Return HardLimitBytes from GCMemoryInfo.TotalAvailableMemoryBytes (#25437)

* Add property HardLimitBytes to GCMemoryInfo

This adds a new property HardLimitBytes.

Unlike TotalAvailableMemoryBytes,
this will reflect an explicitly set COMPLUS_GCHeapHardLimit.

It will also reflect the fraction of a container's size that we use,
where TotalAvailableMemoryBytes is the total container size.

Normally, though, it is equal to TotalAvailableMemoryBytes.

Fix #38821

* Remove HardLimitBytes; have TotalAvailableMemoryBytes take on its behavior

* Fix typos

* Separate total_physical_mem and heap_hard_limit
so we can compute highMemoryLoadThresholdBytes and memoryLoadBytes

* Do more work in gc.cpp instead of Gc.cs

* Consistently end names in "Bytes"

2 files changed, 27 insertions, 19 deletions

diff --git a/src/System.Private.CoreLib/shared/System/GCMemoryInfo.cs b/src/System.Private.CoreLib/shared/System/GCMemoryInfo.cs
index 72c2aca14d..850f1256eb 100644
--- a/src/System.Private.CoreLib/shared/System/GCMemoryInfo.cs
+++ b/src/System.Private.CoreLib/shared/System/GCMemoryInfo.cs
@@ -17,7 +17,12 @@ namespace System
         public long MemoryLoadBytes { get; }
 
         /// <summary>
-        /// Total available memory for the GC to use when the last GC ocurred. By default this is the physical memory on the machine, but it may be customized by specifying a HardLimit.
+        /// Total available memory for the GC to use when the last GC ocurred.
+        ///
+        /// If the environment variable COMPlus_GCHeapHardLimit is set,
+        /// or "Server.GC.HeapHardLimit" is in runtimeconfig.json, this will come from that.
+        /// If the program is run in a container, this will be an implementation-defined fraction of the container's size.
+        /// Else, this is the physical memory on the machine that was available for the GC to use when the last GC occurred.
         /// </summary>
         public long TotalAvailableMemoryBytes { get; }
 
diff --git a/src/System.Private.CoreLib/src/System/GC.cs b/src/System.Private.CoreLib/src/System/GC.cs
index 89c277e544..e67c043e62 100644
--- a/src/System.Private.CoreLib/src/System/GC.cs
+++ b/src/System.Private.CoreLib/src/System/GC.cs
@@ -54,26 +54,28 @@ namespace System
     public static class GC
     {
         [MethodImplAttribute(MethodImplOptions.InternalCall)]
-        internal static extern void GetMemoryInfo(out uint highMemLoadThreshold,
-                                                  out ulong totalPhysicalMem,
-                                                  out uint lastRecordedMemLoad,
+        internal static extern void GetMemoryInfo(out ulong highMemLoadThresholdBytes,
+                                                  out ulong totalAvailableMemoryBytes,
+                                                  out ulong lastRecordedMemLoadBytes,
+                                                  out uint lastRecordedMemLoadPct,
                                                   // The next two are size_t
-                                                  out UIntPtr lastRecordedHeapSize,
-                                                  out UIntPtr lastRecordedFragmentation);
+                                                  out UIntPtr lastRecordedHeapSizeBytes,
+                                                  out UIntPtr lastRecordedFragmentationBytes);
 
         public static GCMemoryInfo GetGCMemoryInfo()
         {
-            GetMemoryInfo(out uint highMemLoadThreshold,
-                          out ulong totalPhysicalMem,
-                          out uint lastRecordedMemLoad,
-                          out UIntPtr lastRecordedHeapSize,
-                          out UIntPtr lastRecordedFragmentation);
+            GetMemoryInfo(out ulong highMemLoadThresholdBytes,
+                          out ulong totalAvailableMemoryBytes,
+                          out ulong lastRecordedMemLoadBytes,
+                          out uint _,
+                          out UIntPtr lastRecordedHeapSizeBytes,
+                          out UIntPtr lastRecordedFragmentationBytes);
 
-            return new GCMemoryInfo((long)((double)highMemLoadThreshold / 100 * totalPhysicalMem),
-                                    (long)((double)lastRecordedMemLoad / 100 * totalPhysicalMem),
-                                    (long)totalPhysicalMem,
-                                    (long)(ulong)lastRecordedHeapSize,
-                                    (long)(ulong)lastRecordedFragmentation);
+            return new GCMemoryInfo(highMemoryLoadThresholdBytes: (long)highMemLoadThresholdBytes,
+                                    memoryLoadBytes: (long)lastRecordedMemLoadBytes,
+                                    totalAvailableMemoryBytes: (long)totalAvailableMemoryBytes,
+                                    heapSizeBytes: (long)(ulong)lastRecordedHeapSizeBytes,
+                                    fragmentedBytes: (long)(ulong)lastRecordedFragmentationBytes);
         }
 
         [DllImport(JitHelpers.QCall, CharSet = CharSet.Unicode)]
@@ -541,13 +543,14 @@ namespace System
 
         private static float GetMemoryLoad()
         {
-            GetMemoryInfo(out uint _,
+            GetMemoryInfo(out ulong _,
                           out ulong _,
-                          out uint lastRecordedMemLoad,
+                          out ulong _,
+                          out uint lastRecordedMemLoadPct,
                           out UIntPtr _,
                           out UIntPtr _);
 
-            return (float)lastRecordedMemLoad / 100;
+            return (float)lastRecordedMemLoadPct;
         }
 
         private static bool InvokeMemoryLoadChangeNotifications()