HPCwire: The Myth of Cut-Through Switching in Datacenter Networks

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February 10, 2009

The Myth of Cut-Through Switching in Datacenter Networks

by Ori Aruj, General Manager North America and Vice President of Marketing at Dune Networks

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Cut-through switching is mentioned as a requirement for datacenter switches. Originating from high performance computing (HPC) environments, cut-through switching was aimed to reduce network latencies to the minimum. In this article, we review the background behind cut-through switching, and examine the effectiveness of the cut-through scheme in typical datacenter networks.

What is Cut-through Switching?

Cut-through is a switching method for packet-switching systems, wherein the switch starts forwarding a packet before the entire packet has been received, normally, as soon as the destination address is processed. This technique reduces latency through the switch. Cut-through is compared to store-and-forward switching, that, as its name reflects, requires storing the entire packet before its transmission begins.

Cut-through switching became popular again in InfiniBand networks, since these are often deployed in symmetric environments such as supercomputer clusters, where latency may be a concern.

Latency gain for small-medium packets

The overall computing performance of an HPC system may be sensitive to the low latency of control packets. These packets are typically short (used for synchronization), and thus they are the focus of interest for cut-through switching. Long packets have less significance in this discussion.

When evaluating the performance of cut-through switching versus store-and-forward switching for small-medium packets (up to 256B/512B) both methods have about the same performance. This is because even a cut-through switch accumulates/stores 64B-512B chunks, depending on the micro-architecture, before it forwards it to the egress port.

For instance, note that Cisco's Nexus 5000 series switches have cut-through switching and specify a minimum latency of 3.2us, a value that is equivalent to modern store-and-forward switches with packets up to 1KB.

Cut-through between ports of different speeds

Cut-through switching cannot operate when sending traffic from a slow port to a faster port. This, however, is a typical case as packets flow from servers through the datacenter access switches (top-of-rack or end-of-row).

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