Reservation Station Replay Prevention for X86 Load Instructions
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Solution Overview
Problem
Out-of-order processors experience significant power wastage due to frequent load replays when accessing resources like fuse arrays and L2 caches, which are shared among cores, leading to increased power utilization and thermal issues.
Innovation Solution
The implementation of a mechanism that includes reservation stations to detect specified load micro instructions directed to non-core resources, stalling younger micro instructions until the load is completed, thereby preventing replay events and reducing power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If out-of-order execution is used to improve throughput, then instruction execution speed is improved, but load replay frequency increases causing power wastage
Solution Approach 1:
The system performs preliminary detection of load instructions that access shared resources (fuse arrays, L2 caches) before they are dispatched for execution. By identifying these instructions in advance and stalling younger dependent instructions, the system prevents replay events before they occur, thereby eliminating the energy waste associated with re-executing instructions that would fail due to resource unavailability.
2Productivity
If load instructions are dispatched aggressively to improve productivity, then throughput is improved, but replay events increase leading to power consumption
Solution Approach 1:
The system implements feedback by detecting the completion status of load instructions accessing shared resources and using this information to control the dispatch of younger dependent instructions. When a load instruction completes successfully, the system allows dependent instructions to proceed; when it fails or is stalled, the system prevents dispatch of dependent instructions that would result in replays, thereby reducing energy consumption while maintaining throughput.
3Adaptability or versatility
If shared resources are accessed by multiple cores, then resource utilization is improved, but load replay frequency increases causing thermal issues
Solution Approach 1:
The system performs preliminary detection of load instructions accessing shared resources before dispatch. By identifying these instructions in advance and stalling younger dependent instructions, the system prevents replay events before they occur, thereby reducing the thermal impact of repeated executions while maintaining the benefits of shared resource access across multiple cores.
Data Source
AI summary
An apparatus including first and second reservation stations. The first reservation station dispatches a load micro instruction, and indicates on a hold bus if the load micro instruction is a specified load micro instruction directed to retrieve an operand from a prescribed resource other than on-core cache memory, where the specified load instruction comprises a load instruction resulting from execution of an x86 special bus cycle. The second reservation station is coupled to the hold bus, and dispatches one or more younger micro instructions therein that depend on the load micro instruction for execution after a number of clock cycles following dispatch of the first load micro instruction, and if it is indicated on the hold bus that the load micro instruction is the specified load micro instruction, the second reservation station is configured to stall dispatch of the one or more younger micro instructions until the load micro instruction has received the operand, and is configured to preclude assertion of any indications that would otherwise result in a replay event.