How to debug 503 Service Unavailable error
We are using dompdf library which is a php library that converts html to pdf. And for some html documents we are trying to convert, server outputs 503 error (the server is cpanel litespeed).
The error is consistent and always happens for a given document. However if I start taking away some stuff from the document, it starts working at some point. I also tried taking away different pieces of the document and it seems that the error is not specific to some piece, but rather to the total size of the document.
That led me to believe that the server is hitting some memory limit. However when I created a test script that overloads the memory and ran it on the same server, the behavior was normal: it was gradually using up the memory and then died with php's memory error when it did hit the right limit. Then I decided to test against script run-time in case it's an infinite loop. The script in question fails with 503 error after 35 seconds of execution. A test script that just waits for 60 seconds, doesn't fail at all.
So I am a little lost and would appreciate some help debugging this. I tried searching through forum and checking multiple log files but didn't find anything useful. Could you let me know how I should approach the problem, i.e. how can I get from the server anything more descriptive than 503. We tried increasing different configs in litespeed admin area but that didn't help too.