Found the limits of our setup yesterday, with about 400+ concurrent sessions and many of them being logged in, actively polling and chatting. The day before we delivered 138k page views without a hitch.
The bottleneck was the number of unicorns (8). We only reached about 35% CPU loads and there were 2GBs of free RAM with these settings. I changed to 10 unicorns, after which we had no more hiccups and served a peak of 440 sessions, and got CPU loads closer to 50%.
But then it was getting late and things cooled down and I wasn’t able to do any more live performance testing with max load.
Anyway, with UpClouds 6-core / 8GB RAM plan (or similar) I would say that 10 unicorns is better than 8. I’ll be looking at the amount of free RAM and and lets see if even 11-12 would be feasible. Ping @mpalmer?