[hamandcheese]

In my past experience with a large rails monolith, memory usage was always the limiting factor. Just booting the app had significant memory overhead. Using in-process concurrency would have led to massive infrastructure savings, since a lot of that overhead could be shared across threads. Probably 2-3x the density compared to single threaded.

In the end, we never got quite there, due to thread safety issues. We did use a post-boot forking solution to achieve some memory savings thanks to copy-on-write memory, which also led to significant savings, but was a bit more complex.

All that to say, the naive "just let kubernetes scale it for you" is probably quite expensive.