PHP-FPM / FastCGI + exit() causing CPU spikes

Question

I have been having intermittent issues on some servers running Archlinux / php-fpm 5.3.9 in FastCGI on Cherokee 1.2.101. I am using a caching plugin that builds and serves static cache files using logic like:

$cache_file = md5($host . $uri) . '.cache';
if( file_exists($cache_file) ) {
  $cache_file_contents = file_get_contents($cache_file)
  exit( $cache_file_contents );
}
// else build/save the $cache_file

A few processes will end up in the slow log of php-fpm hanging on that exit() call. At that time the load spikes, 100% CPU usage goes (almost) entirely to the webserver and PHP pages start returning 500 - Internal Server errors. Sometimes the server recovers on it's own, others I need to restart php-fpm and cherokee.

• I have the FastCGI settings for PHP-FPM configured to do a

• Even though this is a VPS I would tentatively rule out IO wait on the filesystem as the cache file should already be loaded. I have not been able to catch it in the act to test with vmstat

• I have pm.max_requests set to 500 but wonder if the exit() call is interfering with the cycling of processes.

• The php-fpm log shows a lot of WARNING: [pool www] seems busy (you may need to increase pm.start_servers, or pm.min/max_spare_servers) . This seems to be a normal part of php-fpm regulating the number of child processes in the pool though

Any tips on troubleshooting would be appreciated. Here are 3 things I found that raised some red flags:

http://www.php.net/manual/en/function.exit.php#96930

https://serverfault.com/questions/84962/php-via-fastcgi-terminated-by-calling-exit#85008

Errors when calling exit() function for fastCGI?

Answer 1

This might be related to passing the output over to the server (track I/O as well). You could keep FPM out of the way by making your webserver serving the static cache files. Apart from that, I'd suggest you use this PHP chunk instead to lessen memory/I/O a little bit:

if (file_exists($cache_file))
{
    readfile($cache_file)
    exit;
}

See readfile .

If you don't want to use exit (I personally never encountered an issue with using it with FastCGI in PHP) you should clean your code so that it's not necessary to use exit, eg you could return or look on your code-flow why you need to use exit and eliminate the issue.

Answer 2

I ended up using the Pythonic Exception wrapping method cited in the comments at http://www.php.net/manual/en/function.exit.php

In the main index.php

class SystemExit extends Exception {}

try{ 
    /* Resume loading web-app */
}
catch (SystemExit $e) {}

In the Cache Logic from the Question , replacing exit( $cache_file_contents );

while (@ob_end_flush());
flush();
echo $cache_file_contents;
throw new SystemExit();

This has alleviated the php-fpm slow logs that show hangs on that exit() . I'm not entirely convinced that it solved the underlying problem but it has cleaned up the log files.