Adaptive Queued Locking for Speculative Execution Conflict Control
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Solution Overview
Problem
Transactional memory systems face bottlenecks due to high conflict rates between threads attempting to access shared memory, leading to frequent aborts and reduced throughput, as existing locking techniques either allow all threads to execute speculatively or restrict to a single thread, curbing system performance.
Innovation Solution
An adaptive queued locking (AQL) manager dynamically controls the number of threads allowed to execute in a critical section based on a success rate, adjusting a quota to balance concurrent execution and conflict prevention, enabling fine-grained control and throttling of speculative execution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If all threads are allowed to execute speculatively in parallel, then throughput is improved, but conflict rates increase leading to frequent aborts
Solution Approach 1:
The system dynamically adjusts the degree of parallelism by transitioning between transactional mode (higher parallelism) and exclusive mode (lower parallelism) based on real-time conflict detection. When conflicts are detected, the system reduces parallelism to prevent further conflicts, and when no conflicts occur, it increases parallelism to improve throughput.
Solution Approach 2:
The system changes the execution mode parameter from speculative/parallel to exclusive/serialized based on conflict conditions. This parameter change allows the system to adapt the number of concurrently executing threads, resolving the contradiction between throughput and transaction success rate.
2Reliability
If a single thread is restricted to execute in the critical section, then conflict rates are reduced, but throughput deteriorates
Solution Approach 1:
Instead of permanently restricting to a single thread, the system dynamically adjusts the number of allowed concurrent threads based on conflict conditions. In exclusive mode, only one thread executes at a time, but in transactional mode, multiple threads can execute in parallel, thus resolving the contradiction between reducing conflicts and maintaining throughput.
3Reliability
If locks are used to control access to critical sections, then conflict prevention is improved, but system complexity increases
Solution Approach 1:
The patent extracts the locking mechanism from the critical section code, implementing it as a separate manager component (AQL manager). This separation reduces the complexity within the critical section itself while maintaining conflict prevention capabilities through the dedicated locking manager that handles queue management and thread coordination.
Solution Approach 2:
The AQL manager acts as an intermediary between threads and the critical section, managing the queue of waiting threads and coordinating access. This intermediary approach simplifies the locking mechanism by centralizing control logic outside the critical section, reducing the complexity burden on individual threads.
4Productivity
If speculative execution is enabled for multiple threads, then system performance is improved, but abort frequency increases
Solution Approach 1:
The system implements feedback through conflict detection mechanisms that monitor speculative execution outcomes. When conflicts are detected during speculative execution, the system aborts the transactions and transitions to exclusive mode, preventing further wasted speculation. This feedback loop resolves the contradiction by adapting the level of speculative execution based on actual conflict occurrence.
Data Source
AI summary
Adaptive queued locking for control of speculative execution is disclosed. An example apparatus includes a queue controller to control removal of threads from a queue that contains threads waiting to be permitted to speculatively execute in a critical section of a multi-threaded program. The apparatus also includes a first thread associated with a head node of the queue. The queue controller controls the removal of threads from the queue in response to operations performed by the first thread. In addition, an adjuster to change a number of threads permitted to speculatively execute based on a rate of threads currently speculatively executing transactions in the critical section is included.