Thread Channel Deactivation for Instruction Cache Misses
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Solution Overview
Problem
Computer processors face inefficiencies in thread scheduling, particularly in graphics processors, where saturating datapath units can lead to performance stalls and increased power consumption due to varied instruction latencies caused by cache misses, necessitating effective management of thread allocation and cache operations.
Innovation Solution
A two-stage scheduling technique that arbitrates among threads and channels to assign instructions to execution pipelines, utilizing compiler hints and back-pressure information to prioritize threads and manage cache operations, including deactivation of threads based on instruction cache misses to prevent continuous stalls.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If threads are continuously allocated to saturate datapath units, then utilization of execution resources is improved, but performance stalls occur due to instruction cache misses
Solution Approach 1:
The system dynamically adjusts thread allocation to datapath units based on real-time cache hit/miss detection. When a cache miss is detected, the thread is deactivated from the channel, allowing the system to adapt resource allocation dynamically rather than maintaining static saturation, thereby preventing performance stalls while optimizing utilization.
Solution Approach 2:
The system implements feedback mechanisms by monitoring cache access outcomes (hit/miss) and using this information to control thread activation/deactivation. This feedback loop allows the system to respond to cache performance metrics and adjust thread allocation accordingly, preventing continuous stalls caused by cache misses.
2Productivity
If threads are kept active in channels to maintain execution flow, then forward progress is improved, but power consumption increases due to continuous operation
Solution Approach 1:
Instead of continuous thread activation, the system employs periodic activation based on cache performance. Threads are activated only when cache hits occur, and deactivated when cache misses are detected. This periodic activation pattern maintains execution flow continuity when possible while reducing power consumption by avoiding continuous operation during cache miss conditions.
Solution Approach 2:
The system discards (deactivates) threads from channels when cache misses occur to save power, and recovers (reactivates) them when cache performance improves. This discarding and recovering mechanism allows the system to balance power consumption with execution flow continuity, maintaining productivity when cache performance is good while conserving energy during cache miss periods.
3Speed
If cache operations are optimized for speed, then instruction fetch latency is reduced, but chip area increases due to larger cache structures
Solution Approach 1:
The system uses self-service by leveraging existing cache structures and optimizing their utilization through intelligent thread management. Rather than increasing cache size to improve speed, the system makes the existing cache more effective by detecting misses and deactivating threads, thereby achieving speed optimization without additional chip area investment.
Solution Approach 2:
The system changes the operational parameters of thread execution based on cache performance. By monitoring cache hit/miss ratios and adjusting thread activation states accordingly, the system optimizes the effective use of the cache structure without physical modifications. This parameter-based optimization achieves improved instruction fetch efficiency while maintaining the original cache area.
Data Source
AI summary
Techniques are disclosed relating to instruction scheduling in the context of instruction cache misses. In some embodiments, first-stage scheduler circuitry is configured to assign threads to channels and second-stage scheduler circuitry is configured to assign an operation from a given channel to a given execution pipeline based on decode of an operation for that channel. In some embodiments, thread replacement circuitry is configured to, in response to an instruction cache miss for an operation of a first thread assigned to a first channel, deactivate the first thread from the first channel.