# Speed and Optimization¶

This page contains information and guidelines for improving the speed, stability, and general optimization of your JupyterHub deployment.

## Pre-pulling images¶

If a user is scheduled on a node requesting an image that isn’t available, the user will have to wait. If the image is large, the wait can be 5 to 10 minutes. This can happen for two reasons.

1. A new image is introduced during a helm upgrade

With the hook-image-puller puller enabled, images will be pulled to the nodes before the actual helm upgrade thanks to a Helm hook.

prePuller:
hook:
enabled: false

1. A new node was added

CONTINUE FROM HERE … Cluster size can change through manual addition of nodes or autoscaling. When a new node is added to the cluster, the new node does not yet have the user image. A user using this new node would be forced to wait while the image is pulled from scratch. Ideally, it would be helpful to pre-pull images when the new node is added to the cluster.

With the continuous pre-puller enabled (disabled by default), the user’s container image will be pre-pulled when a new node is added. New nodes can for example be added manually or by a cluster autoscaler. The continuous pre-puller uses a daemonset to force Kubernetes to pull the user image on all nodes as soon as a node is present. The continuous pre-puller uses minimal resources on all nodes and greatly speeds up the user pod start time.

The continuous pre-puller is disabled by default. To enable it, use the following snippet in your config.yaml:

prePuller:
continuous:
enabled: true


### Pre-pulling additional images¶

By default, the pre-puller only pulls the singleuser image & the networktools image (if access to cloud metadata is disabled). If you have customizations that need additional images present on all nodes, you can ask the pre-puller to also pull an arbitrary number of additional images.

prePuller:
extraImages:
ubuntu-xenial:
name: ubuntu
tag: 16.04
policy: IfNotPresent


This snippet will pre-pull the ubuntu:16.04 image on all nodes, for example. You can pre-pull any number of images.