Oversized allocations can cause out-of-memory errors or [heavy swapping](https://github.com/getumbrel/umbrel-os/issues/64#issuecomment-663637321), [grinding the system to a halt](https://x.com/murchandamus/status/1964432335849607224).
`LogOversizedDbCache()` now emits a startup warning if the configured `-dbcache` exceeds a cap derived from system RAM, using the same parsing/clamping as cache sizing via CalculateDbCacheBytes(). This isn't meant as a recommended setting, rather a likely upper limit.
Note that we're not modifying the set value, just issuing a warning.
Also note that the 75% calculation is rounded for the last two numbers since we have to divide first before multiplying, otherwise we wouldn't stay inside size_t on 32-bit systems - and this was simpler than casting back and forth.
We could have chosen the remaining free memory for the warning (e.g. warn if free memory is less than 1 GiB), but this is just a heuristic, we assumed that on systems with a lot of memory, other processes are also running, while memory constrained ones run only Core.
If total RAM < 2 GiB, cap is `DEFAULT_DB_CACHE` (`450 MiB`), otherwise it's 75% of total RAM.
The threshold is chosen to be close to values commonly used in [raspiblitz](https://github.com/raspiblitz/raspiblitz/blob/dev/home.admin/_provision.setup.sh#L98-L115) for common setups:
| Total RAM | `dbcache` (MiB) | raspiblitz % | proposed cap (MiB) |
|----------:|----------------:|-------------:|-------------------:|
| 1 GiB | 512 | 50.0% | 450* |
| 2 GiB | 1536 | 75.0% | 1536 |
| 4 GiB | 2560 | 62.5% | 3072 |
| 8 GiB | 4096 | 50.0% | 6144 |
| 16 GiB | 4096 | 25.0% | 12288 |
| 32 GiB | 4096 | 12.5% | 24576 |
[Umbrel issues](https://github.com/getumbrel/umbrel-os/issues/64#issuecomment-663816367) also mention 75% being the upper limit.
Starting `bitcoind` on an 8 GiB rpi4b with a dbcache of 7 GiB:
> ./build/bin/bitcoind -dbcache=7000
warns now as follows:
```
2025-09-07T17:24:29Z [warning] A 7000 MiB dbcache may be too large for a system memory of only 7800 MiB.
2025-09-07T17:24:29Z Cache configuration:
2025-09-07T17:24:29Z * Using 2.0 MiB for block index database
2025-09-07T17:24:29Z * Using 8.0 MiB for chain state database
2025-09-07T17:24:29Z * Using 6990.0 MiB for in-memory UTXO set (plus up to 286.1 MiB of unused mempool space)
```
Besides the [godbolt](https://godbolt.org/z/EPsaE3xTj) reproducers for the new total memory method, we also tested the warnings manually on:
- [x] Apple M4 Max, macOS 15.6.1
- [x] Intel Core i9-9900K, Ubuntu 24.04.2 LTS
- [x] Raspberry Pi 4 Model B, Armbian Linux 6.12.22-current-bcm2711
- [x] Intel Xeon x64, Windows 11 Home Version 24H2, OS Build 26100.4351
Co-authored-by: stickies-v <stickies-v@protonmail.com>
Co-authored-by: Hodlinator <172445034+hodlinator@users.noreply.github.com>
Co-authored-by: w0xlt <woltx@protonmail.com>
Github-Pull: #33333
Rebased-From: 168360f4ae47cbfdb30a2cc4704435bc67e12f16
