Smoothed Analysis of Population Protocols

TL;DR

This paper studies the smoothed analysis of population protocols, introducing a model with both adversarial and random interactions, and presents a phase clock algorithm enabling leader election with a smooth transition in efficiency.

Contribution

It introduces a smoothed analysis framework for population protocols with a novel phase clock, bridging the gap between random and adversarial schedulers.

Findings

Leader election solvable in O(p^{-2} n log^3 n) steps with high probability.

Provides a phase clock primitive for population protocols.

Establishes a smooth transition in complexity from random to adversarial settings.

Abstract

In this work, we initiate the study of \emph{smoothed analysis} of population protocols. We consider a population protocol model where an adaptive adversary dictates the interactions between agents, but with probability $p$ every such interaction may change into an interaction between two agents chosen uniformly at random. That is, $p$-fraction of the interactions are random, while $(1-p)$-fraction are adversarial. The aim of our model is to bridge the gap between a uniformly random scheduler (which is too idealistic) and an adversarial scheduler (which is too strict). We focus on the fundamental problem of leader election in population protocols. We show that, for a population of size $n$, the leader election problem can be solved in $O(p^{-2}n \log^3 n)$ steps with high probability, using $O((\log^2 n) \cdot (\log (n/p)))$ states per agent, for \emph{all} values of $p\leq 1$. Although our result does not match the best known running time of $O(n \log n)$ for the uniformly random scheduler ($p=1$), we are able to present a \emph{smooth transition} between a running time of $O(n \cdot \mathrm{polylog} n)$ for $p=1$ and an infinite running time for the adversarial scheduler ($p=0$), where the problem cannot be solved. The key technical contribution of our work is a novel \emph{phase clock} algorithm for our model. This is a key primitive for much-studied fundamental population protocol algorithms (leader election, majority), and we believe it is of independent interest.