UCX: provide an option to enable/disable UCX rcache

[ZhenlongMa]

What?

Disable UCX registration caches by default in the UCX backend.

Why?

NIXL always passes the local memory handle (memh) explicitly to UCX operations, so UCX does not need to rely on memory hooks for this path.

Installing memory hooks is intrusive and can lead to correctness or stability issues, especially for dynamic memory workloads. Disabling rcache by default avoids installing those hooks while preserving the existing explicit memory registration flow.

How?

This PR adds a build-time enable_rcache Meson option. When it is not set, NIXL configures UCX with:

• RCACHE_ENABLE=n
• GDR_COPY_RCACHE=n
• ROCM_COPY_RCACHE=n

so that all kinds of rcache are disabled.

Summary by CodeRabbit

• Chores
  • Added build configuration option to control UCX registration caching behavior. Users building from source can now enable or disable this feature during compilation (default: disabled).

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[coderabbitai]

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 49feac6 and a509b23.

📒 Files selected for processing (3)

• meson.build
• meson_options.txt
• src/plugins/ucx/ucx_utils.cpp

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📝 Walkthrough

Walkthrough

The PR adds a build-time configuration option to selectively enable or disable UCX registration caching. A new enable_rcache Meson option defaults to false; when enabled, it injects a preprocessor define that conditionally gates rcache-related configuration in the UCX context initialization.

Changes

UCX Registration Cache Feature Gating

Layer / File(s)	Summary
Build option and preprocessor integration meson_options.txt, meson.build	New boolean enable_rcache option added with default false; when enabled, generates -DNIXL_UCX_ENABLE_RCACHE preprocessor define for C/C++ compilation.
UCX rcache configuration gating src/plugins/ucx/ucx_utils.cpp	UCX rcache overrides (RCACHE_ENABLE, GDR_COPY_RCACHE, ROCM_COPY_RCACHE) are conditionally applied only when the preprocessor define is not defined.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Suggested labels

external-contribution, size/XS

Suggested reviewers

• yosefe
• brminich
• gleon99

Poem

🐰 A cache for UCX, now gated with care,
Build options define what's compiled where,
Preprocessor guards keep features aligned,
Registration caching, thoughtfully designed! ✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'UCX: provide an option to enable/disable UCX rcache' accurately summarizes the main change: adding a build-time option to control UCX rcache behavior.
Description check	✅ Passed	The description includes all required template sections (What, Why, How) with comprehensive details about the rationale, implementation approach, and configuration options.
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.

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✨ Finishing Touches

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[ZhenlongMa]

Hi reviewers,
This PR has been waiting for review for about two weeks. Just wanted to check if there is any additional work or changes needed from my side.
Thanks.

[iyastreb]

LGTM
Disabling GDR/rocm rcache may change registration cost, but I think it's not too risky