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.github.ci: list the shared derivations in the eval job
The shared job dry-ran every machine to find the derivations needed by
more than one, after a runner start and a prefetch of all drv closures
from hestia. In the benchmark run (PR 68, only neoprism uncached) it
built nothing and still cost the machine job 61 s: 5 s queueing, 6 s
setup, 21 s prefetch (mostly the 47 MiB nixpkgs source), 13 s dry-run,
then 6 s queueing of the machine job.
The eval job has the drvs already, so it dry-runs the machines itself,
in parallel, and only for two or more (one machine shares nothing). It
hands the list to the shared job as an artifact; with an empty list the
shared job is skipped, and the machine jobs wait for the manifest
version of the eval job instead of the shared job's.
author
Hermes
committer
lassulus
date
(Oct 17, 2026, 11:40 AM CEST)
commit
7d3f6b7c
7d3f6b7c65596d3381e231c75bfdfcf55330c78d
parent
a6ac39e6
a6ac39e6460d87c459dff117149a719b1b078925
+73
-41
+55
-34
.github/workflows/ci.yml
+55
-34
.github/workflows/ci.yml
···
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# The NixOS machines, built on plain runners with hestia (the GitHub Actions
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# cache) as binary cache next to cache.lassul.us:
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#
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# - eval: evaluates ciToplevels once, uploads the drv closures to hestia
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# and lists the machines whose toplevel is in neither cache.
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# - shared: builds the derivations needed by more than one of them once,
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# so the machine jobs substitute them instead of each building them.
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# - eval: evaluates ciToplevels once, uploads the drv closures to hestia,
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# lists the machines whose toplevel is in neither cache and the
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# derivations needed by more than one of them (.github/ci-shared.sh).
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# - shared: builds those once, so the machine jobs substitute them instead
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# of each building them; skipped when there are none.
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# - one job per listed machine, builds its toplevel by drv path on a lazy
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# /nix/store (.github/lazy-store.sh): the paths it would substitute are
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# only downloaded once a file in them is opened.
···
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matrix: ${{ steps.matrix.outputs.matrix }}
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any-jobs: ${{ steps.matrix.outputs.any-jobs }}
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manifest-version: ${{ steps.matrix.outputs.manifest-version }}
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shared: ${{ steps.plan.outputs.shared }}
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niks3: ${{ steps.tools.outputs.niks3 }}
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lazy-store: ${{ steps.tools.outputs.lazy-store }}
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steps:
···
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run: |
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echo "not cached: $(jq -r '[.include[].name] | join(" ")' <<<"$MATRIX")" |
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tee -a "$GITHUB_STEP_SUMMARY"
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# the derivations the shared job builds, none for a single machine
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- id: plan
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if: steps.matrix.outputs.any-jobs == 'true'
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env:
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MATRIX: ${{ steps.matrix.outputs.matrix }}
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run: |
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matrix=$RUNNER_TEMP/matrix.json
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plans=$RUNNER_TEMP/plans
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printf '%s' "$MATRIX" > "$matrix"
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mkdir -p "$plans"
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: > "$plans/shared.drvs"
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if [ "$(jq '.include | length' "$matrix")" -ge 2 ]; then
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.github/ci-shared.sh "$matrix" "$plans"
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{
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for name in $(jq -r '.include[].name' "$matrix"); do
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echo "- $name: $(wc -l < "$plans/$name.drvs") derivations to build"
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done
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echo "- shared (needed by >= 2 machines): $(wc -l < "$plans/shared.drvs")"
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sed -E 's|^/nix/store/[^-]+-(.*)\.drv$| \1|' "$plans/shared.drvs"
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} | tee -a "$GITHUB_STEP_SUMMARY"
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fi
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if [ -s "$plans/shared.drvs" ]; then
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echo "shared=true" >> "$GITHUB_OUTPUT"
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fi
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- if: steps.plan.outputs.shared == 'true'
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uses: actions/upload-artifact@v4
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with:
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name: plans
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path: ${{ runner.temp }}/plans/shared.drvs
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shared:
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needs: eval
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if: needs.eval.outputs.any-jobs == 'true'
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if: needs.eval.outputs.shared == 'true'
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runs-on: ubuntu-latest
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permissions:
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contents: read
···
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run: |
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nix-store --realise "$NIKS3"
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echo "$NIKS3/bin" >> "$GITHUB_PATH"
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- uses: actions/download-artifact@v4
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with:
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name: plans
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path: ${{ runner.temp }}/plans
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- id: build
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env:
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MATRIX: ${{ needs.eval.outputs.matrix }}
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EVAL_MANIFEST_VERSION: ${{ needs.eval.outputs.manifest-version }}
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run: |
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matrix=$RUNNER_TEMP/matrix.json
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plans=$RUNNER_TEMP/plans
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printf '%s' "$MATRIX" > "$matrix"
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read -ra installables <<<"$(jq -r '[.include[].installables] | join(" ")' "$matrix")"
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"$HESTIA_BIN" prefetch "${installables[@]}"
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.github/ci-shared.sh "$matrix" "$plans"
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mapfile -t shared < "$plans/shared.drvs"
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version=$EVAL_MANIFEST_VERSION
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if [ "${#shared[@]}" -gt 0 ]; then
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# a failed one fails the jobs of the machines that need it
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.github/ci-build.sh "${shared[@]/%/^*}" ||
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echo "::warning::some shared derivations failed to build"
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# upload now instead of in the post step, so the machine jobs can
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# wait for the manifest version that contains the shared outputs
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"$HESTIA_BIN" drain --socket "$HESTIA_SOCKET" 2>&1 | tee "$RUNNER_TEMP/drain.txt"
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drained=$(sed -nE 's/.*; manifest m[0-9]+#([0-9]+)$/\1/p' "$RUNNER_TEMP/drain.txt")
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version=${drained:-$version}
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fi
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echo "manifest-version=$version" >> "$GITHUB_OUTPUT"
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{
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for name in $(jq -r '.include[].name' "$matrix"); do
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echo "- $name: $(wc -l < "$plans/$name.drvs") derivations to build"
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done
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echo "- shared (needed by >= 2 machines): ${#shared[@]}"
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printf '%s\n' "${shared[@]}" | sed -E 's|^/nix/store/[^-]+-(.*)\.drv$| \1|'
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} | tee -a "$GITHUB_STEP_SUMMARY"
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mapfile -t shared < "$RUNNER_TEMP/plans/shared.drvs"
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"$HESTIA_BIN" prefetch "${shared[@]/%/^*}"
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# a failed one fails the jobs of the machines that need it
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.github/ci-build.sh "${shared[@]/%/^*}" ||
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echo "::warning::some shared derivations failed to build"
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# upload now instead of in the post step, so the machine jobs can
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# wait for the manifest version that contains the shared outputs
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"$HESTIA_BIN" drain --socket "$HESTIA_SOCKET" 2>&1 | tee "$RUNNER_TEMP/drain.txt"
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drained=$(sed -nE 's/.*; manifest m[0-9]+#([0-9]+)$/\1/p' "$RUNNER_TEMP/drain.txt")
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echo "manifest-version=${drained:-$EVAL_MANIFEST_VERSION}" >> "$GITHUB_OUTPUT"
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build:
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needs: [eval, shared]
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if: needs.eval.outputs.any-jobs == 'true'
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# also when the shared job was skipped: nothing to share
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if: >-
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!cancelled() && needs.eval.result == 'success' && needs.eval.outputs.any-jobs == 'true'
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&& (needs.shared.result == 'success' || needs.shared.result == 'skipped')
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name: ${{ matrix.name }}
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runs-on: ${{ matrix.os }}
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permissions:
···
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with:
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upstream-cache-filter: true
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upstream-cache-key-names: cache.nixos.org-1 cache.lassul.us-1
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wait-manifest-version: ${{ needs.shared.outputs.manifest-version }}
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wait-manifest-version: ${{ needs.shared.outputs.manifest-version || needs.eval.outputs.manifest-version }}
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- env:
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NIKS3: ${{ needs.eval.outputs.niks3 }}
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run: |