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Breaking the Barrier Of 2 for the Competitiveness of Longest Queue Drop

Authors Antonios Antoniadis, Matthias Englert, Nicolaos Matsakis, Pavel Veselý

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Antonios Antoniadis
  • University of Twente, The Netherlands
Matthias Englert
  • University of Warwick, Coventry, UK
Nicolaos Matsakis
  • Athens, Greece
Pavel Veselý
  • Computer Science Institute of Charles University, Prague, Czech Republic

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Antonios Antoniadis, Matthias Englert, Nicolaos Matsakis, and Pavel Veselý. Breaking the Barrier Of 2 for the Competitiveness of Longest Queue Drop. In 48th International Colloquium on Automata, Languages, and Programming (ICALP 2021). Leibniz International Proceedings in Informatics (LIPIcs), Volume 198, pp. 17:1-17:20, Schloss Dagstuhl - Leibniz-Zentrum für Informatik (2021)


We consider the problem of managing the buffer of a shared-memory switch that transmits packets of unit value. A shared-memory switch consists of an input port, a number of output ports, and a buffer with a specific capacity. In each time step, an arbitrary number of packets arrive at the input port, each packet designated for one output port. Each packet is added to the queue of the respective output port. If the total number of packets exceeds the capacity of the buffer, some packets have to be irrevocably rejected. At the end of each time step, each output port transmits a packet in its queue and the goal is to maximize the number of transmitted packets. The Longest Queue Drop (LQD) online algorithm accepts any arriving packet to the buffer. However, if this results in the buffer exceeding its memory capacity, then LQD drops a packet from the back of whichever queue is currently the longest, breaking ties arbitrarily. The LQD algorithm was first introduced in 1991, and is known to be 2-competitive since 2001. Although LQD remains the best known online algorithm for the problem and is of practical interest, determining its true competitiveness is a long-standing open problem. We show that LQD is 1.707-competitive, establishing the first (2-ε) upper bound for the competitive ratio of LQD, for a constant ε > 0.

Subject Classification

ACM Subject Classification
  • Theory of computation → Online algorithms
  • buffer management
  • online scheduling
  • online algorithms
  • longest queue drop


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