Adversarial contention resolution for simple channels

• Authors: Michael A. Bender;Martin Farach-Colton;Simai He;Bradley C. Kuszmaul;Charles E. Leiserson
• Venue: Proceedings of the seventeenth annual ACM symposium on Parallelism in algorithms and architectures
• Year: 2005

This paper analyzes the worst-case performance of randomized backoff on simple multiple-access channels. Most previous analysis of backoff has assumed a statistical arrival model.For batched arrivals, in which all n packets arrive at time 0, we show the following tight high-probability bounds. Randomized binary exponential backoff has makespan Θ(nlgn), and more generally, for any constant r, r-exponential backoff has makespan Θ(nloglgr n). Quadratic backoff has makespan Θ((n/lg n)3/2), and more generally, for r1, r-polynomial backoff has makespan Θ((n/lg n)1+1/r). Thus, for batched inputs, both exponential and polynomial backoff are highly sensitive to backoff constants. We exhibit a monotone superpolynomial subexponential backoff algorithm, called loglog-iterated backoff, that achieves makespan Θ(nlglg n/lglglg n). We provide a matching lower bound showing that this strategy is optimal among all monotone backoff algorithms. Of independent interest is that this lower bound was proved with a delay sequence argument.In the adversarial-queuing model, we present the following stability and instability results for exponential backoff and loglog-iterated backoff. Given a (λ,T)-stream, in which at most n=λT packets arrive in any interval of size T, exponential backoff is stable for arrival rates of λ=O(1/lgn) and unstable for arrival rates of λ=Ω(lglgn/lgn); loglog-iterated backoff is stable for arrival rates of λ=O(1/(lglgn\lgn)) and unstable for arrival rates of λ=Ω(1/lgn). Our instability results show that bursty input is close to being worst-case for exponential backoff and variants and that even small bursts can create instabilities in the channel.