fix(cmake): Explicitly use the logical CPU count when JOBS is unset.

[jackluo923]

Description

cmake:build always emitted --parallel "{{.JOBS}}" with JOBS defaulting to the empty string, so an unset JOBS rendered cmake --build <dir> --parallel "". cmake turns that into bare gmake -f Makefile -j, i.e. unbounded parallelism — a single build could spawn far more compiler processes than there are cores, oversubscribing the machine and risking OOM on large dependencies like folly.

Bound the default to the number of logical CPUs (nproc/sysctl, falling back to 1), so --parallel always receives a finite, right-sized value and a single build no longer oversubscribes. Callers can still override via JOBS (or CMAKE_JOBS, which flows through install-remote-tar -> build).

This matches boost:build-and-install (b2 defaults to the core count for boost >= 1.76.0). Cross-build scheduling — running several cmake:build invocations concurrently — remains the caller's responsibility; set JOBS lower to avoid oversubscribing across builds.

Cross-platform: nproc (Linux) -> sysctl -n hw.logicalcpu (macOS) -> echo 1.

Validation performed

task lint:check and task test pass.

Scratch Taskfile.yml including utils/utils.yaml, wrapping utils:cmake:build (host has 12 logical CPUs):

Input	Rendered command
no JOBS	cmake --build "build" --parallel "12"
JOBS= (explicit empty)	cmake --build "build" --parallel "12"
JOBS=5	cmake --build "build" --parallel "5" (override respected)

For reference, the pre-fix empty case rendered --parallel "" -> bare gmake -f Makefile -j (unlimited).

Summary by CodeRabbit

• Bug Fixes
  • Build tasks now default to a sensible CPU-count-based parallel job value when no job count is provided, improving performance out of the box.
  • Updated installation task documentation so its job-setting guidance matches the build task’s CPU-count default behavior.

[coderabbitai]

Walkthrough

The CMake taskfile now sets a default JOBS value from the local CPU count when unset, and updates the related CMAKE_JOBS comment to match the build task behaviour.

Changes

CMake JOBS default handling

Layer / File(s)	Summary
Default JOBS from CPU count exports/taskfiles/utils/cmake.yaml	The build task now computes _CPU_COUNT with nproc or sysctl with a fallback of 1, uses it as the default JOBS, and updates the CMAKE_JOBS comment to reference the build task’s JOBS.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly matches the main change: defaulting unset JOBS to the logical CPU count.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/cmake-conditional-parallel

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[jackluo923]

After chatting with @davidlion offline, we've decided on a more comprehensive fix.