My argument is, if two different processes are doing completely different things in the same JVM, I am skeptical that they will share memory pages well, etc. I simply don't know without testing, but I don't have any reason to believe they would.

On the other had, if I was running multiple threads that all cached data from the same data store, it is quite likely that scaling up threads in the same JVM would have a massive advantage over splitting them into multiple JVMs.

Also, in the comments I admit that over 10K applications on 1K machines, it's probably better 99% of the time not to use 10K JVMs. There would be a huge CPU/Memory overhead....