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Lecture 6: The Walls to Sequential Computing. Moore’s Law.

Lecture Summary

  • Wrap up Caches
  • Virtual Memory

Caches

  • Handling a write-hit
    • Write-through
    • Write-back
  • Handling a write-miss
    • Write-allocate
    • No-write-allocate
  • Typical combos in practice
    • Write-back + Write-allocate (more common)
    • Write-through + No-write-allocate
Miss rate is more important than the hit rate: 97% hit rate is ~2 times worse than 99% hit rate
Cache Capacity Effects from Memory Mountain

Case Study: Rearranging Loops to Improve Spatial Locality

Virtual Memory

Why memory virtualization?
  • Ease of use (running programs that require more memory than physically available)
  • Isolation (running multiple programs simultaneously)
  • Protection
  • A page of virtual memory corresponds to a frame of physical memory
  • Page table enables the translation of virtual address into physical addresses
  • The page table is stored in main memory
    • If the page table is accessed for each address translation, this would be very costly
  • Translation Lookaside Buffer (TLB): "Cache" for the addr translation process