[justin_]

This is the kind of debate that has been going on surrounding virtual memory forever[0][1]. If you can keep everything in memory, then you're golden. But eventually you won't, and you'll need to rely on secondary storage.

Is there a performance benefit to be had by managing the memory and paging yourself? Yes. But eventually you will also consider running processes next to your database, for logging, auditing, ingesting data, running backups, etc. Virtual memory across the whole system helps with that, especially if other people will be using your database in ways you can't predict. As for the efficiency of MMUs and the OS, seems like for almost all cases it's "satisfactory" enough[1].

[0] http://denninginstitute.com/pjd/PUBS/bvm.pdf

[1] From 1969! https://dl.acm.org/doi/pdf/10.1145/363626.363629

[benlivengood]

I guess things like mshare could be extended to the entire process address spaces and the kernel could avoid TLB invalidation on context switches between them. Core affinity could be used to keep other programs from scheduling on the cores intended for processes sharing the whole address space.