5.2 Directory protocol

:material-circle-edit-outline: 约 338 个字 :material-clock-time-two-outline: 预计阅读时间 1 分钟

Directory-based Cache coherence

PPT 计算机体系结构（本）2024-12-16第3-5节

Distributed Shared Memory 模式下，内存分为本地内存和远程内存，不是共享内存了，上面的办法就用不了了

我们就得改用 Directory protocol

Directory protocol

基本思路是，用目录记录所有 memory block 的状态，以此调控 cache block

Directory: track state of every block in memory

we change the state of block in cache according to directory

• Information in directory
  • Status of Every block: shared / uncached / exclusive
  • Which processors have copies of the block: bit vector
    • 一个位对应一个处理器是否有对应的 copy
  • Whether the block is dirty or clean

Directory protocol can be implemented with a distributed memory：

Directory protocol implementation

• Block status
  • Shared：≥ 1 processors have data, memory up-to-date
  • Uncached：no processor has it; not valid in any cache
  • Exclusive：1 processor (owner) has data; memory out-of-date
• Directory size = entry number * entry size)

Directory Protocol

• Local node 发起请求的结点
• Home node where the memory location of an address resides
• Remote node has a copy of a cache block, 且数据是最新的

目录协议相互之间的通信都可以抽象为三个结点之间发消息

消息需要至少包括：发送者 P = processor number, 请求的地址 A = address

所有会产生的消息的情况，以及对应的消息含义如下：

invalidate 就是，我要写数据了，请 home 查找目录，通知所有有 copy 的 node将该数据无效化

CPU -Cache State Machine

Directory State Machine

[!EXAMPLE]

PPT 60