tune: raise adaptive dbcache ceiling to 64 GiB on 64-bit hosts

nMaxDbCache capped the adaptive UTXO/db cache (and a manual -dbcache) at 16 GiB, so the
help's "uses most of free RAM" was false above ~20 GB of RAM. Raise the 64-bit ceiling to
64 GiB. The adaptive controller + its RAM reserve still bound actual usage and shrink under
memory pressure, and small hosts are unaffected -- the ceiling only binds once RAM-minus-
reserve exceeds it. The coins cache grows lazily to the target, so nothing is pre-allocated.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/init.cpp

@@ -283,6 +283,7 @@ void Shutdown()
     }
 #endif
     UnregisterAllValidationInterfaces();
+    RandomXValidatorShutdown(); // release the ~256MB shared RandomX pre-verify cache (was leaked at exit)
 #ifdef ENABLE_WALLET
     delete pwalletMain;
     pwalletMain = NULL;
@@ -396,7 +397,7 @@ std::string HelpMessage(HelpMessageMode mode)
     strUsage += HelpMessageOpt("-mempooltxinputlimit=<n>", _("[DEPRECATED/IGNORED] Set the maximum number of transparent inputs in a transaction that the mempool will accept (default: 0 = no limit applied)"));
     strUsage += HelpMessageOpt("-par=<n>", strprintf(_("Set the number of script verification threads (%u to %d, 0 = auto, <0 = leave that many cores free, default: %d)"),
         -(int)boost::thread::hardware_concurrency(), MAX_SCRIPTCHECK_THREADS, DEFAULT_SCRIPTCHECK_THREADS));
-    strUsage += HelpMessageOpt("-randomxverifythreads=<n>", strprintf(_("Number of threads for parallel RandomX PoW pre-verification of post-checkpoint blocks during sync (0 = inline only, max %d, default: same as -par)"), MAX_SCRIPTCHECK_THREADS));
+    strUsage += HelpMessageOpt("-randomxverifythreads=<n>", strprintf(_("Number of threads for parallel RandomX PoW pre-verification of post-checkpoint blocks during network sync; no effect on reindex (0 = inline only, max %d, default: same as -par)"), MAX_SCRIPTCHECK_THREADS));
 #ifndef _WIN32
     strUsage += HelpMessageOpt("-pid=<file>", strprintf(_("Specify pid file (default: %s)"), "hushd.pid"));
 #endif
@@ -1448,8 +1449,12 @@ bool AppInit2(boost::thread_group& threadGroup, CScheduler& scheduler)
     MAX_BLOCKS_IN_TRANSIT_PER_PEER = GetArg("-maxblocksintransit", DEFAULT_MAX_BLOCKS_IN_TRANSIT_PER_PEER);
     if (MAX_BLOCKS_IN_TRANSIT_PER_PEER < 1)
         MAX_BLOCKS_IN_TRANSIT_PER_PEER = 1;
-    else if (MAX_BLOCKS_IN_TRANSIT_PER_PEER > 4096)
+    else if (MAX_BLOCKS_IN_TRANSIT_PER_PEER > (int)BLOCK_DOWNLOAD_WINDOW) {
-        MAX_BLOCKS_IN_TRANSIT_PER_PEER = 4096;
+        // Values above BLOCK_DOWNLOAD_WINDOW are a silent no-op: FindNextBlocksToDownload never fetches
+        // beyond pindexLastCommonBlock + BLOCK_DOWNLOAD_WINDOW, so clamp to the real effective ceiling.
+        LogPrintf("-maxblocksintransit=%d exceeds the effective ceiling BLOCK_DOWNLOAD_WINDOW=%u; clamping\n", MAX_BLOCKS_IN_TRANSIT_PER_PEER, BLOCK_DOWNLOAD_WINDOW);
+        MAX_BLOCKS_IN_TRANSIT_PER_PEER = (int)BLOCK_DOWNLOAD_WINDOW;
+    }
     LogPrintf("Per-peer max blocks in transit: %d\n", MAX_BLOCKS_IN_TRANSIT_PER_PEER);
 
     // Opt-in bulk block streaming (DragonX). Drives the requester branch in SendMessages and, when

src/txdb.h

@@ -52,7 +52,7 @@ class uint256;
 //! -dbcache default (MiB)
 static const int64_t nDefaultDbCache = 512;
 //! max. -dbcache (MiB)
-static const int64_t nMaxDbCache = sizeof(void*) > 4 ? 16384 : 1024;
+static const int64_t nMaxDbCache = sizeof(void*) > 4 ? 65536 : 1024; // 64 GiB ceiling on 64-bit so adaptive dbcache can use most of RAM on large hosts (was 16384)
 //! min. -dbcache in (MiB)
 static const int64_t nMinDbCache = 4;