License: Creative Commons Attribution 3.0 Unported license (CC BY 3.0)
When quoting this document, please refer to the following
DOI: 10.4230/LIPIcs.DISC.2020.5
URN: urn:nbn:de:0030-drops-130831
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Blelloch, Guy E. ; Wei, Yuanhao

LL/SC and Atomic Copy: Constant Time, Space Efficient Implementations Using Only Pointer-Width CAS

LIPIcs-DISC-2020-5.pdf (0.5 MB)


When designing concurrent algorithms, Load-Link/Store-Conditional (LL/SC) is a very useful primitive since it avoids ABA problems. The full semantics of LL/SC are not supported in hardware by any modern architecture, so there has been a significant amount of work on simulations of LL/SC using CAS. However, all previous algorithms that are constant time either use unbounded sequence numbers (and thus base objects of unbounded size), or require Ω(MP) space to implement M LL/SC objects for P processes.
We present the first constant time implementation of LL/SC from bounded-sized CAS objects using only constant space overhead per LL/SC variable. In particular, our implementation uses Θ(M+kP²) space, where k is the number of outstanding LL operations per process, and only requires pointer-width CAS operations. In most algorithms that use LL/SC, k is a small constant which reduces our additive space overhead to Θ(P²). Our algorithm can also be extended to implement L word LL/SC objects in Θ(L) time for LL and SC, O(1) time for VL, and Θ((M+kP²)L) space.
To achieve these bounds, our main technical contribution is implementing a new primitive called Single-Writer Copy which takes a pointer to a word sized memory location and atomically copies its contents into another object. The restriction is that only one process is allowed to write/copy into the destination object at a time. The ability to read from one memory location and write to another atomically, and in constant-time, is very powerful and we believe this primitive will be useful in designing other algorithms.

BibTeX - Entry

  author =	{Guy E. Blelloch and Yuanhao Wei},
  title =	{{LL/SC and Atomic Copy: Constant Time, Space Efficient Implementations Using Only Pointer-Width CAS}},
  booktitle =	{34th International Symposium on Distributed Computing (DISC 2020)},
  pages =	{5:1--5:17},
  series =	{Leibniz International Proceedings in Informatics (LIPIcs)},
  ISBN =	{978-3-95977-168-9},
  ISSN =	{1868-8969},
  year =	{2020},
  volume =	{179},
  editor =	{Hagit Attiya},
  publisher =	{Schloss Dagstuhl--Leibniz-Zentrum f{\"u}r Informatik},
  address =	{Dagstuhl, Germany},
  URL =		{},
  URN =		{urn:nbn:de:0030-drops-130831},
  doi =		{10.4230/LIPIcs.DISC.2020.5},
  annote =	{Keywords: LL/SC, Atomic Copy, CAS, Constant Time}

Keywords: LL/SC, Atomic Copy, CAS, Constant Time
Collection: 34th International Symposium on Distributed Computing (DISC 2020)
Issue Date: 2020
Date of publication: 07.10.2020

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