A look at several memory management units, TLB-refill mechanisms, and page table organizations

Abstract

Virtual memory is a staple in modem systems, though there is little agreement on how its functionality is to be implemented on either the hardware or software side of the interface. The myriad of design choices and incompatible hardware mechanisms suggests potential performance problems, especially since increasing numbers of systems (even embedded systems) are using memory management. A comparative study of the implementation choices in virtual memory should therefore aid system-level designers.This paper compares several virtual memory designs, including combinations of hierarchical and inverted page tables on hardware-managed and software-managed translation lookaside buffers (TLBs). The simulations show that systems are fairly sensitive to TLB size; that interrupts already account for a large portion of memory-management overhead and can become a significant factor as processors execute more concurrent instructions; and that if one includes the cache misses inflicted on applications by the VM system, the total VM overhead is roughly twice what was thought (10--20% rather than 5--10%).