Summary: Time- and Space-E cient Randomized Consensus
School of Computer Science
Pittsburgh, PA 15213
February 10, 1992
A protocol is presented which solves the randomized consensus problem 9] for shared memory.
The protocol uses a total of O(p2
+n) worst-case expected increment, decrement and read oper-
ations on a set of three shared O(logn)-bit counters, where p is the number of active processors
and n is the total number of processors. It requires less space than previous polynomial-time
consensus protocols 6, 7], and is faster when not all of the processors participate in the protocol.
A modi ed version of the protocol yields a weak shared coin whose bias is guaranteed to be in
the range 1=2 regardless of scheduler behavior, and which is the rst such protocol for the
shared-memory model to guarantee that all processors agree on the outcome of the coin.
Consensus is a decision problem in which n processors, each starting with a value (0 or 1) not
known to the others, must collectively agree on a single value. A consensus protocol is a distributed