AJAX Request Slow With PHP? Here's Why

Recently I was working on a project where we had a page which loads tons of data from numerous sources. I decided after a while that we wanted to AJAX each section of data so that the page would load a bit quicker. After splitting up the requests and sending them asyncronously, there was little improvement. I thought at first it may be due to the fact we were pinging a single API for most of the data multiple times, that wasn’t it. Maybe it was a browser limit? Nope was still far below the 6 requests most allow. I setup xdebug and kcachegrind and to my surprise it was the session_start() that was taking the most time on the requests.

I looked around the web for a while trying to figure out what in the world was going on. It turns out that PHP’s default session_start will block future session_starts for the same session until the session is closed. This is because the default method uses a file on the filesystem which it locks until you close it. If you want more information on this and how to close it you can read a bit more here.

We switched over to database based sessions and it fixed it. In symfony 1.4 the default session storage uses the file system, however switching over to http://trac.symfony-project.org/browser/branches/1.4/lib/storage/sfPDOSessionStorage.class.php">sfPDOSessionStorage is very easy and quick.