coins: warn on oversized -dbcache

l0rinc · fanquake · commit 5226a92f282b · 2025-09-23T10:20:06.000-04:00
Oversized allocations can cause out-of-memory errors or [heavy swapping](getumbrel/umbrel-os#64 (comment)), [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](getumbrel/umbrel-os#64 (comment)) 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: 168360f
diff --git a/src/init.cpp b/src/init.cpp
@@ -1733,6 +1733,7 @@ bool AppInitMain(NodeContext& node, interfaces::BlockAndHeaderTipInfo* tip_info)
     // ********************************************************* Step 7: load block chain
 
     // cache size calculations
+    node::LogOversizedDbCache(args);
     const auto [index_cache_sizes, kernel_cache_sizes] = CalculateCacheSizes(args, g_enabled_filter_types.size());
 
     LogInfo("Cache configuration:");
diff --git a/src/node/caches.cpp b/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 {
-CacheSizes CalculateCacheSizes(const ArgsManager& args, size_t n_indexes)
+size_t CalculateDbCacheBytes(const ArgsManager& args)
 {
-    // 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 (auto db_cache{args.GetIntArg("-dbcache")}) {
         if (*db_cache < 0) db_cache = 0;
-        uint64_t db_cache_bytes = SaturatingLeftShift<uint64_t>(*db_cache, 20);
+        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()};
-        total_cache = std::max<size_t>(MIN_DB_CACHE, std::min<uint64_t>(db_cache_bytes, max_db_cache));
+        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)
+{
+    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
diff --git a/src/node/caches.h b/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
diff --git a/src/test/CMakeLists.txt b/src/test/CMakeLists.txt
@@ -25,6 +25,7 @@ add_executable(test_bitcoin
   blockmanager_tests.cpp
   bloom_tests.cpp
   bswap_tests.cpp
+  caches_tests.cpp
   chainstate_write_tests.cpp
   checkqueue_tests.cpp
   cluster_linearize_tests.cpp
diff --git a/src/test/caches_tests.cpp b/src/test/caches_tests.cpp
@@ -0,0 +1,40 @@
+#include <node/caches.h>
+#include <util/byte_units.h>
+
+#include <boost/test/unit_test.hpp>
+
+using namespace node;
+
+BOOST_AUTO_TEST_SUITE(caches_tests)
+
+BOOST_AUTO_TEST_CASE(oversized_dbcache_warning)
+{
+    // memory restricted setup - cap is DEFAULT_DB_CACHE (450 MiB)
+    BOOST_CHECK(!ShouldWarnOversizedDbCache(/*dbcache=*/4_MiB, /*total_ram=*/1024_MiB));    // Under cap
+    BOOST_CHECK( ShouldWarnOversizedDbCache(/*dbcache=*/512_MiB, /*total_ram=*/1024_MiB));  // At cap
+    BOOST_CHECK( ShouldWarnOversizedDbCache(/*dbcache=*/1500_MiB, /*total_ram=*/1024_MiB)); // Over cap
+
+    // 2 GiB RAM - cap is 75%
+    BOOST_CHECK(!ShouldWarnOversizedDbCache(/*dbcache=*/1500_MiB, /*total_ram=*/2048_MiB)); // Under cap
+    BOOST_CHECK( ShouldWarnOversizedDbCache(/*dbcache=*/1600_MiB, /*total_ram=*/2048_MiB)); // Over cap
+
+    if constexpr (SIZE_MAX == UINT64_MAX) {
+        // 4 GiB RAM - cap is 75%
+        BOOST_CHECK(!ShouldWarnOversizedDbCache(/*dbcache=*/2500_MiB, /*total_ram=*/4096_MiB)); // Under cap
+        BOOST_CHECK( ShouldWarnOversizedDbCache(/*dbcache=*/3500_MiB, /*total_ram=*/4096_MiB)); // Over cap
+
+        // 8 GiB RAM - cap is 75%
+        BOOST_CHECK(!ShouldWarnOversizedDbCache(/*dbcache=*/6000_MiB, /*total_ram=*/8192_MiB)); // Under cap
+        BOOST_CHECK( ShouldWarnOversizedDbCache(/*dbcache=*/7000_MiB, /*total_ram=*/8192_MiB)); // Over cap
+
+        // 16 GiB RAM - cap is 75%
+        BOOST_CHECK(!ShouldWarnOversizedDbCache(/*dbcache=*/10'000_MiB, /*total_ram=*/16384_MiB)); // Under cap
+        BOOST_CHECK( ShouldWarnOversizedDbCache(/*dbcache=*/15'000_MiB, /*total_ram=*/16384_MiB)); // Over cap
+
+        // 32 GiB RAM - cap is 75%
+        BOOST_CHECK(!ShouldWarnOversizedDbCache(/*dbcache=*/20'000_MiB, /*total_ram=*/32768_MiB)); // Under cap
+        BOOST_CHECK( ShouldWarnOversizedDbCache(/*dbcache=*/30'000_MiB, /*total_ram=*/32768_MiB)); // Over cap
+    }
+}
+
+BOOST_AUTO_TEST_SUITE_END()
