Non-blocking algorithm: Difference between revisions

Revision as of 09:30, 18 January 2023

Revision as of 02:41, 29 January 2023

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This property is critical for real-time systems and is always nice to have as long as the performance cost is not too high.

It was shown in the 1980s<ref name=imp>{{cite conference |last=Herlihy |first=Maurice P. |conference=Proc. 7th Annual ACM Symp. on Principles of Distributed Computing |isbn=0-89791-277-2 |pages=276–290 |doi=10.1145/62546.62593 |title=Impossibility and universality results for wait-free synchronization |year=1988|doi-access=free }}</ref> that all algorithms can be implemented wait-free, and many transformations from serial code, called ''universal constructions'', have been demonstrated. However, the resulting performance does not in general match even naïve blocking designs. Several papers have since improved the performance of universal constructions, but still, their performance is far below blocking designs.

Several papers have investigated the difficulty of creating wait-free algorithms. For example, it has been shown<ref name=cond-sync>{{cite conference |last1=Fich |first1=Faith|author1-link=Faith Ellen |last2=Hendler |first2=Danny |last3=Shavit |first3=Nir |conference=Proc. 23rd Annual ACM Symp.on Principles of Distributed Computing (PODC) |year=2004 |isbn=1-58113-802-4 |pages=80–87 |doi=10.1145/1011767.1011780 |title=On the inherent weakness of conditional synchronization primitives}}</ref> that the widely available atomic ''conditional'' primitives, [[Compare-and-swap|CAS]] and [[Load-link/store-conditional|LL/SC]], cannot provide starvation-free implementations of many common data structures without memory costs growing linearly in the number of threads.

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But in practice these lower bounds do not present a real barrier as spending a cache line or exclusive reservation granule (up to 2&nbsp;KB on ARM) of store per thread in the shared memory is not considered too costly for practical systems (typically the amount of store logically required is a word, but physically CAS operations on the same cache line will collide, and LL/SC operations in the same exclusive reservation granule will collide, so the amount of store physically required{{citation needed|date=June 2014}} is greater).

Wait-free algorithms were rare until 2011, both in research and in practice. However, in 2011 Kogan and [[Erez Petrank|Petrank]]<ref name=wf-queue>{{cite conference |last1=Kogan |first1=Alex |last2=Petrank |first2=Erez |conference=Proc. 16th ACM SIGPLAN Symp. on Principles and Practice of Parallel Programming (PPOPP) |year=2011 |isbn=978-1-4503-0119-0 |pages=223–234 |doi=10.1145/1941553.1941585 |title=Wait-free queues with multiple enqueuers and dequeuers|url=http://www.cs.technion.ac.il/~erez/Papers/wfquque-ppopp.pdf}}</ref> presented a wait-free queue building on the [[Compare-and-swap|CAS]] primitive, generally available on common hardware. Their construction expanded the lock-free queue of Michael and Scott,<ref name=lf-queue>{{cite conference |last1=Michael |first1=Maged |last2=Scott |first2=Michael |conference=Proc. 15th Annual ACM Symp. on Principles of Distributed Computing (PODC) |year=1996 |isbn=0-89791-800-2 |pages=267–275 |doi=10.1145/248052.248106 |title=Simple, Fast, and Practical Non-Blocking and Blocking Concurrent Queue Algorithms|doi-access=free }}</ref> which is an efficient queue often used in practice. A follow-up paper by Kogan and Petrank<ref name=wf-fpsp>{{cite conference |last1=Kogan |first1=Alex |last2=Petrank |first2=Erez |conference=Proc. 17th ACM SIGPLAN Symp. on Principles and Practice of Parallel Programming (PPOPP) |year=2012 |isbn=978-1-4503-1160-1 |pages=141–150 |doi=10.1145/2145816.2145835 |title=A method for creating fast wait-free data structures}}</ref> provided a method for making wait-free algorithms fast and used this method to make the wait-free queue practically as fast as its lock-free counterpart. A subsequent paper by Timnat and Petrank<ref name=wf-simulation>{{cite conference |last1=Timnat |first1=Shahar |last2=Petrank |first2=Erez |conference=Proc. 17th ACM SIGPLAN Symp. on Principles and Practice of Parallel Programming (PPOPP) |year=2014 | isbn=978-1-4503-2656-8 | pages = 357–368 | doi=10.1145/2692916.2555261 | title= A Practical Wait-Free Simulation for Lock-Free Data Structures}}</ref> provided an automatic mechanism for generating wait-free data structures from lock-free ones. Thus, wait-free implementations are now available for many data-structures.

== Lock-freedom ==