a better solution? Probably. I've thought of writing a FCGI::ProcManager subclass that has child memory limit support built in. Maybe someday I will. For now, this kidreaper cron is working nicely enough that I don't urgently need to pursue another solution. I get to focus on other needs, which I assume you'd like to do as well.


If you find that sending a HUP signal to a FCGI child actually causes the whole manager process tree to die, you may have run into the same strange situation that I did on one of my boxes. It happened on only one of my boxes, which of course had to be a production machine. I finally stopped chasing down the root cause when I got into the POSIX module and decided it was something to do either with my Linux kernel or Perl version. In any case, the bad behavior only happened when I used the -d switch of the myapp_fastcgi.pl script to daemonize the process. On a tip from another Catalyst user, I switched to daemontools for managing my application startup, since it does not require the daemonize switch. Presto! That problem was solved.



Mark Blythe <perl@markblythe.com>