Meta-bit Storage in L1 Cache for Snoop Bandwidth Reduction
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Solution Overview
Problem
In multi-core processor systems, maintaining cache coherency and managing snoop bandwidth is complex due to inconsistent data issues across processing units, which increases the burden on interconnect design and can lead to higher costs and complexity, especially with the integration of meta-bits and error correction codes in cache coherency subsystems.
Innovation Solution
The solution involves a data processing apparatus and method where each processor core has an L1 cache that receives memory requests, stores cache lines with error correction code bits and meta-bits indicating local/global state and special uncorrectable error bits, allowing for efficient forwarding of data blocks with error status and reissuing requests based on snoop filter status, thereby minimizing ECC check bits usage and maintaining cache coherency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If cache coherency mechanisms are implemented in multi-core systems, then data consistency between cores is improved, but snoop bandwidth requirements and interconnect complexity increase
Solution Approach 1:
The cache line is segmented into multiple data blocks, with meta-bits embedded within specific blocks rather than requiring separate signaling for the entire cache line. This segmentation allows snoop filter status to be determined from individual blocks without requiring full cache line snoop operations, reducing interconnect bandwidth requirements while maintaining coherency.
Solution Approach 2:
The patent embeds meta-bits within the data block structure itself (adding a dimension to the data representation) rather than requiring separate control signals or bandwidth allocation for snoop filter status. This dimensional integration allows coherency information to be carried along with data transfers without increasing interconnect width or complexity.
2Reliability
If error correction code bits are added to each data block, then error detection and correction capability is improved, but the number of check bits and storage overhead increases
Solution Approach 1:
The patent merges the function of error correction with the snoop filter status indication by embedding meta-bits within the same data block structure that contains ECC bits. This consolidation allows the system to maintain error correction capability while reducing the total overhead by sharing the same structural space for multiple functions rather than adding separate mechanisms.
Solution Approach 2:
The data block structure is designed to serve multiple purposes: it carries user data, includes ECC bits for error correction, and embeds meta-bits for snoop filter status and coherency information. This multi-functional design eliminates the need for separate structures for each function, reducing overall overhead while maintaining all required capabilities.
3Loss of energy
If meta-bits are stored within data blocks rather than separately, then snoop bandwidth requirements are reduced, but the density and organization of data storage becomes more complex
Solution Approach 1:
The meta-bits are nested within the data block structure, specifically embedded among the data bits and ECC bits. This nesting allows coherency and snoop filter information to be carried within the existing data transfer infrastructure without requiring separate storage locations or additional bandwidth allocation, reducing snoop bandwidth requirements while organizing complexity within the block itself.
4Loss of time
If the snoop filter status is determined from the first data block received, then data forwarding latency is reduced, but accuracy may be compromised if the first block does not contain complete information
Solution Approach 1:
The patent places the snoop filter status meta-bit in the first data block of the cache line, allowing the snoop filter status to be determined immediately upon receipt of the first block without waiting for subsequent blocks. This preliminary positioning of critical coherency information enables early decision-making about cache line handling, reducing latency while ensuring accuracy through deliberate bit placement.
Data Source
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AI summary
Mechanisms are provided for efficient storage of meta-bits within a system memory. The mechanisms combine an L/G bit and an SUE bit to form meta-bits. The mechanisms then determine the local/global state of a cache line on the first cycle of data. The mechanisms forward the data to the requesting cache, and the requesting cache may reissue the request globally based on the local/global state of the cache line. The mechanisms then determine the special uncorrectable error state of the cache line on the second or subsequent cycle of data. The mechanisms perform error processing regardless of whether the request was reissued globally.