andygrove opened a new issue, #6387:
URL: https://github.com/apache/datafusion-comet/issues/6387
### What is the problem the feature request solves?
About half of the merge-queue runs compile the native library from scratch.
Of 155 successful full queue runs between 2026-09-11 and 2026-09-29, 76 missed
the cargo cache in the Spark SQL build job, and those took a median of 115
minutes against 88 for the rest. The same miss costs the Linux `Build Native
Library` job and the Iceberg build about 25 minutes each (~29 cold vs ~4 warm),
and it showed up in 10 of 25 sampled PR runs.
The cause is the repository's 10 GB Actions cache budget. The entries main
needs add up to about 12 GB:
| Entry | Size | Written by |
| --- | --- | --- |
| `Linux-cargo-debug-<lock>-<rs>` | 4.1 to 5.3 GB | `linux-test-rust`, on
every push that changes Rust |
| `setup-java-Linux-x64-maven-*` | 2.5 GB | `docs.yaml` (`setup-java` with
`cache: 'maven'`), read by nothing else |
| `Linux-spark-sql-*` | 2.0 GB | `setup-spark-builder` |
| `Linux-java-maven-*` | 1.5 GB | the TPC-H/TPC-DS jobs |
| `Linux-cargo-ci-<lock>-<rs>` | 1.4 GB | `build-native`, on every push that
changes Rust |
Both cargo keys include `hashFiles('native/**/*.rs')`, so every Rust change
on main writes a new pair, and entries keep evicting each other. On 2026-09-29
the `cargo-ci` entry written at 06:18 was gone by 07:46, and at 13:36
`cargo-debug` alone was 5.29 GB of a 9.99 GB total.
`cargo-debug` is the largest entry and buys no wall-clock time.
`linux-test-rust` runs alongside `build-native`, finishes well before the Linux
test matrix on a pull request, and far before the Spark SQL suites in the
queue. It saves about 7 runner-minutes a run (13 minutes warm against 20 cold).
### Describe the potential solution
1. Key the `cargo-ci` cache on `Cargo.lock` and `Cargo.toml` only, in every
workflow that reads it, and save it on main only when that exact key is
missing. That leaves one entry per dependency set instead of one per Rust
change; the workspace crates recompile either way.
2. Stop caching `native/target` for `linux-test-rust`. Keep the crate
registry (`~/.cargo/registry`, `~/.cargo/git`) warm through a small
registry-only entry that `build-native` writes. `linux-test-rust` then writes
no cache of its own and no longer needs to run on push.
3. Drop `cache: 'maven'` from `docs.yaml`. The site deploy runs after the
merge and gates nothing, so downloading its Maven dependencies costs a few
minutes of a job nobody waits on.
That brings the steady state on main to about 6 GB, which also leaves room
for a macOS native build cache.
### Additional context
#6112 covers the TPC dataset caches, which are also written from
pull-request and queue refs.
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