lect11-cache-replacement - ECE/CS 752:Advanced Computer...

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Unformatted text preview: ECE/CS 752:Advanced Computer Architecture I 1 Cache Replacement Policies Cache Replacement Policies Prof. Mikko H. Lipasti University of Wisconsin Madison ECE/CS 752 Spring 2010 Cache Design: Four Key Issues Cache Design: Four Key Issues These are: Placement Where can a block of memory go? Identification How do I find a block of memory? 2005 Mikko Lipasti 2 How do I find a block of memory? Replacement How do I make space for new blocks? Write Policy How do I propagate changes? Consider these for caches Usually SRAM Also apply to main memory, disks Placement Placement Memory Type Placement Comments Registers Anywhere; Int, FP, SPR Compiler/programmer manages 2005 Mikko Lipasti 3 Cache (SRAM) Fixed in H/W Direct-mapped, set-associative, fully-associative DRAM Anywhere O/S manages Disk Anywhere O/S manages Placement Placement Address Range Exceeds cache capacity Map address to finite capacity SRAM Cache Hash Address Index Block Size 2005 Mikko Lipasti 4 Called a hash Usually just masks high order bits Direct mapped Block can only exist in one location Hash collisions cause problems Data Out Index Offset 32-bit Address Offset Identification Identification Fully associative Block can exist anywhere No more hash collisions Identification SRAM Cache Hash Address Hit Tag Check ?= Tag 2005 Mikko Lipasti 5 Identification How do I know I have the right block? Called a tag check Must store address tags Compare against address Expensive! Tag & comparator per block Data Out Offset 32-bit Address Offset Tag Placement Placement Set associative Block can be in a locations H h lli i SRAM Cache Hash Address Index a Tags a Data Blocks Index 2005 Mikko Lipasti 6 Hash collisions: a still OK Identification Still perform tag check However, only a in parallel Data Out Offset Offset 32-bit Address Tag Index ?= ?= ?= ?= Tag ECE/CS 752:Advanced Computer Architecture I 2 Replacement Replacement Cache has finite size What do we do when it is full? Analogy: desktop full? 2005 Mikko Lipasti 7 Move books to bookshelf to make room Bookshelf full? Move least used to library Etc. Same idea: Move blocks to next level of cache Cache Miss Rates: 3 Cs [Hill] Cache Miss Rates: 3 Cs [Hill] Compulsory miss or Cold miss First ever reference to a given block of memory Measure: number of misses in an infinite cache model Capacity Working set exceeds cache capacity 8 Useful blocks (with future references) displaced Good replacement policy is crucial!...
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This note was uploaded on 03/02/2012 for the course ECE 752 taught by Professor Profgurisohi during the Spring '09 term at Wisconsin.

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lect11-cache-replacement - ECE/CS 752:Advanced Computer...

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