Cache memory
- 1. Cache Memory Prof. Neeraj Bhargava Kapil Chauhan Department of Computer Science School of Engineering & Systems Sciences MDS University, Ajmer
- 2. Introduction Cache memory is a not dynamic RAM its work on static RAM. Cache memory is a small size type of volatile computer memory that provides high-speed data. The basic purpose of cache memory is to store program instruction that are frequently accessed by software during operation.
- 3. Memory Hierarchy Facts Compact Fast Increase performance by having “hierarchy” of memory subsystems “Temporal Locality” and “Spatial Locality” are big ideas
- 4. Memory Hierarchy Terms Cache Miss Cache Hit Hit Rate Miss Rate Index, Offset and Tag
- 6. • Compulsory (cold start or process migration, first reference): first access to a block – “Cold” fact of life: not a whole lot you can do about it – Note: If you are going to run “billions” of instruction, Compulsory Misses are insignificant • Capacity: – Cache cannot contain all blocks access by the program – Solution: increase cache size • Conflict (collision): – Multiple memory locations mapped to the same cache location – Solution 1: increase cache size – Solution 2: increase associativity • Coherence (Invalidation): other process (e.g., I/O) updates memory
- 7. Improving Cache Performance Reduce Miss Rate Associativity Victim Cache Compiler Optimizations Reduce Miss Penalty Faster DRAM Write Buffers Reduce Hit Time
- 8. Compiler Optimizations Compiler Optimizations to reduce miss rate Loop Fusion Loop Interchange Merge Arrays
- 9. Reduce Hit Time Lower Associativity Add L1 and L2 caches L1 cache is small => Hit Time is critical L2 cache has large => Miss Penalty is critical
- 10. PerformanceCPU time = (CPU execution clock cycles + Memory stall clock cycles) x clock cycle time Memory stall clock cycles = (Reads x Read miss rate x Read miss penalty + Writes x Write miss rate x Write miss penalty) Memory stall clock cycles = Memory accesses x Miss rate x Miss penalty Different measure: AMAT Average Memory Access time (AMAT) = Hit Time + (Miss Rate x Miss Penalty)