Tree-Based Thread Management for Divergent Path Execution
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Solution Overview
Problem
Conventional thread management techniques in parallel processing architectures serialize divergent paths, leading to undesirable consequences such as program deadlocks and inefficient execution of instructions that depend on conditional conditions across threads.
Innovation Solution
A method employing a tree divergence table to manage threads, where divergent execution paths are handled by creating child nodes with separate program counters and instructions, allowing concurrent execution of divergent paths within a warp, thereby avoiding serialization and enabling parallel execution of divergent threads.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If divergent paths are serialized in conventional thread management, then program execution control is simplified, but execution speed deteriorates and deadlocks may occur
Solution Approach 1:
The patent segments the single warp execution context into multiple independent thread groups, each capable of following different execution paths simultaneously. Instead of serializing all threads in a warp through a single program counter, the system creates separate execution contexts for divergent paths, allowing parallel execution of multiple instruction streams within the same warp.
Solution Approach 2:
The patent adds a dimension of parallelism by enabling multiple program counters to exist simultaneously within a single warp. Rather than transitioning between paths sequentially in time, the system allows multiple paths to exist concurrently in parallel, effectively moving from a 1D sequential execution model to a 2D parallel execution model.
2Productivity
If divergent paths are executed in parallel, then execution speed improves, but thread management complexity increases
Solution Approach 1:
The patent implements a hierarchical nested structure where thread groups are nested within warps, and each thread group has its own program counter nested within the warp-level execution context. This nesting allows the system to manage complexity at multiple levels simultaneously, with the warp level handling high-level coordination and thread group levels handling specific path execution.
Solution Approach 2:
The patent introduces intermediate control structures that mediate between the simple serialized model and the complex parallel model. These intermediaries manage the coordination between multiple parallel thread groups, handling synchronization and convergence points without requiring full complexity of managing all possible path combinations explicitly.
3Device complexity
If a single program counter is used per warp, then hardware implementation is simplified, but conditional instructions cause deadlocks
Solution Approach 1:
The patent merges multiple program counters into a unified warp execution context, allowing the hardware to treat multiple independent execution paths as a single coordinated entity. This merging enables conditional instructions to be evaluated and executed across all thread groups simultaneously rather than causing serialization or deadlock, while maintaining relatively simple hardware implementation.
Data Source
AI summary
In one embodiment of the present invention, a streaming multiprocessor (SM) uses a tree of nodes to manage threads. Each node specifies a set of active threads and a program counter. Upon encountering a conditional instruction that causes an execution path to diverge, the SM creates child nodes corresponding to each of the divergent execution paths. Based on the conditional instruction, the SM assigns each active thread included in the parent node to at most one child node, and the SM temporarily discontinues executing instructions specified by the parent node. Instead, the SM concurrently executes instructions specified by the child nodes. After all the divergent paths reconverge to the parent path, the SM resumes executing instructions specified by the parent node. Advantageously, the disclosed techniques enable the SM to execute divergent paths in parallel, thereby reducing undesirable program behavior associated with conventional techniques that serialize divergent paths across thread groups.