Ticket #399 (FastCGI performance on high load) does a good job of describing the problem. In short, Lighttpd queues all requests going to a fastcgi server (or group of processes), and if the offerered load is higher than the server can handle, new requests are delayed more and more while old requests for which we can no longer send a response are processed.
This patch gives a simple alternative solution to the problem: set the request-queue-limit option for that server, and when lighttpd recieves a request that would be processed by that server, but that server already has more than request-queue-limit processes queued up, lighttpd itself simply returns a 503 instead of queuing the new request. This ensures that at least some requests will be satisified by the fastcgi server, rather than letting the server get into a backlog situation where no requests are satisified because the user has given up by the time the server gets around to processing his request.
I'd suggest that this patch go into the next 1.4 release, since it's minimally invasive, and easily removed once a better solution is available.
