Reservation Station Segmentation for Reduced Readiness Calculations
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Solution Overview
Problem
Increasing the size of reservation stations in processors to enhance instruction-level parallelism and memory-level parallelism is hindered by performance overhead due to capacitive and drive loads on wakeup signals, which affects clock frequency and area usage.
Innovation Solution
Implementing a dual scheduling pool system with a first scheduling pool for instructions benefiting from fast wakeup and a second pool for those that do not, allowing for efficient resource allocation and reduced silicon area usage by conserving space in the first pool.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the size of the reservation station is increased to enhance instruction-level parallelism and memory-level parallelism, then the ability to exploit ILP and MLP is improved, but the capacitive and drive loads on wakeup signals increase, affecting clock frequency and overall processor performance
Solution Approach 1:
The reservation station is divided into two separate scheduling pools: a first scheduling pool for instructions that benefit from single-cycle wakeup and a second scheduling pool for instructions that do not require fast wakeup. This segmentation allows the system to maintain a larger total pool size for improved ILP while limiting the capacitive load on critical wakeup paths to only the necessary subset of instructions.
Solution Approach 2:
Different parts of the reservation station are given different properties: the first scheduling pool is optimized for fast single-cycle wakeup with lower capacitive load, while the second scheduling pool accepts longer wakeup latency. This local differentiation allows each pool to be optimized for its specific function, resolving the contradiction between overall pool size and critical path speed.
2Productivity
If the size of the reservation station is increased to support more instructions, then the window of selectable instructions is enlarged, but the area usage and silicon footprint increase
Solution Approach 1:
By segmenting the reservation station into two pools with different wakeup characteristics, the system can pack more instructions into the total structure. The second pool can use less area per instruction since it does not require the complex single-cycle wakeup logic, allowing overall area efficiency while maintaining large instruction window size.
3Speed
If individual reservation stations are created for separate functional units to reduce wakeup load, then the critical path performance is improved, but achieving sufficient instruction-level parallelism becomes difficult when there is significant imbalance in the number of various types of instructions
Solution Approach 1:
The first scheduling pool serves a dual function: it acts as a fast wakeup pool for time-critical instructions and also serves as a shared resource that can accept instructions from any functional unit. The common instruction picker can select from both pools, providing universal access while maintaining fast wakeup paths for instructions that need them, thus resolving the contradiction between specialized performance and general parallelism.
Data Source
AI summary
In certain aspects of the disclosure, an apparatus comprises a first scheduling pool associated with a first minimum scheduling latency and a second scheduling pool associated with a second minimum scheduling latency, the second minimum scheduling latency greater than the first minimum scheduling latency. A common instruction picker is coupled to both the first scheduling pool and the second scheduling pool. The common instruction picker may be configured to select a first instruction from the first scheduling pool and a second instruction from the second scheduling pool, and then choose either the first instruction or second instruction for dispatch according to a picking policy.