Artificial Intelligence1 min read

Grace Blackwell needs a new scheduler: 27% more usable GPUs from the same racks

By · Published by Everything Blog

In short

Nvidia’s Grace Blackwell NVL72 architecture groups up to 72 GPUs inside a single rack using a high-bandwidth NVLink fabric, but spilling workloads over standard InfiniBand connections can reduce performance by 43%. Shared infrastructure regularly suffers from node failures and staggered job completions that scatter available capacity across physical boundaries. To counteract this fragmentation, the SkyPilot Platform deploys topology-aware placement, automated defragmentation, and in-place failure recovery. This dynamic management ensures communication-intensive training routines stay within individual racks to maintain optimal hardware utilization.

Key points

  • Multi-node workloads must remain contained within individual racks to prevent severe band…: Multi-node workloads must remain contained within individual racks to prevent severe bandwidth slowdowns from external networking fabrics.
  • Cluster fragmentation and hardware faults naturally scatter compute capacity without spec…: Cluster fragmentation and hardware faults naturally scatter compute capacity without specialized, topology-aware scheduling mechanisms.
  • Communication-intensive routines like tensor parallelism and expert parallelism rely heav…: Communication-intensive routines like tensor parallelism and expert parallelism rely heavily on unified rack-scale fabrics to preserve training throughput.

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Not all high-speed interconnects are created equal, and using the wrong one can be quite costly: running the same distributed training job over InfiniBand rather than NVLink can degrade performance by 43%.

Historically, NVLink was limited to communication within a single node, meaning that multi-node AI workloads were forced to communicate over InfiniBand or a cloud equivalent (EFA on AWS, GPUDirect-TCPX on GCP). NVIDIA’s NVL72 systems remove this limitation by connecting all 72 GPUs in a rack with a unified NVLink fabric, so GPUs on different nodes can exchange data with an order of magnitude more bandwidth.

However, maximizing the benefits of NVL72 systems is highly dependent on workload placement: a multi-node job spread across racks is still bottlenecked by InfiniBand. Preserving rack-locality gets harder as hardware failures take nodes offline and jobs finishing at different times scatter free capacity across racks.

This blog covers the communication bottleneck in large-scale training and inference, the NVL72 rack-scale fabric, and slice sizing for resilience to GPU failures, as well as how the SkyPilot Platform keeps multi-node jobs rack-local through topology-aware placement, in-place…

Artificial Intelligence