Yep! The idea of "free memory" largely doesn't exist in modern operating systems. We haven't gotten to the point of power gating DRAMs (have we?), so we're alwa

Yep! The idea of "free memory" largely doesn't exist in modern operating systems. We haven't gotten to the point of power gating DRAMs (have we?), so we're always paying the cost of the DRAM all the time. As such, not using it means you're just throwing energy away. Keeping lots of purgeable data in these unallocated space like (as you said) disk cache not only speeds up access time but also saves a ton of power since a hit to DRAM is leagues lower than having to access external storage. While this is a non-trivial engineering problem, it's almost always a good trade off since it boost performance and saves power without any extra cost other than the effort of speculative loads (which, when linear, is much less expensive than a random one off load!) and managing it