> Swap ram by itself would be stupid but no one doing this isn’t also turning on compression.

I'm not sure what you mean here? Swapping out infrequently accesses pages to disk to make space for more disk cache makes sense with our without compression.

Swapping out to RAM without compression is stupid - then you’re just shuffling pages around in memory. Compression is key so that you free up space. Swap to disk is separate.

>Because then you have more active working memory as infrequently used pages are moved to compressed swap and can be used for more page cache or just normal resident memory.

Uhh... A VMM that swaps out to disk an allocated page to make room for more disk cache would be braindead. The process has allocated that memory to use it. The kernel doesn't have enough information to deem disk cache a higher priority. The only thing that should cause it to be swapped out is either another process or the kernel requesting memory.

> A VMM that swaps out to disk an allocated page to make room for more disk cache would be braindead

Claiming any decision is “brain dead” in something as heuristic heavy and impossible to compute optimally as resident memory pages is quite the statement to make; this is a form of the knapsack problem (NP-complete at least) with the added benefit of time where the items are needed in some specific indeterminate order in the future and there’s a whole bunch of different workloads and workload permutations that alter this.

To drive this point home in case you disagree, what’s dumber? Swapping out to disk an allocated page (from the kernel’s perspective) that’s just sitting in the free list of the userspace allocator for that process or a page of some frequently accessed page of data?

Now, I agree that VMMs may not do this because it’s difficult to come up with these kinds of scenarios that don’t penalize the general case, more importantly than performance this has to be a mechanism that is explainable to others and understandable for them. But claiming it’s a braindead option to even consider is IMHO a bridge too far.