coins: warn on oversized -dbcache

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>

src/node/caches.cpp

@@ -5,9 +5,12 @@
 #include <node/caches.h>
 
 #include <common/args.h>
+#include <common/system.h>
 #include <index/txindex.h>
 #include <kernel/caches.h>
 #include <logging.h>
+#include <node/interface_ui.h>
+#include <tinyformat.h>
 #include <util/byte_units.h>
 
 #include <algorithm>
@@ -23,16 +26,20 @@ static constexpr size_t MAX_FILTER_INDEX_CACHE{1024_MiB};
 static constexpr size_t MAX_32BIT_DBCACHE{1024_MiB};
 
 namespace node {
+size_t CalculateDbCacheBytes(const ArgsManager& args)
+{
+    if (auto db_cache{args.GetIntArg("-dbcache")}) {
+        if (*db_cache < 0) db_cache = 0;
+        const uint64_t db_cache_bytes{SaturatingLeftShift<uint64_t>(*db_cache, 20)};
+        constexpr auto max_db_cache{sizeof(void*) == 4 ? MAX_32BIT_DBCACHE : std::numeric_limits<size_t>::max()};
+        return std::max<size_t>(MIN_DB_CACHE, std::min<uint64_t>(db_cache_bytes, max_db_cache));
+    }
+    return DEFAULT_DB_CACHE;
+}
+
 CacheSizes CalculateCacheSizes(const ArgsManager& args, size_t n_indexes)
 {
-    // Convert -dbcache from MiB units to bytes. The total cache is floored by MIN_DB_CACHE and capped by max size_t value.
-    size_t total_cache{DEFAULT_DB_CACHE};
-    if (std::optional<int64_t> db_cache = args.GetIntArg("-dbcache")) {
-        if (*db_cache < 0) db_cache = 0;
-        uint64_t db_cache_bytes = SaturatingLeftShift<uint64_t>(*db_cache, 20);
-        constexpr auto max_db_cache{sizeof(void*) == 4 ? MAX_32BIT_DBCACHE : std::numeric_limits<size_t>::max()};
-        total_cache = std::max<size_t>(MIN_DB_CACHE, std::min<uint64_t>(db_cache_bytes, max_db_cache));
-    }
+    size_t total_cache{CalculateDbCacheBytes(args)};
 
     IndexCacheSizes index_sizes;
     index_sizes.tx_index = std::min(total_cache / 8, args.GetBoolArg("-txindex", DEFAULT_TXINDEX) ? MAX_TX_INDEX_CACHE : 0);
@@ -44,4 +51,15 @@ CacheSizes CalculateCacheSizes(const ArgsManager& args, size_t n_indexes)
     }
     return {index_sizes, kernel::CacheSizes{total_cache}};
 }
+
+void LogOversizedDbCache(const ArgsManager& args) noexcept
+{
+    if (const auto total_ram{GetTotalRAM()}) {
+        const size_t db_cache{CalculateDbCacheBytes(args)};
+        if (ShouldWarnOversizedDbCache(db_cache, *total_ram)) {
+            InitWarning(bilingual_str{tfm::format(_("A %zu MiB dbcache may be too large for a system memory of only %zu MiB."),
+                        db_cache >> 20, *total_ram >> 20)});
+        }
+    }
+}
 } // namespace node

src/node/caches.h

@@ -27,6 +27,13 @@ struct CacheSizes {
     kernel::CacheSizes kernel;
 };
 CacheSizes CalculateCacheSizes(const ArgsManager& args, size_t n_indexes = 0);
+constexpr bool ShouldWarnOversizedDbCache(size_t dbcache, size_t total_ram) noexcept
+{
+    const size_t cap{(total_ram < 2048_MiB) ? DEFAULT_DB_CACHE : (total_ram / 100) * 75};
+    return dbcache > cap;
+}
+
+void LogOversizedDbCache(const ArgsManager& args) noexcept;
 } // namespace node
 
 #endif // BITCOIN_NODE_CACHES_H