TY - JOUR

T1 - Pointer jumping requires concurrent read

AU - Nisan, Noam

AU - Bar-Yossef, Ziv

PY - 1997

Y1 - 1997

N2 - We consider the well known problem of determining the k'th vertex reached by chasing pointers in a directed graph of out-degree 1. The famous `pointer doubling' technique provides an O(log k) parallel time algorithm on a Concurrent-Read Exclusive-Write (CREW) PRAM. We prove that this problem requires Ω(k) steps on an Exclusive-Read Exclusive-Write (EREW) PRAM, for every k≤c√log n, where n is the number of vertices and c is a constant. This yields a boolean function which can be computed in O(log log n) time on a CREW PRAM, but requires Ω(√log n) time on even an `ideal' EREW PRAM. This is the first separation known for boolean functions between the power of EREW and CREW PRAMs. Previously, separations between EREW and CREW PRAMs were only known for functions on `huge' input domains, or for restricted types of EREW PRAMs.

AB - We consider the well known problem of determining the k'th vertex reached by chasing pointers in a directed graph of out-degree 1. The famous `pointer doubling' technique provides an O(log k) parallel time algorithm on a Concurrent-Read Exclusive-Write (CREW) PRAM. We prove that this problem requires Ω(k) steps on an Exclusive-Read Exclusive-Write (EREW) PRAM, for every k≤c√log n, where n is the number of vertices and c is a constant. This yields a boolean function which can be computed in O(log log n) time on a CREW PRAM, but requires Ω(√log n) time on even an `ideal' EREW PRAM. This is the first separation known for boolean functions between the power of EREW and CREW PRAMs. Previously, separations between EREW and CREW PRAMs were only known for functions on `huge' input domains, or for restricted types of EREW PRAMs.

UR - http://www.scopus.com/inward/record.url?scp=0030676112&partnerID=8YFLogxK

U2 - 10.1145/258533.258648

DO - 10.1145/258533.258648

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AN - SCOPUS:0030676112

SN - 0734-9025

SP - 549

EP - 558

JO - Conference Proceedings of the Annual ACM Symposium on Theory of Computing

JF - Conference Proceedings of the Annual ACM Symposium on Theory of Computing

T2 - Proceedings of the 1997 29th Annual ACM Symposium on Theory of Computing

Y2 - 4 May 1997 through 6 May 1997

ER -