Non-blocking algorithm: Difference between revisions

Revision as of 06:07, 24 September 2024

Revision as of 17:35, 31 October 2024

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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.

But in practiceHowever, these lower bounds do not present a real barrier in practice, 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. (typicallyTypically, 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).{{Clarification needed|date=October 2024|reason=Does this imply that there actually is a barrier in practice?}}

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.