A couple of hours ago I wrote a post about migrating web services to a Debian VM running Squeeze, from one which had been running Lenny. I said I’d switched to the prefork MPM under Apache2.
Under those conditions, the server needed either so many Apache processes that it filled memory, or it reached the limits I had set and page loads took 20+ seconds.
So, I quickly switched back to the worker MPM and PHP running under Fast CGI, and the page loads are back down to 2 seconds or so on average.
I still have some work to do, to make sure I don’t start too many PHP5 CGI processes, but at least the sites are useable again.