Lock-Free FIFO Queue Elimination Structure
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Solution Overview
Problem
Existing concurrent FIFO queue implementations face scalability issues under high loads due to bottlenecks at the head and tail of the queue, and previous lock-free solutions are not linearizable or perform poorly under low loads.
Innovation Solution
A lock-free, linearizable, and scalable FIFO queue implementation using an elimination structure that allows enqueue and dequeue operations to pair up without synchronizing on centralized data, dynamically determining access to a central queue or elimination based on system load, utilizing a collision array for operation pairing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If lock-free FIFO queue implementation uses centralized synchronization at head and tail (MS-queue algorithm), then enqueue and dequeue operations can complete without interfering with each other when queue is nonempty, but as number of concurrent threads increases, head and tail become bottlenecks and performance suffers
Solution Approach 1:
The queue is divided into multiple segments, each with its own head and tail pointers. Multiple threads can operate on different segments simultaneously without centralized synchronization, eliminating the single-point bottleneck while maintaining FIFO semantics through segment ordering.
Solution Approach 2:
An intermediary elimination structure is introduced that allows enqueue and dequeue operations to pair up and eliminate each other without directly synchronizing on centralized head/tail data. This intermediary mechanism enables operations to complete independently while maintaining correctness.
2Productivity
If elimination technique is used to allow enqueue and dequeue operations to pair up without centralized data synchronization, then scalability is improved, but implementing elimination for FIFO queues is substantially more difficult than for stacks due to ordering requirements
Solution Approach 1:
Enqueue operations preliminarily install entries in the elimination structure before actually adding to the central queue. This preliminary action allows dequeue operations to find and pair with corresponding enqueue operations in the elimination structure, enabling elimination without complex coordination.
Solution Approach 2:
The elimination structure maintains copies of enqueue operation information (pointers, values, metadata) separate from the central queue structure. This copying allows dequeue operations to eliminate against the copy without affecting the central queue, simplifying the elimination mechanism while maintaining FIFO semantics.
3Productivity
If dynamic determination of accessing central queue or attempting elimination is implemented based on system load heuristic, then performance is improved across varying loads, but additional overhead is introduced for deciding whether to access queue or eliminate
Solution Approach 1:
The system automatically monitors its own load conditions and dynamically adjusts operation routing without external intervention. The load heuristic is implemented as a self-service mechanism that threads use to decide whether to access the central queue or attempt elimination, eliminating the need for external load management.
Solution Approach 2:
The system dynamically changes operational parameters (routing decisions) based on load conditions. When load is low, operations route to the central queue for direct access; when load is high, operations route to the elimination structure. This parameter change optimizes performance across varying loads without requiring system redesign.
Data Source
AI summary
A scalable first-in-first-out queue implementation adjusts to load on a host system. The scalable FIFO queue implementation is lock-free and linearizable, and scales to large numbers of threads. The FIFO queue implementation includes a central queue and an elimination structure for eliminating enqueue-dequeue operation pairs. The elimination mechanism tracks enqueue operations and/or dequeue operations and eliminates without synchronizing on the FIFO queue implementation.