Hybrid Reservation Station for Processor Scheduling
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Solution Overview
Problem
Modern processors face a trade-off between power efficiency and execution throughput, with in-order processors being power-efficient but having low instruction-level parallelism and out-of-order processors improving throughput but consuming more power due to complex scheduling tasks.
Innovation Solution
A hybrid reservation station is introduced, combining in-order and out-of-order structures to efficiently schedule instructions, including in-order queues, a variable latency tracking table, and a sink lane, which reduces power consumption and increases scheduling efficiency by minimizing the number of structures considered for allocation and scheduling decisions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If out-of-order architecture is used to improve execution throughput, then instruction-level parallelism increases, but power consumption increases due to complex scheduling
Solution Approach 1:
The reservation station is segmented into multiple lanes (first plurality of lanes) that can be selectively activated. The scheduler circuit accesses only the head entry of specific lanes based on functional unit readiness, rather than checking all entries. This segmentation allows the processor to activate only the necessary scheduling structures for current workloads, reducing power consumption while maintaining throughput capability.
Solution Approach 2:
The scheduler circuit performs partial scheduling actions by accessing only the head entry of lanes when functional units are ready, rather than maintaining full out-of-order scheduling complexity for all instructions. This partial action approach provides sufficient instruction-level parallelism for many workloads while avoiding the full power overhead of complete out-of-order scheduling.
2Use of energy by moving object
If in-order architecture is used to reduce power consumption, then power efficiency improves, but instruction-level parallelism decreases leading to low execution throughput
Solution Approach 1:
The reservation station implements dynamic lane activation where the scheduler circuit selectively accesses head entries of lanes based on functional unit readiness and instruction dependencies. This dynamic approach allows the system to adapt between in-order and out-of-order scheduling behavior as needed, providing power efficiency when full parallelism isn't required while enabling throughput improvement when instructions are ready for execution.
3Productivity
If complex out-of-order structures are used to schedule instructions, then scheduling efficiency improves, but device complexity increases
Solution Approach 1:
The reservation station is divided into multiple lanes with the scheduler circuit accessing only the head entry of relevant lanes. This segmentation reduces the complexity of scheduling decisions by limiting the scope of evaluation to only those instructions that are ready for execution, rather than managing complex interdependencies across all instructions in the pipeline.
Solution Approach 2:
The invention extracts only the essential scheduling function by having the scheduler circuit access merely the head entry of lanes when functional units are ready. This extraction removes unnecessary complexity from full out-of-order scheduling while retaining the core capability to schedule instructions efficiently based on readiness and dependency.
Data Source
AI summary
In one embodiment, a reservation station of a processor includes: a plurality of first lanes having a plurality of entries to store information for instructions having in-order dependencies; a variable latency tracking table including a second plurality of entries to store information for instructions having a variable latency; and a scheduler circuit to access a head entry of the plurality of first lanes to schedule, for execution on at least one execution unit, at least one instruction from the head entry of at least one of the plurality of first lanes. Other embodiments are described and claimed.