Tier RAM budget by host size

[Andy11-cpu]

Summary

• Use 25% of physical RAM on hosts with <=16GB, 35% on <=32GB, and 50% above
• Add cbm_mem_ram_fraction_for_total() with unit tests

Motivation

Split from #625 — reduces default memory reservation on 16GB machines without changing indexing behavior.

Test plan

• mem_ram_fraction_* tier tests in test_mem.c
• make -f Makefile.cbm test (CI)

Signed-off-by: Andy11-cpu canada11@duck.com

[DeusData]

Thanks for the RAM-budget split. Triage: high-priority stability/performance work from the memory-pressure track.

Review will check that the tiered default is conservative on 16GB machines, does not regress larger-host throughput unexpectedly, and composes correctly with explicit memory-budget overrides. Please keep any benchmark or before/after memory observations in the PR description as it evolves.

[DeusData]

Quick status note: this PR is one of four open memory/RAM-policy changes (#833, #752, #586, #685) that we've reviewed individually and found genuinely complementary — so rather than merging them piecemeal, we're doing a combined design pass over the whole memory policy (explicit override, host-tiered defaults, retention bounds, post-index release, and the Windows auto-sync driver in #841) and will respond here with a concrete direction shortly. Your work is very much part of that plan — thanks for your patience!

[DeusData]

Thank you — this is a clean, well-judged fix: tiering the RAM fraction by host size (25% ≤16GB / 35% ≤32GB / 50% above) so small and containerized hosts don't over-commit, and it correctly reads the cgroup-aware total so a memory-limited container gets the right tier. Atomic, green on all platforms, with genuine boundary tests. Merged as a960d3d. It composes cleanly with the CBM_MEM_BUDGET_MB explicit override (override still wins) and the retention layer's re-read fallback covers the smaller-budget case with no lost edges. Appreciated — and thanks for splitting this out from the earlier bundle so it could land on its own merits!