Tradeoffs in Supporting Two Page Sizes

Abstract

As computer system main memories get larger and processor cycles-per-instruction (CPIs) get smaller, the time spent in handling translation lookaside buffer (TLB) misses could become a performance bottleneck. We explore relieving this bottleneck by (a) increasing the page size and (b) supporting two page sizes. We discuss how to build a TLB to support two page sizes and examine both alternatives experimentally with a dozen uniprogrammed, user-mode traces for the SPARC architecture. Our results show that increasing the page size to 32KB causes both a significant increase in average working set size (e.g., 60%) and a significant reduction in the TLB's contribution to CPI, CPI/sub TLB/, (namely a factor of eight) compared to using 4KB pages. Results for using two page sizes, 4KB and 32KB pages, on the other hand, show a small increase in working set size (about 10%) and variable decrease in CPI/sub TLB/, (from negligible to as good as found with the 32KB page size). CPI/sub TLB/ when using two page sizes is consistently better for fully associative TLBs than for set-associative ones. Our results are preliminary, however, since (a) our traces do not include multiprogramming or operating system behavior, and (b) our page-size assignment policy may not reflect a real operating system's policy.