Simone
b5c18a6d70
Measured first, as thread entry #12 requires: a cold furyrs pipeline is 134.7s, target/ is 3.0 GiB raw and 2.5 GiB pruned, and the stored entry is 645 MiB. A warm job pays 8.1s (upload + extract) to skip ~120s of dependency compilation. Publishing costs 15.7s but only on a miss. Build it. The assertion in the plan turned out to be worthless and is replaced. It re-scanned for */.fingerprint/<member>-* after deleting exactly those paths, so asked to catch a planted fury-core fingerprint it exited 0 -- the prune had removed the evidence before the check looked. An assertion sharing its globs with the deletion can only prove that rm ran; it cannot see the failure that matters, which is a glob that never matched the artifact in the first place. The replacement asks a different oracle. Cargo records in each .d depfile the sources an artifact was built from: dependencies point into the registry, anything built from workspace source points back into the workspace. After a correct prune, zero surviving depfiles reference workspace source outside target/. Verified negatively too -- an artifact named totallyunrelated-abc123.d that no prune glob matches, declaring a dependency on fury-core/src/lib.rs, makes the script exit 2 and refuse to publish. zstd is already present on the template, so no rebuild is needed; the gzip fallback stays for the plain LinuxBuild2404.
94 lines
4.1 KiB
Markdown
94 lines
4.1 KiB
Markdown
# Cargo dependency cache — measurements
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Measured 2026-07-22 on a `LinuxRust2404` linked clone (4 vCPU, 3.9 GB RAM,
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`rustc 1.97.1`, `zstd` present at `/usr/bin/zstd`), running furyrs `b8ea567`
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with the `build-command` block from its `.gitea/workflows/ci.yml` verbatim.
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Entry #12 of the furyrs thread set the rule: measure before building. There it
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killed the registry cache — 9.3s of saving for +800 MB on the template. Here
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the numbers point the other way, decisively.
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## Baseline
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| Cold pipeline, empty `target/` | **134.7s** |
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Matches the 133s furyrs reported for job 1021 and the 126.6s measured in #12,
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so the clone is representative.
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## Sizes
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| | Bytes | Human |
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|---|---:|---:|
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| `target/` raw | 3,206,670,175 | 3.0 GiB |
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| `tar` of raw `target/` | 3,215,503,360 | 3.0 GiB |
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| `tar` + `zstd -3 -T0`, raw | 810,108,028 | 773 MiB |
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| **`target/` after prune** | 2,719,971,105 | **2.5 GiB** |
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| **entry as stored** (`tar --zstd`, pruned) | 676,585,779 | **645 MiB** |
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Pruning strips 487 MB (15%) of raw `target/`, which is the workspace crates'
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own artifacts — 42 fingerprint directories across the six members
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(`fury-core` 11, `fury-gui` 12, `fury-ipc` 6, `fury-backend` 5, `fury-cli` 5,
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`furyd` 3). 1,928 dependency artifacts and 903 dependency fingerprints survive.
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## Costs
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| Step | Seconds | Paid by |
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|---|---:|---|
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| prune + assert | 0.21 | publish |
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| pack (`tar --zstd`) | 10.88 | publish |
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| fetch guest→host (140 MiB/s) | 4.62 | publish |
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| **publish total** | **15.7** | **only on a cache miss** |
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| upload host→guest (179 MiB/s) | 3.60 | restore |
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| extract | 4.52 | restore |
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| **restore total** | **8.1** | **every warm job** |
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## Verdict: build it
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A warm job pays **8.1s** to skip roughly **120s** of dependency compilation —
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the ~490 external crates that a committed `Cargo.lock` keeps bit-identical
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between runs. Net saving ≈ 112s on a 134.7s job, about **83%**.
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The publish path costs 15.7s but runs only on a miss, i.e. once per
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`Cargo.lock` change, so it is amortised across every run in between.
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Transfer is not the bottleneck anyone feared: the virtual NIC moves the entry
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at 140–180 MiB/s, so 645 MiB crosses in under 4 seconds each way.
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## Notes that changed the implementation
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**`zstd` is already in the template.** `/usr/bin/zstd` is present on
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`LinuxRust2404`, so the gzip fallback is dead code in practice and no template
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rebuild is needed. Keep the fallback anyway — it costs one branch and keeps the
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feature working on a plain `LinuxBuild2404`.
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**The name-based assertion was worthless and was replaced.** The plan's
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original check re-scanned for `*/.fingerprint/<member>-*` after deleting
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exactly those paths. Asked to catch a planted `fury-core-deadbeefcafe`
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fingerprint, it exited **0** — the prune had deleted the evidence before the
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assertion looked. An assertion built from the same globs as the deletion can
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only prove that `rm` ran; it is blind to the failure that matters, a glob that
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never matched the artifact at all.
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The replacement asks a different oracle. Cargo records in each `.d` depfile the
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source files an artifact was built *from*: a dependency points into
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`/opt/rust/cargo/registry/...`, anything built from workspace source points
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back into the workspace. After a correct prune, **zero** surviving depfiles
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reference workspace source outside `target/`. Verified negatively as well — an
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artifact named `totallyunrelated-abc123.d` (which no prune glob matches)
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declaring a dependency on `fury-core/src/lib.rs` makes the script exit **2**
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and refuse to publish.
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| Check | Clean tree | Planted fingerprint | Planted foreign artifact |
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|---|---|---|---|
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| name-based (original) | 0 | **0 — missed it** | 0 — missed it |
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| depfile-based (shipped) | 0 | 0 (prune removed it, correctly) | **2 — refused** |
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**Feature variants coexist in one entry, as predicted.** `deps/` holds
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`libclap-943062c20e623534.rmeta` and `libclap-abe395d6778d44ce.rmeta` side by
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side — the same crate compiled under two different feature resolutions. This is
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why the feature set is not part of the cache key: cargo already separates the
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variants by hash, so one entry serves all of furyrs's four feature
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configurations instead of fragmenting into four.
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