Measure ram consumed by Nginx proxy cache

Hi folks:

I am currently investigating performance of our production Nginx
instances. Our architecture is a Nginx process in front of 8 node.js
workers. As the memory is running low, we want to determine the RAM
consumed by Nginx processes only.

My understanding is that Nginx cache is on disk by default, whereas
Linux caches a lot of file content on memory. Is there a way to tell
how much RAM Nginx uses overall for proxy cache?