PHP and Async requests with file based sessions
We recently ran into an issue where several of our ajax requests were taking an inordinate amount of time to execute. Digging a little deeper into the queries being executed, I was expecting return times in the order of 200ms, not the several seconds I was seeing. Installing XHGui only furthered my confusion: session_start() was the culprit with incredibly high run times. I started exploring how we were storing sessions, the default file store. My first thought was that perhaps there was so many files in the session directory that directory access was slowed (easily fixed with some options onsession.save_path). But we only had a few active sessions and fewer than 100 files. No dice. After some head pounding, I realized the issue: PHP locks the session file for the duration of script execution. The first ajax request took the expected amount of time, but locked the user’s session file for the duration of the request. The second ajax request was blocked on session_start() until the first call had finished. The third ajax request blocked on the first two, and was left with the incredibly high run length that drew me in in the first place.
