Piggy-Back Snoops for Distributed Memory Coherence
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Solution Overview
Problem
In distributed processing systems, the bandwidth of the snoop bus interconnect is heavily utilized, limiting performance due to the inclusion of non-coherent memory transaction snoop information alongside coherent transactions, which is not relevant to all processing cores.
Innovation Solution
The proposed solution involves combining non-coherent snoop information with coherent snoop information to form expanded snoop messages, allowing them to be piggy-backed onto the coherent snoop bus interconnect using additional side-band wires, thereby reducing the bandwidth usage of the coherent snoop bus interconnect.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If non-coherent snoop information is included in snoop bus transmissions, then coherence maintenance is improved, but snoop bus bandwidth is consumed reducing overall performance
Solution Approach 1:
The patent segments snoop information into two distinct types: coherent snoop information (CSN) and non-coherent snoop information (NSN). CSN is transmitted on the coherent snoop bus interconnect, while NSN is transmitted on the non-coherent snoop bus interconnect. This segmentation allows each bus to be optimized for its specific type of traffic, preventing NSN from consuming bandwidth on the coherent snoop bus while still maintaining coherence where needed.
Solution Approach 2:
The patent introduces a non-coherent snoop bus interconnect as an intermediary channel specifically for transmitting non-coherent snoop information. This intermediary structure allows NSN to be transmitted without competing for bandwidth on the coherent snoop bus, effectively mediating between the need to transmit all snoop information and the need to preserve coherent snoop bandwidth for performance-critical operations.
2Loss of information
If non-coherent snoop information is transmitted on the coherent snoop bus, then all snoop information is delivered, but coherent snoop bandwidth is reduced by up to fifty percent
Solution Approach 1:
The patent creates separate transmission channels for different types of snoop information. The coherent snoop bus interconnect is dedicated to CSN, while the non-coherent snoop bus interconnect handles NSN. This ensures that all snoop information is delivered without mixing traffic types, preventing NSN from reducing coherent snoop bandwidth while maintaining complete information delivery.
Solution Approach 2:
The patent creates a universal snoop information delivery system that uses two specialized bus interconnects working in parallel. Together, these buses provide comprehensive snoop information delivery for both coherent and non-coherent transactions, with each bus optimized for its specific function. This multi-functionality approach ensures complete information delivery while preserving coherent snoop bandwidth for performance-critical operations.
Data Source
AI summary
Piggy-back snoops are used for non-coherent memory transactions in distributed processing systems. Coherent and non-coherent memory transactions are received from a plurality of processing cores within a distributed processing system. Non-coherent snoop information for the non-coherent memory transactions is combined with coherent snoop information for the coherent memory transactions to form expanded snoop messages. The expanded snoop messages are then output to a snoop bus interconnect during snoop cycles for the distributed processing system. As such, when the processing cores monitor the snoop bus interconnect, the processing cores receive the non-coherent snoop information along with coherent snoop information within the same snoop cycle. While this piggy-backing of non-coherent snoop information with coherent snoop information uses an expanded snoop bus interconnect, usage of the coherent snoop bandwidth is significantly reduced thereby improving overall performance of the distributed processing system.