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Surpassing the TLB performance of superpages with less operating system support

Many commercial microprocessor architectures have added translation lookaside buffer (TLB) support for superpages. Superpages differ from segments because their size must be a power of two multiple of the base page size and they must be aligned in both virtual and physical address spaces. Very large superpages (e.g., 1MB) are clearly useful for mapping special structures, such as kernel data or frame buffers. This paper considers the architectural and operating system support required to exploit medium-sized superpages (e.g., 64KB, i.e., sixteen times a 4KB base page size). First, we show that superpages improve TLB performance only after invasive operating system modifications that introduce considerable overhead.
We then propose two subblock TLB designs as alternate ways to improve TLB performance. Analogous to a subblock cache, a complete-subblock TLB associates a tag with a superpage-sized region but has valid bits, physical page number, attributes, etc., for each possible base page mapping. A partial-subblock TLB entry is much smaller than a complete-subblock TLB entry, because it shares physical page number and attribute fields across base page mappings. A drawback of a partial-subblock TLB is that base page mappings can share a TLB entry only if they map to consecutive physical pages and have the same attributes. We propose a physical memory allocation algorithm, page reservation, that makes this sharing more likely. When page reservation is used, experimental results show partial-subblock TLBs perform better than superpage TLBs, while requiring simpler operating system changes. If operating system changes are inappropriate, however, complete-subblock TLBs perform best.