When are page frame specific cache management policies useful?

Question

I'm reading the O'Reilly Linux Kernel book and one of the things that was pointed out during the chapter on paging is that the Pentium cache lets the operating system associate a different cache management policy with each page frame. So I get that there could be scenarios where a program has very little spacial/temporal locality and memory accesses are random/infrequent enough that the probability of cache hits is below some sort of threshold.

I was wondering whether this mechanism is actually used in practice today? Or is it more of a feature that was necessary back when caches where fairly small and not as efficient as they are now? I could see it being useful for an embedded system with little overhead as far as system calls are necessary, are there other applications I am missing?

Answer 1

Having multiple cache management policies is widely used, whether by assigning whole regions using MTRRs (fixed/dynamic, as explained in Intel's PRM), MMIO regions, or through special instructions (e.g. streaming loads/stores, non-temporal prefetches, etc..). The use-cases also vary a lot, whether you're trying to map an external I/O device into virtual memory (and don't want CPU caching to impact its coherence), or whether you want to define a writethrough region for better integrity management of some database, or just want plain writeback to maximize the cache-hierarchy capacity and replacement efficiency (which means performance). These usages often overlap (especially when multiple applications are running), so the flexibility is very much needed, as you said - you don't want data with little to no spatial/temporal locality to thrash out other lines you use all the time.

By the way, caches are never going to be big enough in the foreseeable future (with any known technology), since increasing them requires you to locate them further away from the core and pay in latency. So cache management is still, and is going to be for a long while, one of the most important things for performance critical systems and applications