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Cache the emulator system image (measuring whether it is worth it) #25

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Merged
jpcottin merged 2 commits into from
Jul 26, 2026
Merged

Cache the emulator system image (measuring whether it is worth it) #25

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d8cee33
Cache the emulator system image, with timing instrumentation
jpcottin Jul 26, 2026
1e459ce
Rotate the system-image cache key weekly
jpcottin Jul 26, 2026
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34 changes: 34 additions & 0 deletions .github/actions/preview-emulator/action.yml
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Original file line number Diff line number Diff line change
Expand Up @@ -42,6 +42,34 @@ inputs:
runs:
using: composite
steps:
- name: Resolve SDK path and cache key
id: sdk
shell: bash
run: |
echo "path=${ANDROID_HOME:-${ANDROID_SDK_ROOT:-/usr/local/lib/android/sdk}}" >> "$GITHUB_OUTPUT"
# actions/cache never re-saves on an exact key hit, so a fixed key would
# freeze the image at whatever revision was current when it was first
# cached: sdkmanager would re-fetch each new revision on every run and
# the cache would never catch up, while CI quietly tested a stale image.
# Rotating weekly bounds that. The restore-keys prefix means a rotation
# restores last week's copy and lets sdkmanager fetch just the delta,
# rather than paying for a full download.
echo "week=$(date -u +%G-W%V)" >> "$GITHUB_OUTPUT"

# The system image is the one large download here (system.img alone is
# ~4.1 GiB, ~2 GiB compressed). The preview emulator package is
# deliberately NOT cached: it moves often and pinning a stale build would
# defeat the purpose of these jobs. sdkmanager still runs after a cache
# hit, so a newer image revision is still picked up.
- name: Restore cached system image
id: sysimg-cache
uses: actions/cache@v4
with:
path: ${{ steps.sdk.outputs.path }}/system-images/android-${{ inputs.api-level }}/${{ inputs.target }}/${{ inputs.abi }}
key: sysimg-v1-android-${{ inputs.api-level }}-${{ inputs.target }}-${{ inputs.abi }}-${{ steps.sdk.outputs.week }}
restore-keys: |
sysimg-v1-android-${{ inputs.api-level }}-${{ inputs.target }}-${{ inputs.abi }}

- name: Enable KVM
shell: bash
run: |
Expand Down Expand Up @@ -78,7 +106,13 @@ runs:
SDK="${ANDROID_HOME:-${ANDROID_SDK_ROOT:-/usr/local/lib/android/sdk}}"
AVD='${{ inputs.avd-name }}'
PKG='system-images;android-${{ inputs.api-level }};${{ inputs.target }};${{ inputs.abi }}'
# Timed so the value of caching the image can be read straight off the
# job log rather than inferred.
echo "SYSIMG-CACHE hit=${{ steps.sysimg-cache.outputs.cache-hit || 'false' }}"
T0=$(date +%s)
yes | "$SDK/cmdline-tools/latest/bin/sdkmanager" --channel=3 --install "$PKG" platform-tools > /dev/null
echo "SYSIMG-INSTALL seconds=$(( $(date +%s) - T0 ))"
du -sh "$SDK/system-images/android-${{ inputs.api-level }}/${{ inputs.target }}/${{ inputs.abi }}" 2>/dev/null | sed 's/^/SYSIMG-SIZE /' || true
# Mimic the emulator-runner action's invocation (--abi and --device
# skip the auto-select/prompt path where bare creates silently no-op).
echo no | "$SDK/cmdline-tools/latest/bin/avdmanager" create avd --force -n "$AVD" \
Expand Down
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