> why their calculations show that 6rps (that's what their "17 mln monthly lambda invocations" really means) would require 25 servers I'm guessing that it has

> why their calculations show that 6rps (that's what their "17 mln monthly lambda invocations" really means) would require 25 servers I'm guessing that it has something to do with the average job taking 15 minutes. 6rps represents 6 jobs being created per second, but each one takes 15 minutes to run until completion. Another way to look at it is each second 90 minutes of lambda work is created. If you consider 15 minutes the rolling window, which looks like a fair assumption based on the graph provided, there could be up to 5400 (15 min x 60 sec x 6 rps) functions running at once. Working backwards, 25 medium instance provide (25 instances x 4 GB mem) an 100gb memory pool, or 100,000mb. That leaves around ~18mb for each of those 5400 jobs, if you don't consider OS resource overhead. Looking at averages in this situation very possibly can give a warped perception of reality, but 25 instances doesn't seem out of the realm of possibility. I'm sure they have much more relevant metrics to back up that number as well. Whether the functions really need this much time to run is another issue entirely, and hard to answer with the information given.