App Engine gives you 1.3M free requests per day. Assuming that each peer announces and scrapes every 10 minutes, that's over 4,500 simultaneous peers. You get 6.5 CPU-hours per day, which leaves you 18 CPU-ms per request.

That is what I'm trying to figure out. Atrack is designed to make as efficient use of the quota as possible, and so far so good, but I hope all this publicity helps a bit with the stress testing.