8268127: Shenandoah: Heap size may be too small for region to align to large page size

Reviewed-by: rkennke, shade
Backport-of: 5ad4a91

Author: zhengyu123

src/hotspot/share/gc/shenandoah/shenandoahArguments.cpp

@@ -50,11 +50,13 @@ void ShenandoahArguments::initialize() {
 
   FLAG_SET_DEFAULT(ShenandoahVerifyOptoBarriers,     false);
 #endif
-
-  if (UseLargePages && (MaxHeapSize / os::large_page_size()) < ShenandoahHeapRegion::MIN_NUM_REGIONS) {
-    warning("Large pages size (" SIZE_FORMAT "K) is too large to afford page-sized regions, disabling uncommit",
-            os::large_page_size() / K);
-    FLAG_SET_DEFAULT(ShenandoahUncommit, false);
+  if (UseLargePages) {
+    size_t large_page_size = os::large_page_size();
+    if ((align_up(MaxHeapSize, large_page_size) / large_page_size) < ShenandoahHeapRegion::MIN_NUM_REGIONS) {
+      warning("Large pages size (" SIZE_FORMAT "K) is too large to afford page-sized regions, disabling uncommit",
+              os::large_page_size() / K);
+      FLAG_SET_DEFAULT(ShenandoahUncommit, false);
+    }
   }
 
   // Enable NUMA by default. While Shenandoah is not NUMA-aware, enabling NUMA makes

src/hotspot/share/gc/shenandoah/shenandoahHeapRegion.cpp

@@ -462,7 +462,7 @@ size_t ShenandoahHeapRegion::block_size(const HeapWord* p) const {
   }
 }
 
-void ShenandoahHeapRegion::setup_sizes(size_t max_heap_size) {
+size_t ShenandoahHeapRegion::setup_sizes(size_t max_heap_size) {
   // Absolute minimums we should not ever break.
   static const size_t MIN_REGION_SIZE = 256*K;
 
@@ -537,14 +537,28 @@ void ShenandoahHeapRegion::setup_sizes(size_t max_heap_size) {
     region_size = ShenandoahRegionSize;
   }
 
-  // Make sure region size is at least one large page, if enabled.
-  // The heap sizes would be rounded by heap initialization code by
-  // page size, so we need to round up the region size too, to cover
-  // the heap exactly.
+  // Make sure region size and heap size are page aligned.
+  // If large pages are used, we ensure that region size is aligned to large page size if
+  // heap size is large enough to accommodate minimal number of regions. Otherwise, we align
+  // region size to regular page size.
+
+  // Figure out page size to use, and aligns up heap to page size
+  int page_size = os::vm_page_size();
   if (UseLargePages) {
-    region_size = MAX2(region_size, os::large_page_size());
+    size_t large_page_size = os::large_page_size();
+    max_heap_size = align_up(max_heap_size, large_page_size);
+    if ((max_heap_size / align_up(region_size, large_page_size)) >= MIN_NUM_REGIONS) {
+      page_size = (int)large_page_size;
+    } else {
+      // Should have been checked during argument initialization
+      assert(!ShenandoahUncommit, "Uncommit requires region size aligns to large page size");
+    }
+  } else {
+    max_heap_size = align_up(max_heap_size, page_size);
   }
 
+  // Align region size to page size
+  region_size = align_up(region_size, page_size);
   int region_size_log = log2_long((jlong) region_size);
   // Recalculate the region size to make sure it's a power of
   // 2. This means that region_size is the largest power of 2 that's
@@ -614,6 +628,8 @@ void ShenandoahHeapRegion::setup_sizes(size_t max_heap_size) {
                      byte_size_in_proper_unit(HumongousThresholdBytes), proper_unit_for_byte_size(HumongousThresholdBytes));
   log_info(gc, init)("Max TLAB size: " SIZE_FORMAT "%s",
                      byte_size_in_proper_unit(MaxTLABSizeBytes), proper_unit_for_byte_size(MaxTLABSizeBytes));
+
+  return max_heap_size;
 }
 
 void ShenandoahHeapRegion::do_commit() {

src/hotspot/share/gc/shenandoah/shenandoahHeapRegion.hpp

@@ -250,7 +250,8 @@ class ShenandoahHeapRegion {
 
   static const size_t MIN_NUM_REGIONS = 10;
 
-  static void setup_sizes(size_t max_heap_size);
+  // Return adjusted max heap size
+  static size_t setup_sizes(size_t max_heap_size);
 
   double empty_time() {
     return _empty_time;