I have a (Nodejs 4.3) lambda function that I have tested with several memory limit settings (128, 256, 512).
As I pull up the memory limit, the execution time decreases as expected. However the max memory used also goes down. Every time I reduce the memory limit the execution time and max memory used go back up.
Any thoughts? Trying to figure out how to hit the execution time I need while not over paying.
This is the Node VM utilizing memory. If it's available, it's going to use it (and you could probably reproduce this scenario locally with VMs or Docker). I wouldn't worry too much and I wouldn't recommend trying to instruct the VM on what to do or even thinking about garbage collection (which is not easy with Node.js anyway). Node is very opportunistic about memory is all I can say. I would select the amount you need in order to get a reasonable response time and leave it at that.
I would also probably have to imagine that on "warm" runs your speed will increase when you have a lower amount of memory set, but a "cold" run will be higher. So in production, it may not be as big of a concern.
You may wish to start profiling your code and trying to optimize it...But again, Node doesn't really want the developer worrying about the resources on a machine. It tries to optimize for you. It's a bit unfortunate when you're billed that way though. This is part of why I wish Go was natively supported with Lambda.
