fix lower heap hard limit condition for regions

src/coreclr/gc/gc.cpp

@@ -44894,10 +44894,20 @@ HRESULT GCHeap::Initialize()
  {
  if (gc_heap::heap_hard_limit)
  {
- gc_heap::regions_range = 2 * gc_heap::heap_hard_limit;
+ if (gc_heap::heap_hard_limit_oh[soh])
+ {
+ gc_heap::regions_range = gc_heap::heap_hard_limit;
+ }
+ else
+ {
+ // We use this calculation because it's close to what we used for segments.
+ gc_heap::regions_range = ((gc_heap::use_large_pages_p) ? (2 * gc_heap::heap_hard_limit)
+ : (5 * gc_heap::heap_hard_limit));
+ }
  }
  else
  {
+ // If no hard_limit is configured the reservation size is max of 256gb or 2x physical limit
  gc_heap::regions_range = max(((size_t)256 * 1024 * 1024 * 1024), (size_t)(2 * gc_heap::total_physical_mem));
  }
  gc_heap::regions_range = align_on_page(gc_heap::regions_range);
@@ -45044,7 +45054,30 @@ HRESULT GCHeap::Initialize()
 #ifdef USE_REGIONS
  gc_heap::enable_special_regions_p = (bool)GCConfig::GetGCEnableSpecialRegions();
  size_t gc_region_size = (size_t)GCConfig::GetGCRegionSize();
- if (!power_of_two_p(gc_region_size) || ((gc_region_size * nhp * 19) > gc_heap::regions_range))
+
+ // Adjust GCRegionSize based on how large each heap would be, for smaller heaps we would 
+ // like to keep Region sizes small. We choose between 4, 2 and 1mb based on the calculations 
+ // below (unless its configured explictly) such that there are at least 2 regions available
+ // except for the smallest case. Now the lowest limit possible is 4mb. 
+ if (gc_region_size == 0)
+ {
+ // We have a minimum amount of basic regions we have to fit per heap, and we'd like to have the initial
+ // regions only take up half of the space.
+ if ((gc_heap::regions_range / nhp / min_regions_per_heap) / 2 >= (4 * 1024 * 1024))
+ {
+ gc_region_size = 4 * 1024 * 1024;
+ }
+ else if ((gc_heap::regions_range / nhp / min_regions_per_heap) / 2 >= (2 * 1024 * 1024))
+ {
+ gc_region_size = 2 * 1024 * 1024;
+ }
+ else
+ {
+ gc_region_size = 1 * 1024 * 1024;
+ }
+ }
+
+ if (!power_of_two_p(gc_region_size) || ((gc_region_size * nhp * min_regions_per_heap) > gc_heap::regions_range))
  {
  return E_OUTOFMEMORY;
  }

src/coreclr/gc/gcconfig.h

@@ -104,7 +104,7 @@ class GCConfigStringHolder
  INT_CONFIG (GCHeapHardLimitPercent, "GCHeapHardLimitPercent", "System.GC.HeapHardLimitPercent", 0, "Specifies the GC heap usage as a percentage of the total memory") \
  INT_CONFIG (GCTotalPhysicalMemory, "GCTotalPhysicalMemory", NULL, 0, "Specifies what the GC should consider to be total physical memory") \
  INT_CONFIG (GCRegionRange, "GCRegionRange", NULL, 0, "Specifies the range for the GC heap") \
- INT_CONFIG (GCRegionSize, "GCRegionSize", NULL, 4194304, "Specifies the size for a basic GC region") \
+ INT_CONFIG (GCRegionSize, "GCRegionSize", NULL, 0,  "Specifies the size for a basic GC region") \
  INT_CONFIG (GCEnableSpecialRegions, "GCEnableSpecialRegions", NULL, 0, "Specifies to enable special handling some regions like SIP") \
  STRING_CONFIG(LogFile, "GCLogFile", NULL, "Specifies the name of the GC log file") \
  STRING_CONFIG(ConfigLogFile, "GCConfigLogFile", NULL, "Specifies the name of the GC config log file") \

src/coreclr/gc/gcpriv.h

@@ -5807,6 +5807,8 @@ class heap_segment
 
 #define region_alloc_free_bit (1 << (sizeof (uint32_t) * 8 - 1))
 
+const int min_regions_per_heap = ((ephemeral_generation_count + 1) + ((total_generation_count - uoh_start_generation) * LARGE_REGION_FACTOR));
+
 enum allocate_direction
 {
  allocate_forward = 1,