Near Optimal Jointly Private Packing Algorithms via Dual Multiplicative Weight Update

We present an improved $(ε, δ)$-jointly differentially private algorithm for packing problems. Our algorithm gives a feasible output that is approximately optimal up to an $αn$ additive factor as long as the supply of each resource is at least $\tilde{O}(\sqrt{m} / αε)$, where $m$ is the number of resources. This improves the previous result by Hsu et al.~(SODA '16), which requires the total supply to be at least $\tilde{O}(m^2 / αε)$, and only guarantees approximate feasibility in terms of total violation. Further, we complement our algorithm with an almost matching hardness result, showing that $Ω(\sqrt{m \ln(1/δ)} / αε)$ supply is necessary for any $(ε, δ)$-jointly differentially private algorithm to compute an approximately optimal packing solution. Finally, we introduce an alternative approach that runs in linear time, is exactly truthful, can be implemented online, and can be $ε$-jointly differentially private, but requires a larger supply of each resource.