[multi-vector] Detect L1/L2 cache sizes for matrix kernels

[Wei Wu (wuw92)]

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• Does this PR add any new dependencies?
• Does this PR modify any existing APIs?
• Is the change to the API backwards compatible?
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Adds target-specific dependencies on raw-cpuid and libc. No public API signatures change. Internal cache-budget documentation is updated.

Reference Issues/PRs

Builds on the matrix-kernel design introduced in #1368.

What does this implement/fix? Briefly explain your changes.

matrix_kernels::Cache::detect() still derives its budgets from hardcoded cache-size estimates of 48 KB L1d and 1.25 MB L2. This change replaces those estimates with runtime-detected sizes, memoized once per process.

Detection is architecture-first: CPUID where available, OS APIs otherwise.

Arch / OS	Mechanism
x86_64 (Windows, Linux, macOS)	CPUID via raw-cpuid
aarch64 Linux	sysfs
aarch64 macOS	sysctl hw.perflevel0.*, with L2 divided by cpusperl2
Unsupported targets or unavailable probes	48 KB L1d / 1.25 MB L2 fallback

[Copilot]

Pull request overview

This PR updates the multi-vector distance tiling planner in diskann-quantization to derive its tile budgets from runtime-detected L1d/L2 cache sizes (with per-arch fallbacks), replacing the prior hardcoded Skylake-X assumptions.

Changes:

• Added a memoized cache-size probe (L1d, L2) with x86_64 CPUID detection and aarch64 Linux/macOS OS-specific probes, plus a conservative fallback.
• Updated TileBudget::default() to use detected cache sizes when computing L1/L2-derived budgets for tile planning.
• Introduced target-specific dependencies (raw-cpuid on x86_64; libc on aarch64 macOS) to support probing.

Reviewed changes

Copilot reviewed 7 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
diskann-quantization/src/multi_vector/distance/mod.rs	Wires in the new cache submodule.
diskann-quantization/src/multi_vector/distance/kernels/mod.rs	Switches TileBudget::default() to runtime-detected cache sizes.
diskann-quantization/src/multi_vector/distance/cache/mod.rs	Adds memoized cache probing API and basic plausibility/memoization tests.
diskann-quantization/src/multi_vector/distance/cache/cpuid.rs	Implements x86_64 cache detection using raw-cpuid.
diskann-quantization/src/multi_vector/distance/cache/linux.rs	Implements aarch64 Linux detection via sysfs cache entries.
diskann-quantization/src/multi_vector/distance/cache/macos.rs	Implements aarch64 macOS detection via sysctl hw.perflevel0.*.
diskann-quantization/Cargo.toml	Adds target-specific dependencies for the detection paths.
Cargo.lock	Records the new dependency resolutions.

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[Codecov Comments Bot (codecov-commenter)]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.53%. Comparing base (765c794) to head (92b822b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1134      +/-   ##
==========================================
+ Coverage   91.52%   91.53%   +0.01%     
==========================================
  Files         526      528       +2     
  Lines      103087   103140      +53     
==========================================
+ Hits        94351    94414      +63     
+ Misses       8736     8726      -10     

Flag	Coverage Δ
miri	91.53% <100.00%> (+0.01%)	⬆️
unittests	91.28% <100.00%> (+0.01%)	⬆️

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Files with missing lines	Coverage Δ
...ann-quantization/src/matrix_kernels/cache/cpuid.rs	100.00% <100.00%> (ø)
...skann-quantization/src/matrix_kernels/cache/mod.rs	100.00% <100.00%> (ø)
diskann-quantization/src/matrix_kernels/mod.rs	100.00% <100.00%> (ø)

... and 8 files with indirect coverage changes

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