Revisiting Virtual Memory PDF Download

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Languages : en
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Book Description
Page-based virtual memory (paging) is a crucial piece of memory management in today's computing systems. However, I find that need, purpose and design constraints of virtual memory have changed dramatically since translation lookaside buffers (TLBs) were introduced to cache recently-used address translations: (a) physical memory sizes have grown more than a million-fold, (b) workloads are often sized to avoid swapping information to and from secondary storage, and (c) energy is now a first-order design constraint. Nevertheless, level-one TLBs have remained the same size and are still accessed on every memory reference. As a result, large workloads waste considerable execution time on TLB misses and all workloads spend energy on frequent TLB accesses. In this thesis I argue that it is now time to reevaluate virtual memory management. I reexamine virtual memory subsystem considering the ever-growing latency overhead of address translation and considering energy dissipation, developing three results. First, I proposed direct segments to reduce the latency overhead of address translation for emerging big-memory workloads. Many big-memory workloads allocate most of their memory early in execution and do not benefit from paging. Direct segments enable hardware-OS mechanisms to bypass paging for a part of a process's virtual address space, eliminating nearly 99% of TLB miss for many of these workloads. Second, I proposed opportunistic virtual caching (OVC) to reduce the energy spent on translating addresses. Accessing TLBs on each memory reference burns significant energy, and virtual memory's page size constrains L1-cache designs to be highly associative -- burning yet more energy. OVC makes hardware-OS modifications to expose energy-efficient virtual caching as a dynamic optimization. This saves 94-99% of TLB lookup energy and 23% of L1-cache lookup energy across several workloads. Third, large pages are likely to be more appropriate than direct segments to reduce TLB misses under frequent memory allocations/deallocations. Unfortunately, prevalent chip designs like Intel's, statically partition TLB resources among multiple page sizes, which can lead to performance pathologies for using large pages. I proposed the merged-associative TLB to avoid such pathologies and reduce TLB miss rate by up to 45% through dynamic aggregation of TLB resources across page sizes.