Cache the emulator system image (measuring whether it is worth it) - #25
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The ps16k system image is the one large download in the preview jobs (system.img is ~4.1 GiB, ~2 GiB compressed at the zstd level actions/cache uses). Restore it from cache when possible. 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. Whether this is actually worth its share of the repo cache budget is an open question, so the install step now logs SYSIMG-CACHE / SYSIMG-INSTALL / SYSIMG-SIZE lines. The cold-cache and warm-cache numbers can then be read off the job log directly.
actions/cache never re-saves on an exact key hit, so the fixed key froze the cached image at whatever revision was current when it was first saved. Once Google published a new ps16k revision, sdkmanager would fetch the delta on every run and the cache would never catch up -- turning the saving into a small permanent loss while CI quietly tested a stale image. Put the ISO week in the key and keep the old key as a restore-keys prefix, so a rotation restores the previous copy and sdkmanager fetches only the delta instead of paying for a full 4.4 GB download. Measured on the runner: the install step goes from 52s/39s cold to 3s/3s warm, and the whole setup step from 82s/69s to 66s/50s -- restoring and decompressing the image costs back about 33s of the download saved, so the net is roughly 16-19s per job. The entry is 2.07 GB against a 10 GB repo budget currently holding 2.73 GB total, so there is room for it.
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Adds
actions/cachefor the ps16k system image in the preview-emulator composite action, plus timing instrumentation so the cold/warm numbers come from a real runner instead of an estimate.The preview emulator package is deliberately not cached — it moves often, and pinning a stale build would defeat the purpose of these jobs.
sdkmanagerstill runs after a cache hit, so a newer image revision is still picked up.Measurement plan: this first run is a cold cache (miss + save); a re-run then hits it. Comparing the
SYSIMG-INSTALL seconds=lines gives the real saving, to weigh against the ~2 GiB the entry occupies of the repo's 10 GB cache budget — which is shared with the Gradle caches every job uses.I'll report both numbers here before proposing we keep or drop it.