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

TL;DR

This paper provides a worst-case analysis of Count-Min Sketch with Conservative Updates, revealing its error behavior and bounds, especially when each item appears at most once, and introduces novel theoretical bounds involving Markov processes.

Contribution

It introduces new bounds on estimation error for CMS-CU, analyzes its worst-case behavior, and compares it with vanilla Count-Min Sketch, including convergence results for specific parameters.

Findings

Average estimation error converges to 0.5 when d=m-1.

Bounds on estimation error are tight for small g values.

For large m, bounds coincide when d=m-1 and g=1.

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 $10^{-4}$.