Delayed Cache Writeback Instructions for Manycore Data Sharing
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Solution Overview
Problem
In manycore processors, significant time is lost waiting for cache coherency operations when a requesting core fetches data held in a modified state in another core's cache, leading to performance inefficiencies.
Innovation Solution
Implementing delayed cache writeback instructions that allow speculative writeback of data from a modified state to a shared cache level, enabling faster access by other cores, with instructions like clwb2llc.delayed, clwb2llc.delayed.Iru, and clwb2llc.now, allowing more fine-grained control over cache management.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If immediate cache writeback is performed, then data availability for other cores is improved, but processor performance deteriorates due to unnecessary remote cache accesses and cache coherency waiting
Solution Approach 1:
The patent applies preliminary action by allowing the executing core to speculatively write back cache blocks to the shared cache level before other cores actually need them. The delayed writeback instruction marks cache blocks for future writeback, enabling the system to prepare data in advance without forcing immediate writes, thus avoiding unnecessary remote accesses while maintaining the ability to provide fast data availability when needed.
2Productivity
If cache blocks are kept in modified state, then data sharing efficiency is improved, but cache coherency operations cause significant time loss
Solution Approach 1:
The system performs preliminary action by proactively writing back modified cache blocks to the shared cache level before other cores request them. The delayed writeback mechanism allows the executing core to prepare data in advance, eliminating the need for other cores to wait for cache coherency operations when they need the data, thus reducing time loss while maintaining sharing efficiency.
Solution Approach 2:
The patent implements feedback through performance monitoring that tracks remote-M cache hits and local write hits. This feedback information is used to dynamically adjust the writeback strategy, allowing the system to learn from actual access patterns and optimize future delayed writeback decisions, thereby reducing unnecessary remote accesses while maintaining data sharing efficiency.
3Speed
If speculative writeback is performed, then future data access performance is improved, but unnecessary writeback operations increase device complexity
Solution Approach 1:
The patent applies segmentation by dividing the cache management functionality into distinct components: a delayed writeback instruction for marking blocks, a writeback completion tracker for monitoring, and a performance monitoring unit for analysis. This segmentation allows the complex speculative writeback logic to be broken down into manageable parts, reducing the perceived complexity while enabling improved future data access performance.
Data Source
AI summary
Methods and apparatus relating to one or more delayed cache writeback instructions for improved data sharing in manycore processors are described. In an embodiment, a delayed cache writeback instruction causes a cache block in a modified state in a Level 1 (L1) cache of a first core of a plurality of cores of a multi-core processor to a Modified write back (M.wb) state. The M.wb state causes the cache block to be written back to LLC upon eviction of the cache block from the L1 cache. Other embodiments are also disclosed and claimed.