Bump `-dbcache` default to 300MiB

Also cap the allocation for the leveldb-specific cache for the UTXO set
to 8MiB.
This avoids that the extra cache memory goes to the much less effective
leveldb cache instead of our application-level cache.

src/init.cpp

@@ -1216,10 +1216,10 @@ bool AppInit2(boost::thread_group& threadGroup, CScheduler& scheduler)
     nTotalCache = std::max(nTotalCache, nMinDbCache << 20); // total cache cannot be less than nMinDbCache
     nTotalCache = std::min(nTotalCache, nMaxDbCache << 20); // total cache cannot be greated than nMaxDbcache
     int64_t nBlockTreeDBCache = nTotalCache / 8;
-    if (nBlockTreeDBCache > (1 << 21) && !GetBoolArg("-txindex", DEFAULT_TXINDEX))
-        nBlockTreeDBCache = (1 << 21); // block tree db cache shouldn't be larger than 2 MiB
+    nBlockTreeDBCache = std::min(nBlockTreeDBCache, (GetBoolArg("-txindex", DEFAULT_TXINDEX) ? nMaxBlockDBAndTxIndexCache : nMaxBlockDBCache) << 20);
     nTotalCache -= nBlockTreeDBCache;
     int64_t nCoinDBCache = std::min(nTotalCache / 2, (nTotalCache / 4) + (1 << 23)); // use 25%-50% of the remainder for disk cache
+    nCoinDBCache = std::min(nCoinDBCache, nMaxCoinsDBCache << 20); // cap total coins db cache
     nTotalCache -= nCoinDBCache;
     nCoinCacheUsage = nTotalCache; // the rest goes to in-memory cache
     LogPrintf("Cache configuration:\n");

src/txdb.h

@@ -22,11 +22,19 @@ class CCoinsViewDBCursor;
 class uint256;
 
 //! -dbcache default (MiB)
-static const int64_t nDefaultDbCache = 100;
-//! max. -dbcache in (MiB)
+static const int64_t nDefaultDbCache = 300;
+//! max. -dbcache (MiB)
 static const int64_t nMaxDbCache = sizeof(void*) > 4 ? 16384 : 1024;
-//! min. -dbcache in (MiB)
+//! min. -dbcache (MiB)
 static const int64_t nMinDbCache = 4;
+//! Max memory allocated to block tree DB specific cache, if no -txindex (MiB)
+static const int64_t nMaxBlockDBCache = 2;
+//! Max memory allocated to block tree DB specific cache, if -txindex (MiB)
+// Unlike for the UTXO database, for the txindex scenario the leveldb cache make
+// a meaningful difference: https://github.com/bitcoin/bitcoin/pull/8273#issuecomment-229601991
+static const int64_t nMaxBlockDBAndTxIndexCache = 1024;
+//! Max memory allocated to coin DB specific cache (MiB)
+static const int64_t nMaxCoinsDBCache = 8;
 
 struct CDiskTxPos : public CDiskBlockPos
 {