Parallel computation thesis: Difference between revisions

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In [[computational complexity theory]], the '''parallel computation thesis''' is a [[hypothesis]] which states that the ''time'' used by a (reasonable) parallel machine is polynomially related to the ''space'' used by a sequential machine. The parallel computation thesis was set forth by [[Ashok K. Chandra|Chandra]] and [[Larry Stockmeyer|Stockmeyer]] in 1976.<ref (seename=alternation>{{Cite References)conference|last1=Chandra|first1=Ashok K.|last2=Stockmeyer|first2=Larry J.|title=Alternation|booktitle=FOCS'76: Proceedings of the 17th Annual Symposium on Foundations of Computer Science|pages=98–108|doi=10.1109/SFCS.1976.4|year=1976}}</ref>

In other words, for a [[computational model]] which allows computations to branch and run in parallel without bound, a [[formal language]] which is [[decidable language|decidable]] under the model using no more than <math>t(n)</math> steps for inputs of length ''n'' is decidable by a non-branching machine in the unbranching model using no more than <math>t(n)^k</math> units of storage for some constant ''k''. Similarly, if a machine in the unbranching model decides a language using no more than <math>s(n)</math> storage, a machine in the parallel model can decide the language in no more than <math>s(n)^k</math> steps for some constant ''k''.

The parallel computation thesis is not a rigorous formal statement, as it does not clearly define what constitutes an acceptable parallel model. A parallel machine must be sufficiently powerful to emulate the sequential machine in time polynomially related to the sequential space; compare [[Turing machine]], [[non-deterministic Turing machine]], and [[alternating Turing machine]]. N. Blum (1983) has introduced a model for which the thesis does not hold. However, the model allows <mathref>2^{2^{O(T(n))}}</math>Cite paralleljournal|journal=Information threadsProcessing ofLetters|last=Blum|first=Norbert|title=A computationnote afteron <math>T(n)</math>the steps.'parallel computation (See [[Big O notation]]thesis'|volume=17|issue=4|pages=203–205|year=1983|doi=10.) Parberry (1986) suggested a more "reasonable" bound would be <math>2^{O(T(n))}<1016/math> or <math>2^{T0020-0190(n)^{O(183)90041-8}}</mathref>, in defense of the thesis. Goldschlager (1982) has proposed a model which is sufficiently universal to emulate all "reasonable" parallel models, which adheres to the thesis. Chandra and Stockmeyer originally formalized and proved results related to the thesis for deterministic and alternating Turing machines, which is where the thesis originated.

However, the model allows <math>2^{2^{O(T(n))}}</math> parallel threads of computation after <math>T(n)</math> steps. (See [[Big O notation]].) Parberry (1986) suggested a more "reasonable" bound would be <math>2^{O(T(n))}</math> or <math>2^{T(n)^{O(1)}}</math>, in defense of the thesis.<ref>{{Cite journal|doi=10.1145/8312.8317|last=Parberry|first=I.|title=Parallel speedup of sequential machines: a defense of parallel computation thesis|journal=ACM SIGACT News|volume=18|issue=1|pages=54–67|year=1986}}</ref>

Goldschlager (1982) proposed a model which is sufficiently universal to emulate all "reasonable" parallel models, which adheres to the thesis.<ref>{{Cite journal|doi=10.1145/322344.322353|last=Goldschlager|first=Leslie M.|title=A universal interconnection pattern for parallel computers|journal=[[Journal of the ACM]]|volume=29|issue=3|pages=1073–1086|year=1982}}</ref>

Chandra and Stockmeyer originally formalized and proved results related to the thesis for deterministic and alternating Turing machines, which is where the thesis originated.<ref>{{Cite journal|doi=10.1145/322234.322243|last1=Chandra|first1=Ashok K.|last2=Kozen|first2=Dexter C.|last3=Stockmeyer|first3=Larry J.|title=Alternation|journal=[[Journal of the ACM]]|volume=28|issue=1|pages=114–133|year=1981}}</ref>

== References ==

{{reflist}}

* Blum, N., "A note on the 'parallel computation thesis'," ''Inf. Proc. Lett.'', volume 17, pp. 203-205, 1983.

* [http://portal.acm.org/citation.cfm?id=322243&coll=Portal&dl=ACM&CFID=60283170&CFTOKEN=44928981 Chandra, A.K. and Stockmeyer, L.J., 'Alternation,'] ''Journal of the ACM'', Volume 28, Issue 1, pp. 114-133, 1981.

* [http://portal.acm.org/citation.cfm?id=322353&coll=Portal&dl=ACM&CFID=60284798&CFTOKEN=73324856 Goldschlager, Leslie M., 'A Universal Interconnection Pattern for Parallel Computers,'] ''Journal of the ACM'', Volume 29, Issue 3, pp. 1073-1086, 1982.

* [http://portal.acm.org/citation.cfm?id=8317&coll=Portal&dl=ACM&CFID=60284798&CFTOKEN=73324856 Parberry, I., 'Parallel speedup of sequential machines: a defense of parallel computation thesis,'] ''ACM SIGACT News'', Volume 18, Issue 1, pp. 54-67, 1986.

[[Category:Parallel computing]]