SIMT Thread Group Divergence Handling via Off-Path Program Counter Checks
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Solution Overview
Problem
In Single Instruction Multiple Thread (SIMT) processing systems, the mechanisms for detecting and handling thread divergence and re-convergence become complex and costly in terms of circuit area and performance, as they require checks before resuming execution.
Innovation Solution
An apparatus and method that utilize scheduling circuitry to select thread groups, with thread processing circuitry executing threads based on a common program counter, and program counter checking circuitry that performs updates off the critical path using register storage to determine and manage divergence and convergence, allowing other thread groups to be processed concurrently.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If program counter checking mechanisms are implemented to detect thread divergence and re-convergence in SIMT processing, then thread execution accuracy is improved, but circuit area increases and performance deteriorates due to checks required before resuming execution
Solution Approach 1:
The system segments thread groups into multiple warp groups, where each warp group contains a subset of threads. Program counter checking is performed independently for each warp group rather than for the entire thread group, reducing the complexity and area of checking circuitry while maintaining execution accuracy.
Solution Approach 2:
The patent introduces a hierarchical organization dimension by dividing threads into warps and warps into warp groups. This multi-level segmentation creates additional organizational dimensions that reduce the burden on program counter checking mechanisms at any single level.
2Reliability
If program counter checking mechanisms are implemented to detect thread divergence and re-convergence in SIMT processing, then thread execution accuracy is improved, but system performance deteriorates due to checks required before resuming execution
Solution Approach 1:
By segmenting thread groups into smaller warp groups with independent program counter checking, the system enables parallel checking operations that reduce overall check completion time, thereby improving system performance while maintaining execution accuracy.
Solution Approach 2:
The patent enables continuous execution of other thread groups while program counter checking is performed for the current thread group. This overlapping of operations ensures that the processing unit remains productive and avoids idle cycles waiting for check completion.
3Measurement precision
If program counter checking is performed for all threads in a thread group, then thread divergence detection accuracy is improved, but the time required to complete checks increases
Solution Approach 1:
The patent divides the thread group into multiple warp groups, allowing program counter checking to be performed on smaller subsets of threads simultaneously. This segmentation maintains detection accuracy for each subset while reducing the total time required to complete all checks through parallel processing.
Solution Approach 2:
The system performs program counter checking on warp groups independently rather than waiting to check all threads in a thread group before proceeding. This partial action approach allows the system to make progress on multiple fronts simultaneously, reducing overall check completion time.
Data Source
AI summary
An apparatus and method are provided for executing thread groups. The apparatus comprises scheduling circuitry for selecting for execution a first thread group from a plurality of thread groups, and thread processing circuitry that is responsive to the scheduling circuitry to execute active threads of the first thread group in dependence on a common program counter shared between the active threads. In response to an exit event occurring for the first thread group, the thread processing circuitry determines whether a program counter check condition is present, and this can be used to trigger program counter checking circuitry to perform a program counter check operation to update the common program counter and an active thread indication for the first thread group. The thread processing circuitry is provided with register storage in which program counter information for each thread of the first thread group can be stored, and the program counter checking circuitry is arranged to have access to that register storage when performing the program counter check operation. Further, the scheduling circuitry is arranged to select, for execution by the thread processing circuitry, a different thread group whilst awaiting performance of the program counter check operation by the program counter checking circuitry for the first thread group. This provides an area efficient mechanism for handling divergence and re-convergence of threads within thread groups, in a manner that avoids impacting performance.