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15-447 Computer ArchitectureFall 2007 © November 14th, 2007 Majd F. Sakr email@example.com www.qatar.cmu.edu/~msakr/15447-f07/ CS-447– Computer Architecture M,W 10-11:20am Lecture 21 Set Associative Cache
15-447 Computer ArchitectureFall 2007 © Review… °Mechanism for transparent movement of data among levels of a storage hierarchy set of address/value mappings address => index to set of candidate blocks compare desired address with tag service hit or miss -load new block and binding on miss Valid Tag 0x0-3 0x4-70x8-b0xc-f 0 1 2 3... 1 0abcd 000000000000000000 0000000001 1100 address: tag index offset
15-447 Computer ArchitectureFall 2007 © Review… °Drawbacks of Larger Block Size Larger block size means larger miss penalty -on a miss, takes longer time to load a new block from next level If block size is too big relative to cache size, then there are too few blocks -Result: miss rate goes up °In general, minimize Average Access Time = Hit Time + Miss Penalty x Miss Rate
15-447 Computer ArchitectureFall 2007 © Review… °Hit Time = time to find and retrieve data from current level cache °Miss Penalty = average time to retrieve data on a current level miss (includes the possibility of misses on successive levels of memory hierarchy) °Hit Rate = % of requests that are found in current level cache °Miss Rate = 1 - Hit Rate
15-447 Computer ArchitectureFall 2007 © Types of Cache Misses (1/2) °“Three Cs” Model of Misses °1st C: Compulsory Misses occur when a program is first started cache does not contain any of that program’s data yet, so misses are bound to occur can’t be avoided easily, so won’t focus on these in this course
15-447 Computer ArchitectureFall 2007 © Types of Cache Misses (2/2) °2nd C: Conflict Misses miss that occurs because two distinct memory addresses map to the same cache location two blocks (which happen to map to the same location) can keep overwriting each other big problem in direct-mapped caches how do we lessen the effect of these? °Dealing with Conflict Misses Solution 1: Make the cache size bigger -Fails at some point Solution 2: Multiple distinct blocks can fit in the same cache Index?
15-447 Computer ArchitectureFall 2007 © Fully Associative Cache (1/3) °Memory address fields: Tag: same as before Offset: same as before Index: non-existant °What does this mean? no “rows”: any block can go anywhere in the cache must compare with all tags in entire cache to see if data is there
15-447 Computer ArchitectureFall 2007 © Fully Associative Cache (2/3) °Fully Associative Cache (e.g., 32 B block) compare tags in parallel Byte Offset : Cache Data B 0 0 4 31 : Cache Tag (27 bits long) Valid : B 1B 31 : Cache Tag = = = = = :
15-447 Computer ArchitectureFall 2007 © Fully Associative Cache (3/3) °Benefit of Fully Assoc Cache No Conflict Misses (since data can go anywhere) °Drawbacks of Fully Assoc Cache Need hardware comparator for every single entry: if we have a 64KB of data in cache with 4B entries, we need 16K comparators: infeasible
15-447 Computer ArchitectureFall 2007 © Third Type of Cache Miss °Capacity Misses miss that occurs because the cache has a limited size miss that would not occur if we increase the size of the cache sketchy definition, so just get the general idea °This is the primary type of miss for Fully Associative caches.
15-447 Computer ArchitectureFall 2007 © N-Way Set Associative Cache (1/4) °Memory address fields: Tag: same as before Offset: same as before Index: points us to the correct “row” (called a set in this case) °So what’s the difference? each set contains multiple blocks once we’ve found correct set, must compare with all tags in that set to find our data
15-447 Computer ArchitectureFall 2007 © N-Way Set Associative Cache (2/4) °Summary: cache is direct-mapped w/respect to sets each set is fully associative basically N direct-mapped caches working in parallel: each has its own valid bit and data
15-447 Computer ArchitectureFall 2007 © N-Way Set Associative Cache (3/4) °Given memory address: Find correct set using Index value. Compare Tag with all Tag values in the determined set. If a match occurs, hit!, otherwise a miss. Finally, use the offset field as usual to find the desired data within the block.
15-447 Computer ArchitectureFall 2007 © N-Way Set Associative Cache (4/4) °What’s so great about this? even a 2-way set assoc cache avoids a lot of conflict misses hardware cost isn’t that bad: only need N comparators °In fact, for a cache with M blocks, it’s Direct-Mapped if it’s 1-way set assoc it’s Fully Assoc if it’s M-way set assoc so these two are just special cases of the more general set associative design
15-447 Computer ArchitectureFall 2007 © Associative Cache Example °Recall this is how a simple direct mapped cache looked. °This is also a 1-way set- associative cache! Memory Memory Address 0 1 2 3 4 5 6 7 8 9 A B C D E F 4 Byte Direct Mapped Cache Cache Index 0 1 2 3
15-447 Computer ArchitectureFall 2007 © Associative Cache Example °Here’s a simple 2 way set associative cache. Memory Memory Address 0 1 2 3 4 5 6 7 8 9 A B C D E F Cache Index 0 0 1 1
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