Use real number of cores for default -par, ignore virtual cores

To determine the default for `-par`, the number of script verification
threads, use [boost:🧵:physical_concurrency()](http://www.boost.org/doc/libs/1_58_0/doc/html/thread/thread_management.html#thread.thread_management.thread.physical_concurrency)
which counts only physical cores, not virtual cores.

Virtual cores are roughly a set of cached registers to avoid context
switches while threading, they cannot actually perform work, so spawning
a verification thread for them could even reduce efficiency and will put
undue load on the system.

Should fix issue #6358, as well as some other reported system overload
issues, especially on Intel processors.

The function was only introduced in boost 1.56, so provide a utility
function `GetNumCores` to fall back for older Boost versions.

src/init.cpp

@@ -333,7 +333,7 @@ std::string HelpMessage(HelpMessageMode mode)
     strUsage += HelpMessageOpt("-loadblock=<file>", _("Imports blocks from external blk000??.dat file") + " " + _("on startup"));
     strUsage += HelpMessageOpt("-maxorphantx=<n>", strprintf(_("Keep at most <n> unconnectable transactions in memory (default: %u)"), DEFAULT_MAX_ORPHAN_TRANSACTIONS));
     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));
+        -GetNumCores(), MAX_SCRIPTCHECK_THREADS, DEFAULT_SCRIPTCHECK_THREADS));
 #ifndef WIN32
     strUsage += HelpMessageOpt("-pid=<file>", strprintf(_("Specify pid file (default: %s)"), "zcashd.pid"));
 #endif
@@ -901,7 +901,7 @@ bool AppInit2(boost::thread_group& threadGroup, CScheduler& scheduler)
     // -par=0 means autodetect, but nScriptCheckThreads==0 means no concurrency
     nScriptCheckThreads = GetArg("-par", DEFAULT_SCRIPTCHECK_THREADS);
     if (nScriptCheckThreads <= 0)
-        nScriptCheckThreads += boost::thread::hardware_concurrency();
+        nScriptCheckThreads += GetNumCores();
     if (nScriptCheckThreads <= 1)
         nScriptCheckThreads = 0;
     else if (nScriptCheckThreads > MAX_SCRIPTCHECK_THREADS)

src/miner.cpp

@@ -753,7 +753,7 @@ void GenerateBitcoins(bool fGenerate, int nThreads)
         if (Params().DefaultMinerThreads())
             nThreads = Params().DefaultMinerThreads();
         else
-            nThreads = boost::thread::hardware_concurrency();
+            nThreads = GetNumCores();
     }
 
     if (minerThreads != NULL)

src/util.cpp

@@ -908,3 +908,13 @@ std::string LicenseInfo()
            FormatParagraph(_("This product includes software developed by the OpenSSL Project for use in the OpenSSL Toolkit <https://www.openssl.org/> and cryptographic software written by Eric Young and UPnP software written by Thomas Bernard.")) +
            "\n";
 }
+
+int GetNumCores()
+{
+#if BOOST_VERSION >= 105600
+    return boost::thread::physical_concurrency();
+#else // Must fall back to hardware_concurrency, which unfortunately counts virtual cores
+    return boost::thread::hardware_concurrency();
+#endif
+}
+

src/util.h

@@ -219,6 +219,13 @@ std::string HelpMessageGroup(const std::string& message);
  */
 std::string HelpMessageOpt(const std::string& option, const std::string& message);
 
+/**
+ * Return the number of physical cores available on the current system.
+ * @note This does not count virtual cores, such as those provided by HyperThreading
+ * when boost is newer than 1.56.
+ */
+int GetNumCores();
+
 void SetThreadPriority(int nPriority);
 void RenameThread(const char* name);