体系结构量化研究方法 第五章-2

Directory-based Coherence Protocols

• Revisit snooping-based coherence protocol:

  • Use broadcast to track the state of every cache block
    • No centralized data structure
      • Require communication with all caches on every cache miss
  • Inexpensive, but suffer poor scalability

• Directory-based protocol: alternative to snooping-based protocol

  • Directory keeps track of the information of every memory block:
    • Block states
    • Which caches have each block
      • Support direct communication to the relevant caches
  • [Centralized] Easy for multicore with a shared outermost (L3) cache
    • For each block: simply keep a bit vector of the size equal to the number of cores
  • [Decentralized] A single directory as used in multicore is hard to scale out
    • The directory itself should be distributed

Structure of Multiprocessor with Distributed Directory

• Directory shall be distributed to multiprocessors in a well-mapped manner(one directory is added to each node)
  • Avoid broadcasting
    • The coherence protocol can directly find the directory information for any memory block
  • Increase bandwidth
    • The directory information can be distributed to different cache banks
• The simplest directory implementation:

• The simplest directory implementation:
  • Associate an entry in the directory with each memory block
  • [Cost] Information size is proportional to the product of
    • the number of memory blocks
    • the number of nodes

The Basics

• Three states maintained for each memory block by directory

  • Shared
    • Indicate one or more nodes have the block cached, value in memory is up-to-date
    • Must also track which nodes have copies of that block
      • Those copies will need to be invalidated on a write
  • Uncached
    • Indicate no node has a copy of the cache block
  • Modified
    • Indicate exactly one node has a copy of the cache block, value in memory is out-of-date
    • Also use the bit vector to keep track of the owner of the block

• Compare directory-based protocols with snooping-based protocols:

  • States and transitions for cache blocks are almost identical
  • Broadcasting is replaced by two-stage communication
    • First between the requesting node and the directory
    • Then between the directory and one or more remote nodes

Messages Sent Among Node

原文地址：https://blog.csdn.net/qq_42047140/article/details/144792754

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