> ## Documentation Index
> Fetch the complete documentation index at: https://site.aspect.build/llms.txt
> Use this file to discover all available pages before exploring further.

# Optimizing CI runners

> Tune Aspect Workflows CI runners for CI speed and cost: instance size, runner group scaling and warming.

export const gatedHref = (user, href, group) => {
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Try these on the runners themselves. They're independent, and each one changes speed and cost, so measure after each. For the build running on them, see [Optimizing your Bazel build](/docs/bazel/guides/optimizing-builds).

## Tune instance type size

The right instance size depends on your workload's parallelism and memory profile. An undersized machine serializes actions; an oversized one pays for idle cores.

1. Review available instance types from your cloud provider, noting regional availability.
2. Use a binary search: double or halve the instance size and measure wall-clock CI time against cost. Stop when the speed gain no longer justifies the cost increase.
3. Evaluate Arm instances. They typically offer better price-to-performance for CPU-bound workloads.

<Note>
  A resize replaces the running instances and their local output bases, so the first builds afterwards run cold. On a self-hosted deployment, instance types are runner group settings in Terraform; see the <a href={gatedHref(user, "/docs/aspect-workflows/enterprise/self-hosted/reference/terraform-config-aws#runners", "workflows-subscriber")}>AWS</a> and <a href={gatedHref(user, "/docs/aspect-workflows/enterprise/self-hosted/reference/terraform-config-gcp#runners", "workflows-subscriber")}>GCP</a> references.
</Note>

## Tune runner group scaling

These settings control how a runner group scales. On a deployment hosted by Aspect, [ask for the values you want](/docs/aspect-workflows/enterprise/hosted/requesting-changes); on a self-hosted deployment, they're runner group settings in Terraform:

* `min_runners`: the minimum number of runners always allocated. Set this to absorb burst traffic without a cold start.
* `max_runners`: the upper bound. Set this based on your peak concurrent job count and budget.
* `min_free_runners`: the number of idle runners to keep ready. A value of 1–2 removes the wait for a new runner to provision when several jobs arrive at once.

## Configure runner warming

A scheduled `aspect ci warming` job runs `bazel build --nobuild` over your warming targets, then archives the runner's repository cache and output base. A new runner restores the archive before its first job, so that job starts with external repositories fetched and repository rules already run.

See [Warming](/docs/aspect-workflows/enterprise/ci-runners/warming) for configuration details.