Directory-Based Cache Coherence Protocol Without ACK Messages
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Solution Overview
Problem
Directory-based cache-coherence protocols in shared memory multiprocessor systems require ACK messages, which increase bandwidth and resource allocation time, and eliminating these messages can lead to forward-progress problems without proper ordering constraints.
Innovation Solution
A system that processes cache line requests without waiting for acknowledgment messages from requesting nodes, allowing valid copies to be sent directly or through slave nodes, and using special requests to guarantee forward progress when necessary.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If ACK messages are used to terminate transactions, then coherence reliability is improved, but bandwidth requirements increase
Solution Approach 1:
The patent extracts and eliminates the ACK message from the coherence protocol, demonstrating that coherence can be maintained without these acknowledgment messages by using directory-based tracking of cache line locations and valid copies across processors
Solution Approach 2:
The patent implements feedback mechanisms through the directory structure that tracks which processors have valid copies of cache lines, allowing the system to automatically manage coherence without requiring explicit ACK messages for each transaction
2Reliability
If ACK messages are required to complete transactions, then coherence reliability is improved, but resource allocation time increases
Solution Approach 1:
The patent removes the ACK message requirement from the transaction completion process, allowing home nodes to complete transactions by simply sending the requested cache line without waiting for acknowledgment, thereby reducing resource allocation time
Solution Approach 2:
The directory structure performs preliminary tracking of cache line locations and validity before transactions occur, enabling the system to prepare and complete transactions more quickly without needing to wait for ACK messages
3Quantity of substance
If ACK messages are eliminated to reduce bandwidth, then bandwidth requirements decrease, but forward progress problems occur
Solution Approach 1:
The patent uses the directory structure as a feedback mechanism that continuously tracks which processors have valid copies of cache lines, enabling the system to make forward progress by automatically routing requests to appropriate processors without requiring ACK messages
Solution Approach 2:
The directory acts as an intermediary that mediates between requesting processors and the actual data locations, using this intermediate information to manage coherence and forward progress without direct ACK messages
Data Source
AI summary
Some embodiments of the present invention provide a system that processes a request for a cache line in a multiprocessor system that supports a directory-based cache-coherence scheme. During operation, the system receives the request for the cache line from a requesting node at a home node, wherein the home node maintains directory information for all or a subset of the address space which includes the cache line. Next, the system performs an action at the home node, which causes a valid copy of the cache line to be sent to the requesting node. The system then completes processing of the request at the home node without waiting for an acknowledgment indicating that the requesting node received the valid copy of the cache line.