Alistarh, Dan-AdrianIST Austria; Censor Hillel, Keren; Shavit, Nir
Lock-free concurrent algorithms guarantee that some concurrent operation will always make progress in a finite number of steps. Yet programmers prefer to treat concurrent code as if it were wait-free, guaranteeing that all operations always make progress. Unfortunately, designing wait-free algorithms is generally a very complex task, and the resulting algorithms are not always efficient. Although obtaining efficient wait-free algorithms has been a long-time goal for the theory community, most nonblocking commercial code is only lock-free. This article suggests a simple solution to this problem.We show that for a large class of lock-free algorithms, under scheduling conditions that approximate those found in commercial hardware architectures, lock-free algorithms behave as if they are wait-free. In other words, programmers can continue to design simple lock-free algorithms instead of complex wait-free ones, and in practice, they will get wait-free progress. Our main contribution is a new way of analyzing a general class of lock-free algorithms under a stochastic scheduler. Our analysis relates the individual performance of processes to the global performance of the system using Markov chain lifting between a complex per-process chain and a simpler system progress chain. We show that lock-free algorithms are not only wait-free with probability 1 but that in fact a general subset of lock-free algorithms can be closely bounded in terms of the average number of steps required until an operation completes. To the best of our knowledge, this is the first attempt to analyze progress conditions, typically stated in relation to a worst-case adversary, in a stochastic model capturing their expected asymptotic behavior.
Journal of the ACM
Part of this work was performed while the first author was a postdoctoral associate at MIT CSAIL, where he was supported by the SNF Postdoctoral Fellows Program, NSF grant CCF-1217921, DoE ASCR grant ER26116/DE-SC0008923, and by grants from the Oracle and Intel corporations. The second author was supported in part by ISF grant 1696/14. The third author was supported in part by NSF grants CCF-1217921, CCF-1301926, IIS-1447786, and CCF-1561807, and the U.S. Department of Energy under grant DE-SC0008923, and by equipment grants from Intel Corporation.
Alistarh D-A, Censor Hillel K, Shavit N. Are lock free concurrent algorithms practically wait free . Journal of the ACM. 2016;63(4). doi:10.1145/2903136
Alistarh, D.-A., Censor Hillel, K., & Shavit, N. (2016). Are lock free concurrent algorithms practically wait free . Journal of the ACM, 63(4). https://doi.org/10.1145/2903136
Alistarh, Dan-Adrian, Keren Censor Hillel, and Nir Shavit. “Are Lock Free Concurrent Algorithms Practically Wait Free .” Journal of the ACM 63, no. 4 (2016). https://doi.org/10.1145/2903136.
D.-A. Alistarh, K. Censor Hillel, and N. Shavit, “Are lock free concurrent algorithms practically wait free ,” Journal of the ACM, vol. 63, no. 4, 2016.
Alistarh D-A, Censor Hillel K, Shavit N. 2016. Are lock free concurrent algorithms practically wait free . Journal of the ACM. 63(4).
Alistarh, Dan-Adrian, et al. “Are Lock Free Concurrent Algorithms Practically Wait Free .” Journal of the ACM, vol. 63, no. 4, ACM, 2016, doi:10.1145/2903136.