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Recap: Memory Hierarchy of a Modern Computer System

Recap: Memory Hierarchy of a Modern Computer System. By taking advantage of the principle of locality: Present the user with as much memory as is available in the cheapest technology. Provide access at the speed offered by the fastest technology. Processor. Control. Tertiary Storage (Disk).

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Recap: Memory Hierarchy of a Modern Computer System

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  1. Recap: Memory Hierarchy of a Modern Computer System • By taking advantage of the principle of locality: • Present the user with as much memory as is available in the cheapest technology. • Provide access at the speed offered by the fastest technology. Processor Control Tertiary Storage (Disk) Secondary Storage (Disk) Second Level Cache (SRAM) Main Memory (DRAM) On-Chip Cache Datapath Registers Speed (ns): 1s 10s 100s 10,000,000s (10s ms) 10,000,000,000s (10s sec) Size (bytes): 100s Ks Ms Gs Ts

  2. 4 Questions for Memory Hierarchy • Q1: Where can a block be placed in the cache? (Block placement) • Q2: How is a block found if it is in the cache?(Block identification) • Q3: Which block should be replaced on a miss? (Block replacement) • Q4: What happens on a write? (Write strategy)

  3. Summary of caches: • The Principle of Locality: • Program likely to access a relatively small portion of the address space at any instant of time. • Temporal Locality: Locality in Time • Spatial Locality: Locality in Space • Three Major Categories of Cache Misses: • Compulsory Misses: sad facts of life. Example: cold start misses. • Conflict Misses: increase cache size and/or associativity. Nightmare Scenario: ping pong effect! • Capacity Misses: increase cache size • Cache Design Space • total size, block size, associativity • replacement policy • write-hit policy (write-through, write-back)

  4. Q1: Where can a block be placed in the cache? • Direct mapped – every block has only one possible location, shared by others • Set associative – every block has several possible locations, shared by others in its set • Fully associative – a block may be located anywhere • Why not always use full associativity?

  5. Q2: How is a block found if it is in the cache? • Index is used to find possible location(s) • Tag on each block within possible locations must be compared against tag in address we’re looking for • Increasing associativity shrinks index, expands tag so it becomes more expensive to determine whether block is in cache

  6. Q3: Which block should be replaced on a miss? • Easy for Direct Mapped (why?) • Set Associative or Fully Associative: • Random • LRU (Least Recently Used)

  7. Q4: What happens on a write? • Write through—The information is written to both the block in the cache and to the block in the lower-level memory. • Write back—The information is written only to the block in the cache. The modified cache block is written to main memory only when it is replaced. • Have to check whether block is clean or dirty • Pros and Cons of each? • WT: read misses cannot result in writes • WB: no writes of repeated writes • WT always combined with write buffers so that don’t wait for lower level memory

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