Reply To: Memory

Sound questions.

1) Frame exit invalidation is done in the Well Known Region for the stack. It is not necessary to clear the valid bits, because the exited frame is no longer in the user’s address space; return automatically cuts back the stack permission WKR. However, there is an issue with inter-core references to foreign stacks. Core B has no WKR for core A’s stack, so in normal mode threads cannot reference each others stacks and threads cannot reference their own exited frames. However, there is a legacy mode in which inter-stack references are permitted. In legacy mode the entire stack, both used and unused, has a PLB entry for the turf. So if there are two legacy threads running in the same turf then they can browse each others stack, and can browse the rubble in their own stack. We have never found a way to restrict interstack references to only the live portion; all practical implementations seem to require exposing rubble too.

2) The implementation of the TLB is per-member; it is transparent to the programmer. As currently in the sim, a miss in the TLB requires a search of the PTE tables. However, unallocated regions of the 60-bit address space do not need PTEs and do not occupy physical memory. If the search leads to an address where a PTE might be but is not, then the Implicit Zero logic will return a synthetic line of zeroes. The PTE format is carefully arranged to that a zero PTE means that the region is unallocated in DRAM – and the load in turn returns implicit zero. Thus the page table is in virtual memory, not physical, and there are only entries for mapped space.

Thus the miss will trigger a search, which may miss, which triggers a search… The Tiny Alice algorithm we use bounds the recursion to the number of distinct range sizes (current 5), but in practice the intermediate entries are in the LLC and search depth is nearly always <= 2.