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We present a randomized approach for wait-free locks with strong bounds on time and fairness in a context in which any process can be arbitrarily delayed. Our approach supports a tryLock operation that is given a set of locks, and code to run when all the locks are acquired. A tryLock operation, or attempt, may fail if there is contention on the locks, in which case the code is not run. Given an upper bound κ known to the algorithm on the point contention of any lock, and an upper bound L on the number of locks in a try- Lock’s set, a tryLock will succeed in acquiring its locks and running the code with probability at least 1/(κL). It is thus fair. Furthermore, if the maximum step complexity for the code in any lock is T , the attempt will take O(κ^2L^2T ) steps, regardless of whether it succeeds or fails. The attempts are independent, thus if the tryLock is repeat- edly retried on failure, it will succeed in O(κ^3L^3T ) expected steps, and with high probability in not much more.
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Lock-free locks revisited
This paper presents a new and practical approach to lock-free locks based on helping, which allows the user to write code using fine-grained locks, but run it in a lock-free manner. Although lock-free locks have been suggested in the past, they are widely viewed as impractical, have some key limitations, and, as far as we know, have never been implemented. The paper presents some key techniques that make lock-free locks practical and more general. The most important technique is an approach to idempotence—i.e. making code that runs multiple times appear as if it ran once. The idea is based on using a shared log among processes running the same protected code. Importantly, the approach can be library based, requiring very little if any change to standard code—code just needs to use the idempotent versions of memory operations (load, store, LL/SC, allocation, free). We have implemented a C++ library called Flock based on the ideas. Flock allows lock-based data structures to run in either lock-free or blocking (traditional locks) mode. We implemented a variety of tree and list-based data structures with Flock and compare the performance of the lock-free and blocking modes under a variety of workloads. The lock-free mode is almost as fast as blocking mode under almost all workloads, and significantly faster when threads are oversubscribed (more threads than processors). We also compare with several existing lock-based and lock-free alternatives.
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- PAR ID:
- 10416516
- Publisher / Repository:
- ACM
- Date Published:
- Journal Name:
- Proceedings of the 27th ACM SIGPLAN Symposium on Principles and Practice of Parallel Programming
- ISBN:
- 9781450392044
- Page Range / eLocation ID:
- 278 to 293
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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