--- title: "Reduce the setup floor before increasing shard count" description: "Each extra shard repeats checkout, dependencies and container setup." url: https://starsling.dev/ci-playbooks/techniques/setup-cost-limits-sharding canonicalUrl: https://starsling.dev/ci-playbooks/techniques/setup-cost-limits-sharding --- # Reduce the setup floor before increasing shard count Each extra shard repeats checkout, dependencies and container setup. ## How to choose an approach This technique currently has one documented company example. Validate its prerequisites, tradeoffs and original CI environment against your repository before adapting it. - **[Linear](https://starsling.dev/ci-playbooks/linear#setup-cost-limits-sharding)**: Use it when More shards would repeat substantial fixed setup work. Check before adopting: Remeasure per-shard setup before changing shard count. ## Apply this technique ````text Evaluate and apply this CI technique to my repository: Reduce the setup floor before increasing shard count. Each extra shard repeats checkout, dependencies and container setup. Read the technique and its source-backed company examples: https://starsling.dev/ci-playbooks/techniques/setup-cost-limits-sharding.md Inspect this repository’s CI workflows, build and test commands, dependencies, and recent run timings. Identify where this technique applies and explain the expected benefit using this repository’s evidence. Measure a baseline from recent comparable runs: elapsed time, queue time, runner minutes and failures where available. If data is unavailable, say what cannot be measured. Compare relevant company approaches, checking prerequisites, platform differences and tradeoffs. Rank applicable options by evidence, fit and measurable opportunity; do not present company results as predictions for this repo. Recommend the smallest safe change, show what checks and fallback paths remain, and ASK ME WHICH CHANGE TO APPROVE BEFORE EDITING ANY FILES. Only after approval, work on a new feature branch, make one reversible change, and open a reviewable pull request. Never merge or push to a default or protected branch. Keep required checks, test coverage, deployment and release behavior intact. Validate the change and compare equivalent runs where possible; report limitations and what should be monitored after merge. ```` ## How Linear used this technique Read what Linear changed and follow the original sources. ### Linear: Reduce the setup floor before increasing shard count Each extra shard repeats checkout, dependencies and container setup. Before its setup work, Linear’s test job used 4 shards and spent 8.3 minutes on setup; afterward it ran 8 shards with 7.5 minutes of setup. Evidence: Documented technique Use it when: More shards would repeat substantial fixed setup work. Before applying it: Remeasure per-shard setup before changing shard count. [Read more in the Linear playbook](https://starsling.dev/ci-playbooks/linear#setup-cost-limits-sharding) [Primary source](https://linear.app/now/ci-bottleneck-reworked#sharding-is-limited-by-setup-overhead) ## CI optimization is continuous work. Apply this technique yourself, or use StarSling Runners for faster GitHub Actions and optional agent-driven optimization PRs, which teams on eligible paid plans can turn on. [Get self-improving CI](https://starsling.dev/products/runners) ## Related techniques ### [Install and build only the workspace packages a job uses](https://starsling.dev/ci-playbooks/techniques/scope-workspace-builds) Scope dependency installation and builds to the package or lane under test, including its actual dependencies. 3 implementations · 3 companies (Linear, Mastra, Shopify) ### [Bake stable dependencies into runner images](https://starsling.dev/ci-playbooks/techniques/prebuild-runner-images) Prepare versioned system dependencies and build downloads once in the runner image instead of installing them on every job. 2 implementations · 2 companies (Anthropic, Linear) ### [Balance test work across shards](https://starsling.dev/ci-playbooks/techniques/balance-test-shards) Use measured runtimes or smaller test files to reduce the slowest shard, which determines when the suite finishes. 4 implementations · 4 companies (Linear, OpenAI, Ramp, Stripe) ## Company Playbooks ### [Anthropic](https://starsling.dev/ci-playbooks/anthropic) Run only affected tests ### [DoorDash](https://starsling.dev/ci-playbooks/doordash) Run build and test in parallel, sharded across CI agents ### [Linear](https://starsling.dev/ci-playbooks/linear) Benchmark the same job on faster hardware ### [Mastra](https://starsling.dev/ci-playbooks/mastra) StarSling runners for the slowest test workflow ### [OpenAI](https://starsling.dev/ci-playbooks/openai) Rank CI fixes by the minutes they save ### [Ramp](https://starsling.dev/ci-playbooks/ramp) Move a CPU-bound test suite to a faster CPU ### [Shopify](https://starsling.dev/ci-playbooks/shopify) Let an agent iterate against one CI metric ### [Spotify](https://starsling.dev/ci-playbooks/spotify) Build arm64 on native arm64 runners ### [Stripe](https://starsling.dev/ci-playbooks/stripe) Run only the tests a change can reach ### [Uber](https://starsling.dev/ci-playbooks/uber) Build the merge-queue speculations most likely to be needed ## Related guides Implementation guides for this technique. ### [Cancel superseded CI runs](https://starsling.dev/best-practices/github-actions/cancel-superseded-runs) Use concurrency groups to keep PR verification focused on the latest commit. ### [Build and test only what changed](https://starsling.dev/best-practices/github-actions/build-only-affected) Scope work with your dependency graph and keep a full-run fallback. ### [Cut repeated test setup](https://starsling.dev/best-practices/github-actions/cut-repeated-test-setup) Move shared fixture work out of individual test cases and keep necessary cleanup. ### [Shard tests across parallel jobs](https://starsling.dev/best-practices/github-actions/shard-tests) Split a long suite with a matrix and your test framework’s sharding support. ### [Filter workflows by changed files](https://starsling.dev/best-practices/github-actions/path-filter-workflows) Use paths and job conditions while keeping required checks reporting. ### [Cut CI queue time](https://starsling.dev/best-practices/github-actions/cut-queue-time) Measure waiting time and identify concurrency or capacity constraints.