Restartable Sequence Memory Barrier Elision
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Solution Overview
Problem
Existing computer systems face significant overhead due to memory ordering requirements in multi-threaded workloads, particularly when threads execute on different processors, leading to inefficient use of CPU cycles.
Innovation Solution
The system employs a method where a first thread executes a restartable sequence and, upon detection that a second thread communicating with it has been preempted, the first thread restarts the sequence before reaching a memory barrier, thereby avoiding unnecessary memory barriers when threads run on the same physical CPU.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If memory barriers are used to ensure memory ordering in multi-threaded workloads, then data integrity is maintained, but CPU cycle efficiency deteriorates due to computationally expensive barrier operations
Solution Approach 1:
The patent applies local quality by differentiating memory ordering requirements based on the physical processor relationship between threads. When threads execute on the same physical processor, relaxed ordering is safe and applied. When threads execute on different physical processors, full ordering with memory barriers is applied. This localized approach to memory ordering ensures data integrity where needed while eliminating unnecessary barriers where threads share the same physical CPU, thus resolving the contradiction between reliability and productivity.
2Reliability
If memory barriers are always executed to maintain memory ordering, then correct data access is ensured across processors, but system performance deteriorates due to overhead
Solution Approach 1:
The patent implements dynamics by making memory barrier execution conditional rather than static. The system dynamically determines whether to execute memory barriers based on real-time information about thread placement on physical processors. Threads check their physical processor ID and compare it with the target thread's physical processor ID, executing barriers only when necessary. This dynamic approach reduces execution time by eliminating redundant barriers while maintaining memory ordering correctness when required.
3Reliability
If restartable sequences are used for updates, then atomicity is achieved, but complexity increases due to preemption handling and memory barrier requirements
Solution Approach 1:
The patent applies preliminary action by having threads check their physical processor relationship with the target thread at the beginning of the restartable sequence, before executing update operations. This early determination of whether memory barriers are needed simplifies the overall sequence management by establishing the ordering requirements upfront. The thread stores the target's physical processor ID and compares it with its own, deciding on barrier execution before the actual update, thus reducing the complexity of managing atomicity while maintaining it.
Data Source
AI summary
A system includes a memory, at least one physical processor in communication with the memory, and a plurality of threads executing on the at least one physical processor. A first thread of the plurality of threads is configured to execute a plurality of instructions that includes a restartable sequence. Responsive to a different second thread in communication with the first thread being pre-empted while the first thread is executing the restartable sequence, the first thread is configured to restart the restartable sequence prior to reaching a memory barrier.