Limit arena size in C++ tests

[bdice]

Description

Use an explicit 8 GiB arena in the C++ arena MR reference tests instead of the default half of available device memory. This prevents 116 fixture constructions from each reserving roughly 125 GiB on GB300 while preserving the capacity available on the original 16 GiB test target.

Measured on DGX Spark (128GB integrated memory), total test time for the arena MR reference tests decreased from 64.41 seconds to 11.88 seconds (81.6% reduction).

Closes #2505. Follows the Python test precedent in #2273.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

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📝 Walkthrough

Summary by CodeRabbit

• Tests

  • Increased test arena capacity to 8 GiB for more extensive memory-resource testing.
  • Updated continuous integration to run the nightly test matrix for the GB300 GPU configuration.
  • Added diagnostics for kernel memory initialization and NVIDIA driver settings during C++ test runs.

• Chores

  • Streamlined continuous integration by disabling selected build, test, packaging, and telemetry jobs.
  • Updated copyright and licensing notices to include NVIDIA affiliates.

Walkthrough

Arena memory resource tests now use an explicit 8 GiB capacity. The C++ nightly workflow targets GB300 and disables selected CI jobs. The C++ test script reports kernel and NVIDIA driver allocation settings.

Changes

Arena test sizing and CI workflow

Layer / File(s)	Summary
Configure explicit arena capacity cpp/tests/mr/mr_ref_arena_tests.cpp, cpp/tests/mr/mr_ref_test.hpp	Both arena test fixtures pass an explicit 8 GiB initial size. Copyright notices include NVIDIA affiliates.
Target GB300 nightly tests .github/workflows/pr.yaml	The conda-cpp-tests job uses the nightly matrix and filters it to gb300. Selected CI jobs are disabled with if: false.
Report allocation settings ci/test_cpp.sh	The script reports /proc/cmdline’s init_on_alloc setting and the NVIDIA driver’s Coherent parameter. It reports explicit unavailable messages when either value cannot be read.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

• rapidsai/rmm#2468: Both PRs update arena memory-resource code and related tests, but that PR adds [[nodiscard]] allocation annotations.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Out of Scope Changes check	⚠️ Warning	The workflow disables multiple unrelated CI jobs, and ci/test_cpp.sh adds diagnostics not required by issue #2505.	Remove unrelated workflow changes and CI diagnostics, or link them to separate issues with clear justification.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly describes the primary change: limiting the arena size in C++ tests.
Description check	✅ Passed	The description explains the explicit arena size, the performance issue, measured improvement, and linked issue.
Linked Issues check	✅ Passed	The PR passes an explicit 8 GiB arena size to both arena test fixtures, addressing issue #2505 without increasing the timeout.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

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[bdice]

/ok to test

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Actionable comments posted: 2

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In @.github/workflows/pr.yaml:
- Line 59: Replace each constant if: false condition in the workflow with an
appropriate feature flag, or remove the disabled jobs and corresponding
pr-builder.needs entries. Ensure no unconditional constant-false conditions
remain so actionlint passes.
- Around line 282-283: Update the GB300 nightly matrix entry by setting its
build_workflow_name input to pr.yaml, ensuring artifact lookup targets the C++
artifacts produced by conda-cpp-build in pr.yaml while preserving the existing
matrix_type and matrix_filter values.

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Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@ci/test_cpp.sh`:
- Line 39: Update the `/proc/cmdline` check in `ci/test_cpp.sh` to validate
readability before searching for `init_on_alloc`. Report the setting as
unavailable with a meaningful error message when the file is missing or
unreadable, and retain the existing “not set” message only when a readable file
lacks the parameter.

🪄 Autofix

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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📥 Commits

Reviewing files that changed from the base of the PR and between 221ee4e and 20b3dd1.

📒 Files selected for processing (1)

• ci/test_cpp.sh

[bdice]

I have some temporary testing changes for GB300 so that I can figure out why the tests are so slow. I think it's an issue with missing the init_on_alloc=0 config. I'm verifying that.

[wence-]

Approving C++ changes, which seem reasonable independent of the runner setup issues.