A16_MemMgt-SuperPages - Practical, transparent operating...

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1 Practical, transparent operating system support for superpages Juan Navarro, Sitaram Iyer, Peter Druschel, Alan Cox OSDI 2002
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2 What’s a Superpage? A very large page size, much greater than the base page size Supported by most computer architectures today Machines that support superpages usually have several different page sizes, beginning with the base page and then in increasing sizes, each a power of 2 – today, some as large as a gigabyte.
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3 Background Summary Virtual memory automates the movement of a process’s address space (code and data) between disk and primary memory. Virtual addresses are translated using information stored in the page table. Page tables are stored in primary memory. Extra memory references due to page table degrades performance
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4 Translation Lookaside Buffer TLB (translation lookaside buffer) – faster memory; caches portions of the page table If most memory references “hit” in the TLB, the overhead of address translation is acceptable. TLB coverage : the amount of memory that can be accessed strictly through TLB entries.
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5 The Problem Computer memories have increased in size faster than TLBs. TLB coverage as a percentage of total memory has decreased over the years. At the time this paper was written, most TLBs covered a megabyte or less of physical memory Many applications have working sets that are not completely covered by the TLB Result: more TLB misses, poorer performance.
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6 The Solution Superpages! Increase coverage without increasing TLB size. How? By increasing amount of memory each TLB entry can map
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7 Hardware-Imposed Constraints Must have enough contiguous free memory to store each superpage Superpage addresses (physical and virtual) must be aligned on the superpage size: e.g., a 64KB SP must start at address 0, or 64KB, or 128KB, etc. TLB entry only has one set of bits (R, M, etc.) and thus can only provide coarse-grained info – not good for efficient page management.
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8 Issues Issues for a superpage management system: Allocation and fragmentation control Promotion Demotion Eviction
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9 Issues: Frame Allocation When a page fault occurs, must choose a frame In non-superpage systems any frame will do In a superpage system we may later decide to include this page in a superpage – how does this affect the decision? Possible approaches to allocation: Reservation based Relocation based
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Reservation-based Allocation When a page is initially loaded choose a superpage size and reserve aligned, contiguous frames to hold it. As other pages are referenced, load them into the previously reserved frames Will adjoining pages ever be needed by the program? How is the superpage size chosen?
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This note was uploaded on 12/14/2011 for the course CS 690 taught by Professor Staff during the Fall '11 term at University of Alabama - Huntsville.

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A16_MemMgt-SuperPages - Practical, transparent operating...

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