The Scalable Commutativity Rule: Designing Scalable Software for Multicore Processors

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The Scalable Commutativity RuleDesigning
Scalable Software for Multicore Processors

• A. T. Clements, M. F. Kaashoek, N.Zeldovich,R.
  T. Morris, and E. Kohler
• MIT CSAIL and Harvard

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Paper Highlights

• Presents an general efficient mechanism to manage
  pages of different sizes in a VM system
• Superpages
• Objective is to address the limitations of extant
  translation lookaside buffers (TLB).

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MESI Coherency protocol (I)

• Write-through cache and memory consistency
  protocol
• Name refers to four possible states for cache
  lines
• Modified (locked and dirty)
• Exclusive (locked and clean)
• Shared (unlocked and clean)
• Invalid
• Common-sense transitions between states

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Snooping

• Any cache that hold a line in the modified state
  must intercept and service all attempted read
  accesses to that line from other caches
• Any cache that holds a line in the shared state
  must act on invalidate or request-for-ownership
  broadcasts from other caches
• A cache that holds a line in the Exclusive state
  must snoop all read requests to that line from
  all other caches, and move the line to shared
  state if needed

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Optimizations

• Store buffers allow caches to write into invalid
  lines
• Invalidation queues contain invalidation
  requests that have not yet been acted upon
• Weaken consistency and require barriers
• Store barriers flush the store buffers
• Read barrier flush the invalidation queue