【2014-11-23】《The Hardware/Software Interface》– Section 7

  1. Cache
    • Definition: computer memory with short access time used for the storage of frequently or recently used instructions or data(i-cachhe and d-cache).
    • more generally, used to optimize data transfers between system elements with different characteristics (network interface cache, I/O cache, etc.)
    • Hit:
    • Miss:
  2. Locality: programs tend to use data and instructions with addresses near or equal to those they have used recently
    • Temporal locality:
      • Recentlly referenced items are likely to be referenced again in the near future.
    • Spatial locality:
      • Items with nearby addresses tend to be referenced close together in time
  3. Cost of Cache Misses
    • Huge difference between a hit and a miss
      • could be 100x, if just L1 and main memory
    • 99% hits is twice as good as 97%
  4. Cache Performance Metrics
    • Miss Rate:
      • Fraction of memory references not found in cache (misses/ accesses) = 1 – hit rate
      • Typical numbers(in percentage)
        • 3% – 10% for L1
    • Hit Time
      • Time to deliver a line in the cache to the processor
        • Includes time to determine whether the line is in the cache
      • Typical hit times: 1 –2 clock cycles for L1
    • Miss Penalty
      • Additional time required because of a miss
      • Typically 50 – 200 cycles
  5. Memory Hierarchies
    • Well-written programs tend to exhibit good locality
    • These properties complement each other beautifully
    • Fundamental idea of a memory hierarchy:
      • Each level k serves as a cache for the larger, slower, level k+1 below
  10. Optimizations for the Memory Hierarchy
    • Write code that has locality
      • Spatial: access data contiguously
      • Temporal: make sure access to the same data is not too far apart in time
    • How to achieve
      • Proper choice of algorithm
      • Loop transformations
时间： 2024-10-03 23:14:53

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