node: smooth oversized `dbcache` warnings #35200

Problem

The oversized -dbcache warning threshold has a sharp formula cliff when detected RAM crosses the cutoff used by the warning logic.

This was reported during review of #34641, where an earlier version could jump from the auto default at 4095 MiB RAM to 75% of RAM at 4096 MiB. That made 1 MiB of extra detected RAM raise the warning threshold from about 511 MiB to 3072 MiB.

The surviving warning-only code has the same shape at a different boundary. Below 2 GiB RAM the cap is DEFAULT_DB_CACHE (450 MiB), but at 2 GiB it switches to 75% of total RAM, so a tiny increase in detected RAM can suddenly raise the warning threshold from 450 MiB to about 1536 MiB.

<img width="1484" height="881" alt="Image" src="https://github.com/user-attachments/assets/b51d5d24-31b8-4a2a-8f70-e7536481f855" />

Fix

Base the warning on a reserved non-dbcache memory budget instead:

\text{warn if } \mathit{dbcache} > \max\left(\mathit{DEFAULT\_DB\_CACHE}, 0.75 \cdot \max(\text{total RAM} - \mathit{DBCACHE\_WARNING\_RESERVED\_RAM}, 0)\right)

DBCACHE_WARNING_RESERVED_RAM is 2 GiB, so the fixed DEFAULT_DB_CACHE cap remains the floor below that reserve and the warning threshold grows monotonically above it.

This keeps the warning conservative around low-memory boundaries and avoids treating a boundary-crossing 1 MiB RAM difference as a reason to allow a much larger explicit -dbcache.

Quick reference

On 32-bit builds, effective -dbcache values are still capped to 1024 MiB before the warning check, so thresholds above that cap are not reachable there.

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🚧 At least one of the CI tasks failed. _{Task i686, no IPC: https://github.com/bitcoin/bitcoin/actions/runs/25279789848/job/74115183233} _{LLM reason (✨ experimental): CI failed because the CTest “Bitcoin Core Test Suite” had failing tests, specifically sock_tests (1 test failed).}

<details><summary>Hints</summary>

Try to run the tests locally, according to the documentation. However, a CI failure may still happen due to a number of reasons, for example:

• Possibly due to a silent merge conflict (the changes in this pull request being incompatible with the current code in the target branch). If so, make sure to rebase on the latest commit of the target branch.

• A sanitizer issue, which can only be found by compiling with the sanitizer and running the affected test.

• An intermittent issue.

Leave a comment here, if you need help tracking down a confusing failure.

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