8372861: Genshen: Override parallel_region_stride of ShenandoahResetBitmapClosure to a reasonable value for better parallelism

Reviewed-by: kdnilsen, shade, wkemper

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

@@ -76,6 +76,9 @@ public:
     }
   }
 
+  // Bitmap reset task is heavy-weight and benefits from much smaller tasks than the default.
+  size_t parallel_region_stride() override { return 8; }
+
   bool is_thread_safe() override { return true; }
 };
 

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

@@ -1962,7 +1962,7 @@ void ShenandoahHeap::parallel_heap_region_iterate(ShenandoahHeapRegionClosure* b
   assert(blk->is_thread_safe(), "Only thread-safe closures here");
   const uint active_workers = workers()->active_workers();
   const size_t n_regions = num_regions();
-  size_t stride = ShenandoahParallelRegionStride;
+  size_t stride = blk->parallel_region_stride();
   if (stride == 0 && active_workers > 1) {
     // Automatically derive the stride to balance the work between threads
     // evenly. Do not try to split work if below the reasonable threshold.

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

@@ -113,6 +113,7 @@ public:
 class ShenandoahHeapRegionClosure : public StackObj {
 public:
   virtual void heap_region_do(ShenandoahHeapRegion* r) = 0;
+  virtual size_t parallel_region_stride() { return ShenandoahParallelRegionStride; }
   virtual bool is_thread_safe() { return false; }
 };
 

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

@@ -44,6 +44,10 @@ public:
     }
   }
 
+  size_t parallel_region_stride() override {
+    return _closure->parallel_region_stride();
+  }
+
   bool is_thread_safe() override {
     return _closure->is_thread_safe();
   }
@@ -64,6 +68,10 @@ public:
     }
   }
 
+  size_t parallel_region_stride() override {
+    return _closure->parallel_region_stride();
+  }
+
   bool is_thread_safe() override {
     return _closure->is_thread_safe();
   }