php, multithreading and other doubts

Question

morning

I have some doubts about the the way php works. I cant find the answer anywhere on books so I thought to hit the stack ;)

so here it goes:

lets assume we have one single server with php+apache installed. Here are my beliefs:

1 - php can handle one request at a time. Doesn't matter if apache can handle more than 1 thread at a time because eventually the invoked php interpreter is single threaded.

2 - from belief 1 follows that I believe if the server receives 4 calls at the same very time these calls are queued up and executed 1 at a time. Who makes the request last gets the response last.

3 - from 1 and 2 follows that if I cron-call a url corresponding to a script that does some heavy-lifting/time consuming stuff I slow down the server up to the moment the script returns.

Whats true? whats false?

cheers

Answer 1

I see no reason why would PHP be not able to handle multiple requests at the same time. That said, it may be semi-true for handling requests of single client, depending on the type of script.

Many scripts use sessions. When session_start() is called, session is being opened and locked. When execution of script ends, session is being closed and unlocked (this can be done manually). When there are multiple requests for the same session, first requests opens and locks session, and the second request has to wait until session is unlocked. This might make an impression that multiple PHP scripts cannot be executed at the same time, but that's true (partly) only for requests that use the same session (in other words - requests from the same browser). Requests from two clients (browsers) may be processed parallelly as long as they don't use resources (files, DB tables etc) that are being locked/unlocked in other requests.

Answer 2

My crystal ball suggests that you are using PHP sessions and you have having simultaneous requests (either iframes or AJAX) getting queued. The problem is that the default session handler uses files and session_start() locks the data file. You should read your session data quickily and then call session_write_close() to release the file.