Balance evacuation

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

@@ -484,6 +484,8 @@ void ShenandoahHeap::initialize_heuristics() {
   if (mode()->is_generational()) {
     _young_generation->initialize_heuristics(_gc_mode);
     _old_generation->initialize_heuristics(_gc_mode);
+
+    ShenandoahEvacWaste = ShenandoahGenerationalEvacWaste;
   }
 }
 
@@ -847,6 +849,12 @@ HeapWord* ShenandoahHeap::allocate_from_gclab_slow(Thread* thread, size_t size)
 
   // Figure out size of new GCLAB, looking back at heuristics. Expand aggressively.
   size_t new_size = ShenandoahThreadLocalData::gclab_size(thread) * 2;
+
+  // Limit growth of GCLABs to ShenandoahMaxEvacLABRatio * the minimum size.  This enables more equitable distribution of
+  // available evacuation buidget between the many threads that are coordinating in the evacuation effort.
+  if (ShenandoahMaxEvacLABRatio > 0) {
+    new_size = MIN2(new_size, PLAB::min_size() * ShenandoahMaxEvacLABRatio);
+  }
   new_size = MIN2(new_size, PLAB::max_size());
   new_size = MAX2(new_size, PLAB::min_size());
 
@@ -897,6 +905,11 @@ HeapWord* ShenandoahHeap::allocate_from_plab_slow(Thread* thread, size_t size, b
 
   // Figure out size of new PLAB, looking back at heuristics. Expand aggressively.
   size_t new_size = ShenandoahThreadLocalData::plab_size(thread) * 2;
+  // Limit growth of PLABs to ShenandoahMaxEvacLABRatio * the minimum size.  This enables more equitable distribution of
+  // available evacuation buidget between the many threads that are coordinating in the evacuation effort.
+  if (ShenandoahMaxEvacLABRatio > 0) {
+    new_size = MIN2(new_size, PLAB::min_size() * ShenandoahMaxEvacLABRatio);
+  }
   new_size = MIN2(new_size, PLAB::max_size());
   new_size = MAX2(new_size, PLAB::min_size());
 

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

@@ -281,11 +281,42 @@
           "Larger values make evacuations more resilient against "          \
           "evacuation conflicts, at expense of evacuating less on each "    \
           "GC cycle.  Smaller values increase the risk of evacuation "      \
-          "failures, which will trigger stop-the-world Full GC passes.  "   \
-          "A minimum value of 1.6 is recommended for Generational "         \
-          "mode of Shenandoah.")                                            \
+          "failures, which will trigger stop-the-world Full GC passes.")    \
           range(1.0,100.0)                                                  \
                                                                             \
+  product(double, ShenandoahGenerationalEvacWaste, 2.0, EXPERIMENTAL,       \
+          "For generational mode, how much waste evacuations produce "      \
+          "within the reserved space.  Larger values make evacuations "     \
+          "more resilient against evacuation conflicts, at expense of "     \
+          "evacuating less on each GC cycle.  Smaller values increase "     \
+          "the risk of evacuation failures, which will trigger "            \
+          "stop-the-world Full GC passes.  The default value for "          \
+          "generational mode is 2.0.  The reason for the higher default "   \
+          "value in generational mode is because generational mode "        \
+          "enforces the evacuation budget, triggering degenerated GC "      \
+          "which upgrades to full GC whenever the budget is exceeded.")     \
+          range(1.0,100.0)                                                  \
+                                                                            \
+  product(uintx, ShenandoahMaxEvacLABRatio, 16, EXPERIMENTAL,               \
+          "Potentially, each running thread maintains a PLAB for "          \
+          "evacuating objects into old-gen memory and a GCLAB for "         \
+          "evacuating objects into young-gen memory.  Each time a thread "  \
+          "exhausts its PLAB or GCLAB, a new local buffer is allocated. "   \
+          "By default, the new buffer is twice the size of the previous "   \
+          "buffer.  The sizes are reset to the minimum at the start of "    \
+          "each GC pass.  This parameter limits the growth of evacuation "  \
+          "buffer sizes to its value multiplied by the minimum buffer "     \
+          "size.  A higher value allows evacuation allocations to be more " \
+          "efficient because less synchronization is required by "          \
+          "individual threads.  However, a larger value increases the "     \
+          "likelihood of evacuation failures, leading to long "             \
+          "stop-the-world pauses.  This is because a large value "          \
+          "allows individual threads to consume large percentages of "      \
+          "the total evacuation budget without necessarily effectively "    \
+          "filling their local evcauation buffers with evacuated "          \
+          "objects.  A value of zero means no maximum size is enforced.")   \
+          range(0, 1024)                                                    \
+                                                                            \
   product(bool, ShenandoahEvacReserveOverflow, true, EXPERIMENTAL,          \
           "Allow evacuations to overflow the reserved space. Enabling it "  \
           "will make evacuations more resilient when evacuation "           \