Count-Min Sketch with Conservative Updates: Worst-Case Analysis

Abstract

Count-Min Sketch with Conservative Updates (CMS-CU) is a memory-efficient hash-based data structure used to estimate the occurrences of items within a data stream. CMS-CU stores $m$ counters and employs $d$ hash functions to map items to these counters. We first argue that the estimation error in CMS-CU is maximal when each item appears at most once in the stream. Next, we study CMS-CU in this setting. In the case where $d=m-1$, we prove that the average estimation error and the average counter rate converge almost surely to $\frac{1}{2}$, contrasting with the vanilla Count-Min Sketch, where the average counter rate is equal to $\frac{m-1}{m}$. For any given $m$ and $d$, we prove novel lower and upper bounds on the average estimation error, incorporating a positive integer parameter $g$. Larger values of this parameter improve the accuracy of the bounds. Moreover, the computation of each bound involves examining an ergodic Markov process with a state space of size $\binom{m+g-d}{g}$ and a sparse transition probabilities matrix containing $\mathcal{O}(m\binom{m+g-d}{g})$ non-zero entries. For $d=m-1$, $g=1$, and as $m\to \infty$, we show that the lower and upper bounds coincide. In general, our bounds exhibit high accuracy for small values of $g$, as shown by numerical computation. For example, for $m=50$, $d=4$, and $g=5$, the difference between the lower and upper bounds is smaller than $10^{-4}$.