Is it possible to assign parts of the shared L2 caches to different cores

Question

Lets say, 4 threads are running on 4 separate cores of a Multicore x86 processor, and they do not share any data, is it possible to progammatically make the 4 cores use separate and predefined portions of the shared L2 cache.

Answer 1

Let's use two terms, exclusive and shared caches instead of L1, L2, L3, L4 caches. Different CPU families start to share cache on different levels. In the presented terms the original question is - is it possible split shared cache into the parts, each of which will be used exclusively by one of the CPU/cores? There is no clear answer. Furthermore there are two answers opposite to each other.

1) First and general answer: NO. Cache is by design managed in hardware. There are only few control levers of cache accessible in software such as enable/disable cache for whole memory or defined memory region, apply specified policy for cache flushing (write through/ write back). NO basically due to the fact, that it was designed to be managed in hardware. So there are no useful interface that will allow manage it gracefully in software.

2) Second answer: Yes. In fact, cache designed in such a way, that each line of the cache can save data from specified set of memory lines. Due to this if memory manager provides guaranty, that the same CPU one CPU/core own and use all memory lines assigned to the same cache line exclusively, then memory manager provides guaranty that that cache line will be used by that CPU exclusively. It is a very tricky workaround. And it have very limited benefits, and have serious drawbacks: memory layout is very fragmented, cache usage is unbalanced, complicated memory management, very hadrware-dependent (Details can be found in the paper provided by "MetallicPriest").

Resume: it is possible in theory and almost impossible on practice.