title: PHP Warning: exec() unable to fork
body: <p>So here is a little background info on my setup. Running Centos with apache and php 5.2.17.  I have a website that lists products from many different retailers websites.  I have crawler scripts that run to grab products from each website.  Since every website is different, each crawler script had to be customized to crawl the particular retailers website.  So basically I have 1 crawler per retailer.  At this time I have 21 crawlers that are constantly running to gather and refresh the products from these websites.  Each crawler is a php file and once the php script is done running it checks to ensure its the only instance of itself running and at the very end of the script it uses exec to start itself all over again while the original instance closes.  This helps protect against memory leaks since each crawler restarts itself before it closes. However recently I will check the crawler scripts and notice that one of them Isnt running anymore and in the error log I find the following.</p>
  
  <pre><code>PHP Warning:  exec() [&lt;a href='function.exec'&gt;function.exec&lt;/a&gt;]: Unable to fork [nice -n 20 php -q /home/blahblah/crawler_script.php &amp;gt;/dev/null &amp;amp;]
  </code></pre>
  
  <p>This is what is supposed to start this particular crawler over again however since it was "unable to fork" it never restarted and the original instance of the crawler ended like it normally does.</p>
  
  <p>Obviously its not a permission issue because each of these 21 crawler scripts runs this exec command every 5 or 10 minutes at the end of its run and most of the time it works as it should.  This seems to happen maybe once or twice a day.  It seems as though its a limit of some sort as I have only just recently started to see this happen ever since I added my 21st crawler.  And its not always the same crawler that gets this error it will be any one of them at a random time that are unable to fork its restart exec command.</p>
  
  <p>Does anyone have an idea what could be causing php to be unable to fork or maybe even a better way to handle these processes as to get around the error all together?  Is there a process limit I should look into or something of that nature?  Thanks in advance for help!</p>