I’m not sure I understand. In your equations, why is swap taking away from RAM? It’s purely additive.
Say you have 64GB of RAM, of which 4GB are dirty pages that are never, or at least very rarely, accessed. With 64GB RAM + 4GB swap, you filled up your swap with them and have 64GB RAM left to use actively. With 64GB RAM + 0GB swap, you only have 60GB RAM left to use actively.
In my examples nothing is taking away from nothing, I'm just asking if, if two situations have the same total amount of GB, what's the actual difference between RAM+swap vs entirely RAM.
Use the 32+32 numbers if you're more comfortable with those round numbers.
If I have a system running comfortably with 32GB RAM + 32GB swap (no OOM, no swap thrashing), and I upgrade to 64GB RAM + 0 swap, what actual negative effects would I be getting for the lack of swap?
That's my question, to know if swap has any benefits other than being additional slow RAM.
Adding 32GB of RAM is vastly better than adding 32GB of swap, it’s not even close. However, it’s also vastly more expensive. That has always been the case, not just in the current insane situation.
We use swap because we have plenty of disk space, but (comparably) very little DRAM.
The same goes for another level on the hierarchy: If you could choose between 1GB of DRAM vs. 1GB of SRAM (which your CPU cache is), you’d definitely choose the SRAM. But you’d be paying out of your nose both with money and area, it’s not economical at all. My CPU has 96MB of cache, and that’s considered massive.
Say you have 64GB of RAM, of which 4GB are dirty pages that are never, or at least very rarely, accessed. With 64GB RAM + 4GB swap, you filled up your swap with them and have 64GB RAM left to use actively. With 64GB RAM + 0GB swap, you only have 60GB RAM left to use actively.