Cache Line Invalidation for Reduced Memory Write Latency
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Solution Overview
Problem
In modern computer systems with distributed cache memories, the existing cache coherency protocols require writers to complete bulk transfers before readers can access data, leading to increased latency and missed opportunities for parallel processing.
Innovation Solution
The system allows readers to access cache lines that have already been written by the writer while the writer is still completing the bulk transfer, by prefetching sole ownership of cache lines and relinquishing control to the reader as soon as the data is written, enabling parallelization of reading and writing operations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the writer completes all bulk transfers before the reader can access data, then data consistency is maintained, but the observed latency increases and parallel processing opportunities are lost
Solution Approach 1:
The system performs preliminary invalidation of cache lines before the bulk transfer is complete. The writer invalidates individual cache lines as they are written, allowing readers to access them sooner without waiting for the entire bulk transfer to complete. This preliminary action on individual cache lines resolves the contradiction by enabling early reader access while maintaining consistency through controlled invalidation.
Solution Approach 2:
The bulk transfer operation is segmented into individual cache line operations. Instead of treating the bulk transfer as a single atomic operation that must complete before reader access, the system processes and invalidates cache lines individually. This segmentation allows readers to access completed cache lines while the writer continues processing remaining cache lines, reducing observed latency while maintaining data consistency through selective invalidation.
2Reliability
If the writer waits for all writes to complete before notifying the reader, then data integrity is ensured, but productivity decreases due to serialization of operations
Solution Approach 1:
The writer performs preliminary invalidation of cache lines as they are completed, rather than waiting for all writes to finish. This allows readers to begin accessing data earlier in the transfer process, enabling parallel processing between writer and reader operations. Data integrity is maintained through the invalidation mechanism that prevents readers from accessing stale data.
Solution Approach 2:
The system enables continuous useful action by allowing the reader to operate in parallel with the writer. As the writer completes individual cache line writes and invalidates them, the reader can continuously access and process the invalidated cache lines without idle waiting periods. This continuity eliminates serialization delays and improves overall operation throughput while maintaining data integrity through controlled invalidation.
3Stability of the object's composition
If the system enforces strict ordering of operations, then coherence is maintained, but the critical path latency increases beyond minimums
Solution Approach 1:
The system segments the bulk transfer into individual cache line operations with independent invalidation. Instead of enforcing a single strict ordering for the entire bulk transfer, each cache line can be invalidated and made accessible to readers independently as soon as its write is complete. This segmentation maintains cache coherence through individual invalidation while reducing critical path latency by eliminating unnecessary serialization.
Solution Approach 2:
The system introduces dynamic behavior where the coherence protocol adapts to the actual state of cache line writes. Rather than following a static, predetermined ordering, the writer dynamically invalidates cache lines as they become ready, allowing readers to access them in real-time. This dynamic approach maintains coherence while minimizing latency by responding to actual write completion events rather than following a rigid sequence.
Data Source
AI summary
A system including: a reader; a writer; and a shared memory shared by the reader and the writer, wherein the writer is configured to: specify, in the shared memory, first and second cache lines as unsafe to read; prefetch sole ownership of the first and second cache lines; specify, after the prefetching, that the first and second prefetched cache lines are safe to read; write data to the first prefetched cache line in the shared memory; and in response to completing writing data to the first prefetched cache, relinquish control of the first prefetched cache line to a reader.