Non-blocking algorithm: Difference between revisions

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{{Distinguish|non-blocking I/O}}

In [[computer science]], an [[algorithm]] is called '''non-blocking''' if failure or [[Scheduling (computing)|suspension]] of any [[thread (computing)|thread]] cannot cause failure or suspension of another thread;<ref>{{cite book|last1=Göetz|first1=Brian|last2=Peierls|first2=Tim|last3=Bloch|first3=Joshua|last4=Bowbeer|first4=Joseph|last5=Holmes|first5=David|last6=Lea|first6=Doug|title=Java concurrency in practice|date=2006|publisher=Addison-Wesley|___location=Upper Saddle River, NJ|isbn=9780321349606|page=[https://archive.org/details/javaconcurrencyi00goet/page/41 41]|url-access=registration|url=https://archive.org/details/javaconcurrencyi00goet/page/41}}</ref> for some operations, these algorithms provide a useful alternative to traditional [[lock (computer science)|blocking implementations]]. A non-blocking algorithm is '''lock-free''' if there is guaranteed system-wide [[Resource starvation|progress]], and '''wait-free''' if there is also guaranteed per-thread progress. "Non-blocking" was used as a synonym for "lock-free" in the literature until the introduction of obstruction-freedom in 2003.<ref name=obs-free>{{cite conference|last1=Herlihy|first1=M.|last2=Luchangco|first2=V.|last3=Moir|first3=M.|title=Obstruction-Free Synchronization: Double-Ended Queues as an Example|conference=23rd [[International Conference on Distributed Computing Systems]]|year=2003|pages=522|url=httphttps://www.cs.brown.edu/people/mph/HerlihyLM03/main.pdf}}</ref>

The word "non-blocking" was traditionally used to describe [[telecommunications network]]s that could route a connection through a set of relays "without having to re-arrange existing calls"{{Quote without source|date=November 2024}} (see [[Clos network]]). Also, if the telephone exchange "is not defective, it can always make the connection"{{Quote without source|date=November 2024}} (see [[nonblocking minimal spanning switch]]).

== Motivation ==

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Blocking a thread can be undesirable for many reasons. An obvious reason is that while the thread is blocked, it cannot accomplish anything: if the blocked thread had been performing a high-priority or [[real-time computing|real-time]] task, it would be highly undesirable to halt its progress.

Other problems are less obvious. For example, certain interactions between locks can lead to error conditions such as [[Deadlock (computer science)|deadlock]], [[livelock]], and [[priority inversion]]. Using locks also involves a trade-off between coarse-grained locking, which can significantly reduce opportunities for [[parallel computing|parallelism]], and fine-grained locking, which requires more careful design, increases locking overhead and is more prone to bugs.

Unlike blocking algorithms, non-blocking algorithms do not suffer from these downsides, and in addition are safe for use in [[interrupt handler]]s: even though the [[Pre-emptive multitasking|preempted]] thread cannot be resumed, progress is still possible without it. In contrast, global data structures protected by mutual exclusion cannot safely be accessed in an interrupt handler, as the preempted thread may be the one holding the lock—butlock. While this can be rectified easily by masking the interrupt requestrequests during the critical section, this requires the code in the critical section to have bounded (and preferably short) running time, or excessive [[interrupt latency]] may be observed.<ref name="monit">{{cite journal | doi = 10.1145/358818.358824 | url = http://research.microsoft.com/lampson/23-ProcessesInMesa/Abstract.html | title = Experience with Processes and Monitors in Mesa | author = Butler W. Lampson | author-link = Butler W. Lampson |author2=David D. Redell |author2-link=David D. Redell | journal = Communications of the ACM | volume = 23 | issue = 2 | pages = 105–117 |date=February 1980| citeseerx = 10.1.1.142.5765 | s2cid = 1594544 }}</ref>

A lock-free data structure can be used to improve performance.

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Several libraries internally use lock-free techniques,<ref>

[httphttps://libcds.sourceforge.net/ libcds] - C++ library of lock-free containers and safe memory reclamation schema

</ref><ref>

[https://www.liblfds.org/ liblfds] - A library of lock-free data structures, written in C

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[[C++11]] programmers can use <code>std::atomic</code> in <code>&lt;atomic&gt;</code>,

and [[C11 (C standard revision)|C11]] programmers can use <code>&lt;stdatomic.h&gt;</code>,

both of which supply types and functions that tell the [[compiler]] not to re-arrange such instructions, and to insert the appropriate memory barriers.<ref>

Bruce Dawson.

[https://randomascii.wordpress.com/2020/11/29/arm-and-lock-free-programming/ "ARM and Lock-Free Programming"].

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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.

Under reasonable assumptions, Alistarh, Censor-Hillel, and Shavit showed that lock-free algorithms are practically wait-free.<ref name=lf-wf>{{cite conference |last1=Alistarh |first1=Dan |last2=Censor-Hillel |first2=Keren |last3=Shavit |first3=Nir |conference=Proc. 46th Annual ACM Symposium on Theory of Computing (STOC’14) | year=2014 | isbn=978-1-4503-2710-7 | pages = 714–723 | doi=10.1145/2591796.2591836 | title=Are Lock-Free Concurrent Algorithms Practically Wait-Free?|arxiv=1311.3200 }}</ref> Thus, in the absence of hard deadlines, wait-free algorithms may not be worth the additional complexity that they introduce.

== Lock-freedom ==

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All wait-free algorithms are lock-free.

In particular, if one thread is suspended, then a lock-free algorithm guarantees that the remaining threads can still make progress. Hence, if two threads can contend for the same mutex lock or [[spinlock]], then the algorithm is ''not'' lock-free. (If we suspend one thread that holds the lock, then the second thread will block.)

An algorithm is lock-free if infinitely often operation by some processors will succeed in a finite number of steps. For instance, if {{var|N}} processors are trying to execute an operation, some of the {{var|N}} processes will succeed in finishing the operation in a finite number of steps and others might fail and retry on failure. The difference between wait-free and lock-free is that wait-free operation by each process is guaranteed to succeed in a finite number of steps, regardless of the other processors.

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== See also ==

* [[Deadlock (computer science)|Deadlock]]

* [[Java ConcurrentMap#Lock-free atomicity]]

* [[Liveness]]

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* [[Priority inversion]]

* [[Resource starvation]]

* [[Non-lock concurrency control]]

* [[Optimistic concurrency control]]

== References ==