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What is allocatable in vCluster virtual scheduler

Supported Configurations
Running the control plane as a container with:

What are allocatable resources on a node?​

Allocatable on a Kubernetes node represents the compute resources available for pods. The scheduler prevents over-subscribing resources beyond the allocatable value. It supports three primary resource types:

  • CPU
  • Memory
  • Ephemeral storage

What is vCluster virtual scheduler?​

By default, vCluster uses the control plane cluster scheduler to determine pod placement. This approach offers two key benefits:

  • Saves computing resources.
  • Allows pods to run on any node.

However, the control plane cluster scheduler has limitations detailed in the virtual scheduler documentation. The virtual scheduler provides an alternative by:

  • Making independent pod allocation decisions.
  • Allowing taints and labels on tenant cluster nodes.
  • Preventing modifications to the control plane cluster nodes.

The challenge with allocatable values​

When using the vCluster virtual scheduler, you'll notice a significant difference between node allocatable values in the control plane cluster and vCluster context.

Consider this scenario:

  • Kubernetes cluster with three nodes
  • vCluster assigned to use only two nodes
  • Focus on CPU allocation for simplicity

Control plane cluster perspective​

In the control plane Kubernetes cluster context, the first node shows 62.5 allocatable cpus. This value is calculated by:

  • Total node cpus: 64
  • Minus kubeReserved and systemReserved values

vCluster perspective​

In the vCluster context, the same node shows an allocatable value of 30.5 or lower.

Why the allocatable values differ​

The vCluster scheduler uses the allocatable value to prevent over-subscribing resources.

  • Must reflect resources available specifically to vCluster
  • Cannot use the entire control plane cluster's allocatable value
  • Node details show only what vCluster knows and controls

Using the full allocatable value of the control plane cluster would cause the vCluster scheduler to over-subscribe the node.

How vCluster manages allocatable values​

The vCluster syncer implements a dynamic update mechanism:

  • Regularly reads node information from the control plane cluster.
  • Updates node information in the vCluster.
  • Ensures allocatable values change as pods start and stop in the control plane cluster context.