PHP stack occasionally showing blank pages

Question

I've started getting a really strange error on a server of mine and I've no idea how to fix it. The basic setup is a Wordpress (not v3, the previous stable release) site on PHP-FPM + APC on nginx on Ubuntu Lucid.

For five or so minutes at a time, some pages will just return straight out, blank page, no content. I'll wait for a bit, try again and it'll work fine.

It's only been happening for the past week or so but there hasn't really been any change in code, platform or number of users.

I'm banging my head against a wall trying to figure out how it could be happening... I'm open to all suggestions and as ever, if you need extra detail, just drop a comment and I'll get on it ASAP.

Answer 1

I have a varnish proxy in front of my wordpress installation. A error which occurred some month ago: wordpress transfers many information in the request header and the header size was limited on a old version of varnish. Maybe something similar with nginx. But it is just a wild guess.

As Artefacto said: Check the log.

Answer 2

I don't know the exact answer, but check your error reporting as well as your logs files. If you have error reporting turned entirely off, you may be missing something critical that blocks page execution. Also, if Apache or PHP segfaults, sometimes it will return blank content - you should see that in your log files.

Answer 3

A suggestion would be to check for OOM (out of memory). This can be hard to spot sometimes because PHP usually runs just for a few seconds.

Check out APC, it's also known to segfaults (see google), try without it.

Answer 4

If you are running out of memory, try increasing the memory for Wordpress by putting this line near the top of your wp-config.php file:

define('WP_MEMORY_LIMIT', '64M');

or in your php.ini, if you have access to it:

memory_limit = 64M;