LiteSpeed / PHP server parallel requests

Question

On our PHP website we have noticed, that if I request pages in 2 separate tabs, the second tab will never start loading until the first tab finishes processing the PHP side (uses LiteSpeed webserver). It's a cloud webhosting that should handle a notable number of visitors. We observed the same behavior on previous hosting with Apache.

Could the issue be in webserver configuration? Or could it be something in our application blocking the processing?

Answer 1

If your application uses PHP sessions (intentionally or not), this will generally be the behavior.

When an application calls session_start() it generates a session for you as a user, and will do a file lock on this session until the execution of the site is done, then the lock will be removed, and the next script waiting for the same session can then proceed.

If you're not going to write any data into the session, it's beneficial to call session_write_close() early on, since this will free the lock, and let other processes (Your other tab) allow being processed.

The locking happens because you obviously don't want two processes trying to write to the same file at once, possibly overriding one of the processes data that was written to the session.

Obviously it could also be something else in your application causing this, but you'd have to actually debug the code to figure out. You could possibly use strace on the command-line as well to try to gather more information.

PHP session locking is explained in depth in https://ma.ttias.be/php-session-locking-prevent-sessions-blocking-in-requests/ - it simply goes into a lot more detail than I did above.