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  DAP Spr.‘98  ©  UCB 1 Lecture 12: Memory Hierarchy — Ways to Reduce Misses  DAP Spr.‘98  ©  UCB 2 Block Replacement When a miss occurs, the cache controller must select a block to be replaced with the desired data. A replacement policy determines which block should be replaced.With direct-mapped placement the decision is simple because there is no choice: only one block frame is checked for a hit and only that block can be replaced.With fully-associative or set-associative placement , there are more than one block to choose from on a miss. Primary strategies: Random -to spread allocation uniformly, candidate blocks are randomly selected.  Advantage: simple to implement in hardware  Disadvantage: ignores principle of locality Least-Recently Used (LRU) -to reduce a chance of throwing out information that will be needed soon, accesses to blocks are recorded. The block replaced is the one that has been unused for the longest time.  Advantage: takes locality into account  Disadvantage: as the number of blocks to keep track of increases, LRU becomes more expensive (harder to implement, slower and often just approximatedOther strategies: First In First Out (FIFO) Most-Recently Used (MRU)Least-Frequently Used (LFU) Most-Frequently Used (MFU)  DAP Spr.‘98  ©  UCB 3 Review: Four Questions for Memory Hierarchy Designers ã Q1: Where can a block be placed in the upper level? (Block placement)  – Fully Associative, Set Associative, Direct Mapped ã Q2: How is a block found if it is in the upper level? (Block identification)  – Tag/Block ã Q3: Which block should be replaced on a miss? (Block replacement)  – Random, LRU ã Q4: What happens on a write? (Write strategy)  – Write Back or Write Through (with Write Buffer)  DAP Spr.‘98  ©  UCB 4 Review: Cache Performance CPUtime = Instruction Count x (CPI execution + Mem accesses per instruction x Miss rate x Miss penalty) x Clock cycle timeMisses per instruction = Memory accesses per instruction x Miss rateCPUtime = IC x (CPI execution + Misses per instruction x Miss penalty) x Clock cycle timeTo Improve Cache Performance:1. Reduce the miss rate 2. Reduce the miss penalty3. Reduce the time to hit in the cache.
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