Snoop Request Forwarding in Multiprocessor Systems
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Solution Overview
Problem
In multiprocessor systems, accessing data from another node involves multiple communications, leading to latency issues due to the need for snoop requests and responses through a home node, even when the data is available at another node.
Innovation Solution
A data processing apparatus and method that includes receiving and transmitting circuitry to handle snoop requests with an indication of forwarding, allowing direct data transfer from the responding node to the requesting node when the data is cached, reducing the need for intermediate communication through the home node.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If snoop requests are sent through the home node to maintain data coherency, then data coherency is maintained throughout the system, but access latency increases due to multiple communication steps
Solution Approach 1:
The patent introduces a forwarding indicator as an intermediary mechanism that enables direct peer-to-peer data transfer between nodes while maintaining home node coordination. The forwarding indicator, when set in the snoop request, authorizes the responding node to bypass the home node for data transmission, thus maintaining data coherency through the home node's request initiation while reducing latency by enabling direct transfer paths.
Solution Approach 2:
The patent segments the data access path into two distinct phases: the coherency management phase handled by the home node (sending snoop requests with forwarding indicators), and the data transfer phase that can occur directly between nodes when forwarding is authorized. This segmentation allows the system to separate the functions of coherency maintenance and data transfer, enabling optimized direct transfers while preserving coherency guarantees.
2Reliability
If multiple communication steps are used for data access through the home node, then data coherency is ensured, but the number of communications increases leading to higher latency
Solution Approach 1:
The patent implements dynamic data access paths by introducing the forwarding indicator mechanism. When the forwarding indicator is set in a snoop request, the system dynamically switches from the standard home-node-mediated path to a direct peer-to-peer path. This dynamic adaptation allows the system to optimize for speed when direct transfer is safe, while maintaining the traditional path when coherency requires home node involvement, thus improving overall data access efficiency.
Solution Approach 2:
The patent changes the state parameter of the snoop request by introducing the forwarding indicator bit. This parameter change fundamentally alters the communication behavior: when the forwarding indicator is set, the responding node is authorized to send data directly to the requesting node, reducing the number of communication steps from multiple (through home node) to a single direct transfer, thereby improving productivity.
3Loss of time
If direct data forwarding is enabled between nodes, then access latency is reduced, but system complexity increases due to forwarding management
Solution Approach 1:
The patent extracts the forwarding authorization logic from the complex home node mediation process and encapsulates it in a simple forwarding indicator bit within the snoop request message. This extracted indicator carries all necessary authorization information, allowing nodes to make forwarding decisions based on a simple bit check rather than complex home node coordination, thus reducing the perceived complexity while enabling direct transfers.
Solution Approach 2:
The patent enables nodes to autonomously determine whether to forward data directly by checking the forwarding indicator in received snoop requests. This self-service mechanism allows nodes to independently make forwarding decisions without requiring complex inter-node negotiation or home node involvement in each transfer decision, simplifying the forwarding management while reducing latency.
Data Source
AI summary
A data processing apparatus is provided, which includes receiving circuitry to receive a snoop request in respect of requested data on behalf of a requesting node. The snoop request includes an indication as to whether forwarding is to occur. Transmitting circuitry transmits a response to the snoop request and cache circuitry caches at least one data value. When forwarding is to occur and the at least one data value includes the requested data, the response includes the requested data and the transmitting circuitry transmits the response to the requesting node.