Spectrum-X: It's Bigger Than Software

There's been a lot of confusion around Spectrum-X, some of which NVIDIA seems to have created intentionally. The company's branding is part of the issue, as it seems to conflate Spectrum-X with the Spectrum line of Ethernet switch chips. In fact, Spectrum-X is simply a software license that enables new features across a set of existing hardware products.

The reality that Spectrum-X is a set of software, however, devalues what NVIDIA has actually delivered. Working on top of the company's end-to-end Ethernet hardware, the software creates the first merchant congestion-managed Ethernet fabric. Minimizing tail latency is critical to AI-training workloads, as detailed in our recent white paper. We use the merchant qualifier because some hyperscalers have developed their own congestion-management schemes that work with standard Ethernet-switch hardware. One example is Amazon, which developed the scalable reliable datagram (SDP) protocol for use with its internally-developed Nitro smart-NIC along with merchant switch silicon.

Spectrum-X (Source: NVIDIA)

Spectrum-X comprises software for BlueField-3 DPUs, residing in servers, and Ethernet switches based on Spectrum-4. The Spectrum-X announcement included the news that the Spectrum-4 51.2Tbps switch chip is now in production and available in new SN5000 Series switch systems. That follows BlueField-3 production availability, which NVIDIA revealed at GTC in March. At its core, Spectrum-X implements ROCEv2 adaptive routing and congestion management to create a lossless Ethernet fabric with predictable and bounded latency. As is the case with Amazon's SDP, adaptive routing enables optimal path selection on a per-packet basis, also known as packet spraying. This can result in out-of-order packets at the destination, which the DPU in the endpoint must handle. On the transmit side, the DPU can also limit the packet-injection rate in response to congestion notifications. Spectrum-4 provides realtime telemetry data (branded NetQ), such as queue depths.

NVIDIA's full-stack solution provides an Ethernet-based alternative to InfiniBand for AI training. The company positions Spectrum-X for multitenant clouds where AI isn't the only workload, whereas InfiniBand provides ultimate scaling for "AI factories." In other words, you won't be using Spectrum-X to train the next ChatGPT, which is the domain of the DGX GH200. Spectrum-X is also NVIDIA's answer to Ethernet scheduled fabrics available from Broadcom and Cisco. In Cisco's parlance, Spectrum-X is a telemetry-assisted Ethernet solution, which also describes Amazon's implementation. Fully-scheduled fabrics instead handle congestion management in the fabric and present standard Ethernet ports at ingress/egress. This approach enables servers to use standard NICs, such as NVIDIA's ConnectX-7, rather than smart-NICs (or DPUs). Although the scheduled fabric is vendor-specific (or proprietary), it decouples the switch fabric from the NIC.

We might think Spectrum-X is simply a defensive response to Ethernet competitors, aimed at protecting NVIDIA's sizable InfiniBand business. But the company is building what it says will be Israel's most powerful supercomputer, dubbed Israel-1, around the Spectrum-X fabric. Hyperscalers including Amazon and Meta are already using Ethernet fabrics in AI systems, but Spectrum-X will serve the next tier of cloud providers. In the race to build AI infrastructure, there's no one-size-fits-all network architecture, meaning neither NVIDIA's InfiniBand nor Broadcom's Tomahawk will be the dominant solution.

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