inst eecs berkeley edu cs61c CS61C Machine Structures Lecture 34 Caches III 2004 11 17 Lecturer PSOE Dan Garcia www cs berkeley edu ddgarcia The Incredibles The Biggest digital photo of all time is a huge 78 797 x 31 565 I e 2 5 Gibipixels 7 5 GiB Zoom away www tpd tno nl smartsite966 html Garcia Fall 2004 UCB CS61C L34 Caches III 1 Review Mechanism for transparent movement of data among levels of a storage hierarchy set of address value bindings address index to set of candidates compare desired address with tag service hit or miss load new block and binding on miss address tag index offset 000000000000000000 0000000001 1100 Valid 0x4 7 0x8 b 0xc f 0x0 3 Tag 0 1 1 2 3 0 a b c d CS61C L34 Caches III 2 Garcia Fall 2004 UCB Memorized this table yet Blah blah Cache size 16KB blah blah 223 blocks blah blah how many bits Answer 2XY means X 0 X 1 X 2 X 3 X 4 X 5 X 6 X 7 X 8 no suffix kibi Kilo 103 mebi Mega 106 gibi Giga 109 tebi Tera 1012 pebi Peta 1015 exbi Exa 1018 zebi Zetta 1021 yobi Yotta 1024 CS61C L34 Caches III 3 Y 0 Y 1 Y 2 Y 3 Y 4 Y 5 Y 6 Y 7 Y 8 Y 9 1 2 4 8 16 32 64 128 256 512 Garcia Fall 2004 UCB How Much Information IS that www sims berkeley edu research projects how much info 2003 Print film magnetic and optical storage media produced about 5 exabytes of new information in 2002 92 of the new information stored on magnetic media mostly in hard disks Amt of new information stored on paper film magnetic optical media doubled in last 3 yrs Information flows through electronic channels telephone radio TV and the Internet contained 18 exabytes of new information in 2002 3 5x more than is recorded in storage media 98 of this total is the information sent received in telephone calls incl voice data on fixed lines wireless WWW 170 Tb of information on its surface in volume 17x the size of the Lib of Congress print collections Garcia Fall 2004 UCB CS61C L34 Caches III 4 Block Size Tradeoff 1 3 Benefits of Larger Block Size Spatial Locality if we access a given word we re likely to access other nearby words soon Very applicable with Stored Program Concept if we execute a given instruction it s likely that we ll execute the next few as well Works nicely in sequential array accesses too CS61C L34 Caches III 5 Garcia Fall 2004 UCB Block Size Tradeoff 2 3 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 x Hit Rate Miss Penalty x Miss Rate CS61C L34 Caches III 6 Garcia Fall 2004 UCB Block Size Tradeoff 3 3 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 CS61C L34 Caches III 7 Garcia Fall 2004 UCB Extreme Example One Big Block Valid Bit Cache Data Tag B3 B2 B1 B0 Cache Size 4 bytes Block Size 4 bytes Only ONE entry in the cache If item accessed likely accessed again soon But unlikely will be accessed again immediately The next access will likely to be a miss again Continually loading data into the cache but discard data force out before use it again Nightmare for cache designer Ping Pong Effect CS61C L34 Caches III 8 Garcia Fall 2004 UCB Block Size Tradeoff Conclusions Miss Rate Exploits Spatial Locality Miss Penalty Block Size Average Access Time Fewer blocks compromises temporal locality Block Size Increased Miss Penalty Miss Rate Block Size CS61C L34 Caches III 9 Garcia Fall 2004 UCB Administrivia Project 2 grades are frozen Details on Midterm clobbering Final exam will contain midterm labeled questions covering weeks 1 7 called FinalMid On these questions if your st dev is greater than your on the Midterm you have clobbered your grade and we ll replace your Midterm w equivalent grade from FinalMid E g Mid x 50 12 you got 38 Your Mid grade is 1 0 FinalMid x 60 10 you get 65 Your FinalMid grade is 0 5 Your new Mid grade is now 0 5 or 50 0 5 56 WooHoo CS61C L34 Caches III 10 Garcia Fall 2004 UCB 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 CS61C L34 Caches III 11 Garcia Fall 2004 UCB 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 CS61C L34 Caches III 12 Garcia Fall 2004 UCB 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 CS61C L34 Caches III 13 Garcia Fall 2004 UCB Fully Associative Cache 2 3 Fully Associative Cache e g 32 B block compare tags in parallel 31 Cache Tag 27 bits long Valid Cache Data B 31 B1 B 0 Cache Tag 4 0 Byte Offset CS61C L34 Caches III 14 Garcia Fall 2004 UCB 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 CS61C L34 Caches III 15 Garcia Fall 2004 UCB 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 CS61C L34 Caches III 16 Garcia Fall 2004 UCB N Way Set Associative Cache 1 4 Memory address fields Tag same as before Offset same …
View Full Document
Unlocking...