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We are having Apache processes get hung up after seeing a POST request, and wondering what the best path might be to troubleshooting this. When I look at the extended server status for the requests that hang, I see entries like:

53-31 28616 9/13/74232 W 0.71 7174 0 7.6 0.01 961.47 xxx.xxx.xxx.xxx www.mysite.com POST xxxxx HTTP/1.1

27-31 7629 1/34/107074 W 0.96 4480 0 0.0 0.39 1394.10 xxx.xxx.xxx.xxx www.mysite.com POST xxxxx HTTP/1.1

The process itself is always doing something, usually consuming significant CPU resources until we kill it.

Originally I assumed it was some bad code -- everything is running PHP scripts -- but after awhile we saw hung processes resulting from a variety of different requests, many of them simple, but always showing POST as the last type of request handled.

I don't expect that the community would instantly be able to solve this without seeing the actual code running; what I am really looking for is the best path to debug this as I am out of ideas. Some things that do come to mind:

  • Is there any kind of a tool similar to jstack that will allow us to dump and inspect a stack trace of a hung process, that would give useful information?

  • Is there a way to track useful input that might help troubleshoot this? I have tried setting the Apache logs to debug and having it output POST query parameters but the amount of data logged is overwhelming and I have a hard time knowing what to look for. Any best practice here?

  • The server is using prefork with MaxClients set to 150, and a KeepAlive of 5. Under normal load scenarios there are usually 30-50 processes up and running; at high load closer to 100. We have seen the hung process behavior occur under any load scenario. Is there any piece of the configuration that I might specifically look at?

The server is Apache 2 running under Debian Lenny on EC2.

Many thanks!

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2 Answers 2

up vote 0 down vote accepted

One very useful utility is apachetop. It's like top, but is keeping its eye on your Apache's access log in real time. You can sort the results by many different criterias -- amount of requests, amount of kilobytes transferred and so on.

Don't know what's going on in your case, but I've spotted some broken rewrite rules with Apachetop. For example, in one case a rewrite rule was pointing to a local, but non-existing page, and did so by performing a redirect (R in RewriteRule line). Unfortunately also the Apache's Error Document was misconfigured -- ErrorDocument 404 http://thesiteimtalkingabout.com/404.html and that 404 page was missing, too.

This combination lead to a death-spiraling experience: a seemingly innocent page was causing Apache to recursively call itself and Apache became totally unresponsive.

Probably that war story does not help you at all, so I point you to couple of other debugging techniques:

  • use strace. See the pid of misbehaving Apache process from server-status, and then do something like strace -fF -s 128 -p thatbastardpid -o /tmp/wtf and see what the heck the process is doing by reading the /tmp/wtf file. Or, to see statistics, strace -fF -p thatbastardpid -c -o /tmp/wtf

  • Capture the network traffic with tcpdump or wireshark and see if there's something odd going on with that PHP process.

  • use PHP's Xdebug together with (for example) KCachegrind and see a very thorough breakdown what's going on with your PHP script.

I hope this helps. Good luck!

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Thanks for the assistance and sorry for the slow response; using strace did eventually help me find a solution, it was related to the mod_dumpio module which had been enabled in the server config and was occasionally just spinning. It seems like it is a known-but-very-archaic bug with that module, which we had only enabled long ago for a single debug session and no longer needed, so we just disabled it and the problem went away. –  futureal Nov 24 '10 at 19:03

Create a simple php page which takes just one post variable and prints it on screen. Block all clients so that only you can access server. Restart web server. Now use your simple script and send only one post variable.

If apache still manages to hang then problem with POST method. If this works then problem with large number of scripts may be due to common included file in all scripts.

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