Simone
9f78742111
Measured through the real pipeline, not the unit tests: a Rust job goes from 186s to 84s (build phase 150s -> 47s), and 75s with the source modified, which is the scenario CI actually runs. The entry is 645 MB and costs 8s to restore. The test that decides whether this ships is row 4: a deliberate type error in fury-core, built warm. The cache restored 645 MB of dependencies and the build still went red with E0308, exit 1, publishing nothing. A cache that can turn a red build green is the worst failure a CI system has because it is silent -- re-run that row whenever the prune rules change. AGENTS.md #17 records something found while validating: `vmrun list` is per-user. Run as the wrong user it answers "Total running VMs: 0" while a clone is very much alive, which makes the "no VMs running" check before a deploy --force (#14) unreliable. `ps | grep vmware-vmx` is user-independent and was already the recommended diagnosis.
142 lines
6.1 KiB
Markdown
142 lines
6.1 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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|---|---|
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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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## End-to-end, through `ci job` (2026-07-22)
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Full pipeline against `LinuxRust2404`, furyrs `b8ea567`, `--skip-artifact`.
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"Build" is the build phase alone; "total" includes clone, boot (~35s) and
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teardown.
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| # | Scenario | Build | Total | Cache line in the log |
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|---|---|---:|---:|---|
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| 1 | Cold, empty store | 150s | 186s | `miss` → `prune: ok` → `published … (645 MB)` |
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| 2 | Warm, same commit | **47s** | **84s** | `hit … — restoring` |
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| 3 | Warm, source modified | **38s** | **75s** | `hit … — restoring` |
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| 4 | Warm, deliberate compile error | — | — | `hit`, then **build fails, exit 1** |
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| 5 | `--no-cargo-cache` | 133s | 170s | none — cache never touched |
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**Saving: 103s on the build phase (150 → 47), 102s on the total (186 → 84).**
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Row 3 is the real CI scenario and is the fastest of the warm runs, because the
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touched crate is small.
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Row 2's log names every workspace crate being rebuilt —
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```
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Compiling fury-core, fury-gui, fury-ipc, fury-backend, furyd, fury-cli
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```
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— and no dependency. That is the design working: dependencies restored,
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workspace recompiled from source, every time.
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### Row 4 is the one that decides whether this ships
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A deliberate type error was pushed to a scratch branch and built warm:
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```
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[cache] hit for v1-1311… (645 MB) — restoring
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error[E0308]: mismatched types
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| --- ^^^^^^^^^^^^^^^^^^^ expected `u32`, found `&str`
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error: could not compile `fury-core` (lib) due to 1 previous error
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JOB_EXIT=1
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```
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The cache restored 645 MB of dependencies and the regression still went red. A
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cache that can turn a red build green is the worst failure a CI system has,
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because it is silent; this is the test that rules it out, and it is worth
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re-running whenever the prune rules change.
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The failed job also published nothing — the store still held exactly one entry
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and `.incoming/` was empty — because publishing sits after the build-failure
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check. A broken tree cannot become an entry.
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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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