Per-thread Scoreboard Segmentation Eliminates Alias Stall Delays
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Solution Overview
Problem
Conventional scoreboard mechanisms in computing platforms are limited by hardware constraints, leading to scoreboard aliasing issues in interleaved shard execution environments, which result in inefficient warp occupancy and execution performance.
Innovation Solution
The proposed solution involves implementing per-thread scoreboards using spills and refills in per-thread registers, along with blocking dependency barriers, to mitigate scoreboard aliasing without requiring additional hardware resources. This approach maintains per-thread counters of outstanding memory accesses and ensures that each shard waits only for its own memory accesses to complete, rather than relying on the completion of other shards.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a shared scoreboard is used among multiple divergent shards in a warp, then hardware resources are conserved, but scoreboard aliasing occurs causing false dependencies between instructions in different shards
Solution Approach 1:
The shared scoreboard is segmented into per-shard scoreboard entries, where each shard has its own dedicated scoreboard counter. This segmentation eliminates scoreboard aliasing by ensuring that each shard tracks only its own instruction dependencies, preventing false dependencies between divergent shards while maintaining efficient resource utilization through shared scoreboard structure.
2Extent of automation
If conventional scoreboarding is implemented automatically in processor hardware, then dependency management is automated, but circuit complexity, layout footprint, and power consumption increase
Solution Approach 1:
The scoreboard structure is designed to serve multiple functions: it automatically manages instruction dependencies through hardware automation, while also supporting per-shard tracking to eliminate aliasing. The unified scoreboard design handles both automatic dependency management and shard-specific tracking without requiring separate dedicated structures, reducing overall circuit complexity while maintaining full automation benefits.
3Device complexity
If a shared scoreboard is used among all threads in a warp, then resource sharing is maximized, but execution performance deteriorates due to unnecessary stalls when independent instructions from different shards complete
Solution Approach 1:
The scoreboard is segmented to provide per-shard tracking within the shared resource framework. Each shard maintains its own scoreboard counter that independently tracks only its issued instructions, allowing shards to proceed without unnecessary stalls when other shards complete their instructions. This segmentation preserves resource sharing efficiency while eliminating performance degradation from false dependencies.
Data Source
AI summary
A process to ameliorate scoreboard aliasing in multi-threaded data processors whereby, in response to executing at least one long-latency instruction in a first thread, a shared hardware scoreboard is incremented. A shared software register is incremented and the shared software register is spilled to a first per-thread register, and execution is switched to a second thread. After execution switches back to the first thread, execution of the first thread is suspended until the shared hardware scoreboard reaches a value at or below a difference between a value in the shared software register and the value spilled into the first per-thread register.