Scaling Kubernetes to 7,500 nodes

Scaling a single Kubernetes cluster to this size is rarely done and requires some special care, but the upside is a simple infrastructure that allows our machine learning research teams to move faster and scale up without changing their code.

Since our last post on scaling to 2,500 nodes⁠ we’ve continued to grow our infrastructure to meet researcher needs, in the process learning many additional lessons. This post summarizes those lessons so that others in the Kubernetes community can benefit from them, and ends with problems we still face that we’ll be tackling next.

Our workload

Before we get too far, it’s important to describe our workload. The applications and hardware we run with Kubernetes are pretty different from what you may encounter at a typical company. Our problems and corresponding solutions may, or may not, be a good match to your own setup!

A large machine learning job spans many nodes and runs most efficiently when it has access to all of the hardware resources on each node. This allows GPUs to cross-communicate directly using NVLink⁠(opens in a new window), or GPUs to directly communicate with the NIC using GPUDirect⁠(opens in a new window). So for many of our workloads, a single pod occupies the entire node. Any NUMA, CPU, or PCIE resource contention aren’t factors for scheduling. Bin-packing or fragmentation is not a common problem. Our current clusters have full bisection bandwidth, so we also don’t make any rack or network topology considerations. All of this means that, while we have many nodes, there’s relatively low strain on the scheduler.

That said, strain on the kube-scheduler is spiky. A new job may consist of many hundreds of pods all being created at once, then return to a relatively low rate of churn.

Networking

Once marked, iptables will start counters to track the number of bytes and packets that match this rule. You can eyeball these counters by using iptables itself: