Auto merge of #2873 - bitcartel:2793_dbcache, r=bitcartel

Closes #2793 to improve sync performance by increasing default value of dbcache.

doc/reducing-memory-usage.md

@@ -0,0 +1,8 @@
+In-memory caches
+----------------
+
+The size of some in-memory caches can be reduced. As caches trade off memory usage for performance, usually reducing these have a negative effect on performance.
+
+- `-dbcache=<n>` - the UTXO database cache size, this defaults to `450` (`100` before 1.0.15). The unit is MiB (where 1 GiB = 1024 MiB).
+  - The minimum value for `-dbcache` is 4.
+  - A lower dbcache make initial sync time much longer. After the initial sync, the effect is less pronounced for most use-cases, unless fast validation of blocks is important such as for mining.

doc/release-notes.md

@@ -4,3 +4,9 @@ release-notes at release time)
 Notable changes
 ===============
 
+UTXO memory accounting
+----------------------
+
+The default -dbcache has been changed in this release to 450MiB. Users can set -dbcache to a higher value (e.g. to keep the UTXO set more fully cached in memory). Users on low-memory systems (such as systems with 1GB or less) should consider specifying a lower value for this parameter.
+
+Additional information relating to running on low-memory systems can be found here: [reducing-memory-usage.md](https://github.com/zcash/zcash/blob/master/doc/reducing-memory-usage.md).

src/txdb.h

@@ -20,8 +20,8 @@ struct CDiskTxPos;
 class uint256;
 
 //! -dbcache default (MiB)
-static const int64_t nDefaultDbCache = 100;
-//! max. -dbcache in (MiB)
+static const int64_t nDefaultDbCache = 450;
+//! max. -dbcache (MiB)
 static const int64_t nMaxDbCache = sizeof(void*) > 4 ? 16384 : 1024;
 //! min. -dbcache in (MiB)
 static const int64_t nMinDbCache = 4;