Load Instruction Replay Scheduling in Out-of-Order Microprocessors
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Solution Overview
Problem
Existing microprocessors face challenges with high power consumption and complex design due to dynamic scheduling of instructions, particularly with load instructions having long latencies and requiring speculative execution, which complicates the execution pipeline.
Innovation Solution
A microprocessor design employing a time counter for static scheduling of instructions, allowing for out-of-order execution based on a time count, with mechanisms for replaying instructions when source operands become invalid, using a time-resource matrix to manage resource availability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If dynamic scheduling of instructions is used in out-of-order superscalar microprocessors, then instruction execution flexibility and performance are improved, but power consumption and design complexity increase significantly
Solution Approach 1:
The processor is divided into in-order and out-of-order execution domains. The in-order execution unit handles simple instructions sequentially, while the out-of-order execution unit handles complex instructions with dependencies. This segmentation allows each unit to be optimized independently, reducing overall complexity while maintaining high performance.
Solution Approach 2:
Instructions are pre-scheduled with static timing information before execution. The scheduler determines the execution order in advance based on instruction dependencies and resource availability, allowing the execution units to operate more predictably and reducing the complexity of dynamic scheduling decisions during runtime.
2Productivity
If load instructions are executed with high priority to handle long latency, then instruction throughput is improved, but power consumption increases due to frequent replaying
Solution Approach 1:
The processor implements a load instruction replay mechanism with feedback control. When a load instruction completes (either hit or miss), the status is fed back to the scheduler, which then determines whether dependent instructions should be replayed. This feedback mechanism allows the system to minimize replaying by only replaying when necessary, reducing power consumption while maintaining throughput.
Solution Approach 2:
The processor dynamically adjusts the replay threshold parameter based on cache hit rate and workload characteristics. When cache performance is good, the replay threshold is raised to reduce replay frequency. When cache performance degrades, the threshold is lowered to maintain throughput. This parameter adaptation optimizes the balance between throughput and power consumption.
3Speed
If speculative execution is implemented to handle load latencies, then execution speed is improved, but design complexity and power consumption increase
Solution Approach 1:
The processor implements dynamic speculative execution where the level of speculation is adjusted based on runtime conditions. When cache hit rates are high, speculative execution is minimized. When dependencies would otherwise cause stalls, speculation is enabled. This dynamic approach maintains high execution speed while avoiding the constant overhead of full speculative execution, reducing complexity.
4Reliability
If data cache miss handling is implemented with full replay mechanism, then correctness is maintained, but performance degradation occurs due to execution stalls
Solution Approach 1:
Instead of replaying all instructions after a cache miss, the processor implements partial replay by identifying and replaying only the subset of instructions that are dependent on the missed load. This selective replay approach maintains execution correctness while minimizing performance degradation from stalls.
Data Source
AI summary
A processor includes a time counter and a time-resource matrix and provides a method for statically dispatching load/store instructions if the required resources are available based on data stored in the time-resource matrix, and wherein execution times for the load/store instructions use a time count from the time counter to specify when the load/store instructions may be provided to a load-store unit pipeline. The execution times of the load instruction is based on the data cache hit latency time. A delay of the load/store instruction causes the load/store instruction to be replayed with known or estimated time. A load-store unit employs multiple bank queues to access multiple data banks of a data cache.