fix node size estimation (XRPLF#4536)

Fix a bug in the `NODE_SIZE` auto-detection feature in `Config.cpp`.
Specifically, this patch corrects the calculation for the total amount
of RAM available, which was previously returned in bytes, but is now
being returned in units of the system's memory unit. Additionally, the
patch adjusts the node size based on the number of available hardware
threads of execution.

src/ripple/core/impl/Config.cpp

@@ -68,10 +68,8 @@ namespace detail {
 [[nodiscard]] std::uint64_t
 getMemorySize()
 {
-    struct sysinfo si;
-
-    if (sysinfo(&si) == 0)
-        return static_cast<std::uint64_t>(si.totalram);
+    if (struct sysinfo si; sysinfo(&si) == 0)
+        return static_cast<std::uint64_t>(si.totalram) * si.mem_unit;
 
     return 0;
 }
@@ -128,7 +126,7 @@ sizedItems
     {SizedItem::lgrDBCache,         {{      4,       8,      16,      32,     128 }}},
     {SizedItem::openFinalLimit,     {{      8,      16,      32,      64,     128 }}},
     {SizedItem::burstSize,          {{      4,       8,      16,      32,      48 }}},
-    {SizedItem::ramSizeGB,          {{      8,      12,      16,      24,      32 }}},
+    {SizedItem::ramSizeGB,          {{      6,       8,      12,      24,       0 }}},
     {SizedItem::accountIdCacheSize, {{  20047,   50053,   77081,  150061,  300007 }}}
 }};
 
@@ -265,7 +263,8 @@ getEnvVar(char const* name)
 }
 
 Config::Config()
-    : j_(beast::Journal::getNullSink()), ramSize_(detail::getMemorySize())
+    : j_(beast::Journal::getNullSink())
+    , ramSize_(detail::getMemorySize() / (1024 * 1024 * 1024))
 {
 }
 
@@ -290,22 +289,18 @@ Config::setupControl(bool bQuiet, bool bSilent, bool bStandalone)
             threshold.second.begin(),
             threshold.second.end(),
             [this](std::size_t limit) {
-                return (ramSize_ / (1024 * 1024 * 1024)) < limit;
+                return (limit == 0) || (ramSize_ < limit);
             });
 
+        assert(ns != threshold.second.end());
+
         if (ns != threshold.second.end())
             NODE_SIZE = std::distance(threshold.second.begin(), ns);
 
         // Adjust the size based on the number of hardware threads of
         // execution available to us:
-        if (auto const hc = std::thread::hardware_concurrency())
-        {
-            if (hc == 1)
-                NODE_SIZE = 0;
-
-            if (hc < 4)
-                NODE_SIZE = std::min<std::size_t>(NODE_SIZE, 1);
-        }
+        if (auto const hc = std::thread::hardware_concurrency(); hc != 0)
+            NODE_SIZE = std::min<std::size_t>(hc / 2, NODE_SIZE);
     }
 
     assert(NODE_SIZE <= 4);