During the server crash right now, I had the opportunity to observe the HTTP server handling more than 100 requests per second. Apart from one bug (using a too small backlog on the listening socket; now fixed), it handled it better than I expected. However, it turns out that the MySQL daemon suddenly turned into the bottleneck for the forum, which surprises me, because it has always seemed to handle every load without even taking a breather.
Does someone with more experience of phpBB perhaps know if there's any known, common way to tweak MySQL to make it handle these loads better? Judging from "mysqladmin processlist", it seems the main bottleneck is the "sessions" table. In particular, there seems to be a lot of DELETE operations waiting for its lock, which I also find a bit peculiar -- do that many sessions really need to be deleted all the time?