8312610: GenShen: Old generation available is unintentionally restric…

…ted by mutator's available memory

Reviewed-by: kdnilsen, ysr

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

@@ -885,17 +885,7 @@ size_t ShenandoahGeneration::soft_available() const {
 
 size_t ShenandoahGeneration::available(size_t capacity) const {
   size_t in_use = used() + get_humongous_waste();
-  size_t available = in_use > capacity ? 0 : capacity - in_use;
-  // The collector reserve may eat into what the mutator is allowed to use. Make sure we are looking
-  // at what is available to the mutator when deciding whether to start a GC.
-  size_t usable = ShenandoahHeap::heap()->free_set()->available();
-  if (usable < available) {
-    log_debug(gc)("Usable (" SIZE_FORMAT "%s) is less than available (" SIZE_FORMAT "%s)",
-                  byte_size_in_proper_unit(usable), proper_unit_for_byte_size(usable),
-                  byte_size_in_proper_unit(available), proper_unit_for_byte_size(available));
-    available = usable;
-  }
-  return available;
+  return in_use > capacity ? 0 : capacity - in_use;
 }
 
 void ShenandoahGeneration::increase_capacity(size_t increment) {

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

@@ -58,6 +58,15 @@ size_t ShenandoahGlobalGeneration::available() const {
   return ShenandoahHeap::heap()->free_set()->available();
 }
 
+size_t ShenandoahGlobalGeneration::soft_available() const {
+  size_t available = this->available();
+
+  // Make sure the code below treats available without the soft tail.
+  assert(max_capacity() >= soft_max_capacity(), "Max capacity must be greater than soft max capacity.");
+  size_t soft_tail = max_capacity() - soft_max_capacity();
+  return (available > soft_tail) ? (available - soft_tail) : 0;
+}
+
 void ShenandoahGlobalGeneration::set_concurrent_mark_in_progress(bool in_progress) {
   ShenandoahHeap* heap = ShenandoahHeap::heap();
   if (in_progress && heap->mode()->is_generational()) {

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

@@ -43,6 +43,7 @@ class ShenandoahGlobalGeneration : public ShenandoahGeneration {
   virtual size_t used_regions() const override;
   virtual size_t used_regions_size() const override;
   virtual size_t available() const override;
+  virtual size_t soft_available() const override;
 
   virtual void set_concurrent_mark_in_progress(bool in_progress) override;
 

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

@@ -95,3 +95,15 @@ void ShenandoahYoungGeneration::add_collection_time(double time_seconds) {
     ShenandoahGeneration::add_collection_time(time_seconds);
   }
 }
+
+size_t ShenandoahYoungGeneration::available() const {
+  // The collector reserve may eat into what the mutator is allowed to use. Make sure we are looking
+  // at what is available to the mutator when reporting how much memory is available.
+  size_t available = this->ShenandoahGeneration::available();
+  return MIN2(available, ShenandoahHeap::heap()->free_set()->available());
+}
+
+size_t ShenandoahYoungGeneration::soft_available() const {
+  size_t available = this->ShenandoahGeneration::soft_available();
+  return MIN2(available, ShenandoahHeap::heap()->free_set()->available());
+}

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

@@ -64,6 +64,9 @@ class ShenandoahYoungGeneration : public ShenandoahGeneration {
   }
 
   virtual void add_collection_time(double time_seconds) override;
+
+  size_t available() const override;
+  size_t soft_available() const override;
 };
 
 #endif // SHARE_VM_GC_SHENANDOAH_SHENANDOAHYOUNGGENERATION_HPP