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I have a PHP website which uses $_SESSION variables. It runs on Apache, Mysql and Ubuntu 14.04 on a very resourceful server (Amazon EC2 - M3.X-large)

For some unknown reason, the website virtually grinds to a halt every so often, and I don't know why. When this occurs, very few people are visiting the website, and server load is very low. Restarting Apache & MySQL does nothing, however, after about an hour the website speed returns to normal. Restarting my entire server also temporarily fixes the problem, but after a few days the problem reoccurs.

My question is - what could be causing this problem, and what can I do to debug / fix it? Like I said - traffic is very low when the problem occurs so I don't think apache / php require more resources. I think the problem runs deeper. Could the amount of session data being saved be causing the issue?

Thanks in advance.

Andrew Schulman
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richelliot
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  • i doubt it's the amount of session data. while the problem is happening try to analyze what's going on. check the memory usage with `top` check the web logs for hits to particular scripts; check mysql with `> show processlist;` check your bandwidth by testing the web server from the local server vs remote, etc, hopefully you find the next clue, good luck. – user16081-JoeT Jan 06 '15 at 16:28
  • I think the downvotes here are responses to the "why" question, which is broad and impossible for us to answer from the information you provided. But "how can I debug" seems like a good question to me, and it should be the focus here. Title revised to reflect that. – Andrew Schulman Jan 06 '15 at 17:46
  • Ok fair enough. So Andrew - any ideas on how to de-bug?? – richelliot Jan 06 '15 at 17:48
  • Have you checked any logs at the time of the issue? @user16081-JoeT has given some information on how to debug, you could also look at using ```strace``` to get more information on what processes are doing at this time. I'll be happy to help more but you'll probably need to provide a little more information. – Chris Davidson Jan 06 '15 at 18:14
  • Yes, I have checked the logs but I can see anything that is giving me any clues. Looking at 'top' and 'apachetop' doesn't shed any light either. However, next time it occurs I will look at using 'strace' and check whats going on with bandwidth etc. Thanks for the tips guys. – richelliot Jan 07 '15 at 09:32

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Another useful tool is iotop, although its possible that (since this is a VM) that its nothing on your system causing the slowdown but resource consumption on the virtualization host / storage. In addition to the tools already mentioned you might want to have a look at hprof and compare the profiles outside and during a performance incident.

symcbean
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