diff a/src/hotspot/share/gc/shenandoah/shenandoahDegeneratedGC.cpp b/src/hotspot/share/gc/shenandoah/shenandoahDegeneratedGC.cpp
--- a/src/hotspot/share/gc/shenandoah/shenandoahDegeneratedGC.cpp
+++ b/src/hotspot/share/gc/shenandoah/shenandoahDegeneratedGC.cpp
@@ -1,7 +1,8 @@
 /*
  * Copyright (c) 2021, Red Hat, Inc. All rights reserved.
+ * Copyright Amazon.com Inc. or its affiliates. All Rights Reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
  * under the terms of the GNU General Public License version 2 only, as
  * published by the Free Software Foundation.
@@ -27,30 +28,43 @@
 #include "gc/shared/collectorCounters.hpp"
 #include "gc/shenandoah/shenandoahCollectorPolicy.hpp"
 #include "gc/shenandoah/shenandoahConcurrentMark.hpp"
 #include "gc/shenandoah/shenandoahDegeneratedGC.hpp"
 #include "gc/shenandoah/shenandoahFullGC.hpp"
+#include "gc/shenandoah/shenandoahGeneration.hpp"
+#include "gc/shenandoah/shenandoahGenerationalHeap.hpp"
 #include "gc/shenandoah/shenandoahHeap.inline.hpp"
 #include "gc/shenandoah/shenandoahMetrics.hpp"
 #include "gc/shenandoah/shenandoahMonitoringSupport.hpp"
+#include "gc/shenandoah/shenandoahOldGeneration.hpp"
 #include "gc/shenandoah/shenandoahOopClosures.inline.hpp"
 #include "gc/shenandoah/shenandoahRootProcessor.inline.hpp"
 #include "gc/shenandoah/shenandoahSTWMark.hpp"
 #include "gc/shenandoah/shenandoahUtils.hpp"
 #include "gc/shenandoah/shenandoahVerifier.hpp"
+#include "gc/shenandoah/shenandoahYoungGeneration.hpp"
 #include "gc/shenandoah/shenandoahWorkerPolicy.hpp"
 #include "gc/shenandoah/shenandoahVMOperations.hpp"
 #include "runtime/vmThread.hpp"
 #include "utilities/events.hpp"
 
-ShenandoahDegenGC::ShenandoahDegenGC(ShenandoahDegenPoint degen_point) :
+ShenandoahDegenGC::ShenandoahDegenGC(ShenandoahDegenPoint degen_point, ShenandoahGeneration* generation) :
   ShenandoahGC(),
-  _degen_point(degen_point) {
+  _degen_point(degen_point),
+  _generation(generation),
+  _abbreviated(false) {
 }
 
 bool ShenandoahDegenGC::collect(GCCause::Cause cause) {
   vmop_degenerated();
+  ShenandoahHeap* heap = ShenandoahHeap::heap();
+  if (heap->mode()->is_generational()) {
+    bool is_bootstrap_gc = heap->old_generation()->is_bootstrapping();
+    heap->mmu_tracker()->record_degenerated(GCId::current(), is_bootstrap_gc);
+    const char* msg = is_bootstrap_gc? "At end of Degenerated Bootstrap Old GC": "At end of Degenerated Young GC";
+    heap->log_heap_status(msg);
+  }
   return true;
 }
 
 void ShenandoahDegenGC::vmop_degenerated() {
   TraceCollectorStats tcs(ShenandoahHeap::heap()->monitoring_support()->full_stw_collection_counters());
@@ -62,11 +76,10 @@
 void ShenandoahDegenGC::entry_degenerated() {
   const char* msg = degen_event_message(_degen_point);
   ShenandoahPausePhase gc_phase(msg, ShenandoahPhaseTimings::degen_gc, true /* log_heap_usage */);
   EventMark em("%s", msg);
   ShenandoahHeap* const heap = ShenandoahHeap::heap();
-
   ShenandoahWorkerScope scope(heap->workers(),
                               ShenandoahWorkerPolicy::calc_workers_for_stw_degenerated(),
                               "stw degenerated gc");
 
   heap->set_degenerated_gc_in_progress(true);
@@ -77,11 +90,30 @@
 void ShenandoahDegenGC::op_degenerated() {
   ShenandoahHeap* const heap = ShenandoahHeap::heap();
   // Degenerated GC is STW, but it can also fail. Current mechanics communicates
   // GC failure via cancelled_concgc() flag. So, if we detect the failure after
   // some phase, we have to upgrade the Degenerate GC to Full GC.
-  heap->clear_cancelled_gc();
+  heap->clear_cancelled_gc(true /* clear oom handler */);
+
+#ifdef ASSERT
+  if (heap->mode()->is_generational()) {
+    ShenandoahOldGeneration* old_generation = heap->old_generation();
+    if (!heap->is_concurrent_old_mark_in_progress()) {
+      // If we are not marking the old generation, there should be nothing in the old mark queues
+      assert(old_generation->task_queues()->is_empty(), "Old gen task queues should be empty");
+    }
+
+    if (_generation->is_global()) {
+      // If we are in a global cycle, the old generation should not be marking. It is, however,
+      // allowed to be holding regions for evacuation or coalescing.
+      assert(old_generation->is_idle()
+             || old_generation->is_doing_mixed_evacuations()
+             || old_generation->is_preparing_for_mark(),
+             "Old generation cannot be in state: %s", old_generation->state_name());
+    }
+  }
+#endif
 
   ShenandoahMetricsSnapshot metrics;
   metrics.snap_before();
 
   switch (_degen_point) {
@@ -93,21 +125,57 @@
       // We have degenerated from outside the cycle, which means something is bad with
       // the heap, most probably heavy humongous fragmentation, or we are very low on free
       // space. It makes little sense to wait for Full GC to reclaim as much as it can, when
       // we can do the most aggressive degen cycle, which includes processing references and
       // class unloading, unless those features are explicitly disabled.
-      //
 
+      // Note that we can only do this for "outside-cycle" degens, otherwise we would risk
+      // changing the cycle parameters mid-cycle during concurrent -> degenerated handover.
+      heap->set_unload_classes(_generation->heuristics()->can_unload_classes() &&
+                                (!heap->mode()->is_generational() || _generation->is_global()));
+
+      if (heap->mode()->is_generational() &&
+            (_generation->is_young() || (_generation->is_global() && ShenandoahVerify))) {
+        // Swap remembered sets for young, or if the verifier will run during a global collect
+        // TODO: This path should not depend on ShenandoahVerify
+        _generation->swap_remembered_set();
+      }
+
+    case _degenerated_roots:
       // Degenerated from concurrent root mark, reset the flag for STW mark
-      if (heap->is_concurrent_mark_in_progress()) {
-        ShenandoahConcurrentMark::cancel();
-        heap->set_concurrent_mark_in_progress(false);
+      if (!heap->mode()->is_generational()) {
+        if (heap->is_concurrent_mark_in_progress()) {
+          heap->cancel_concurrent_mark();
+        }
+      } else {
+        if (_generation->is_concurrent_mark_in_progress()) {
+          // We want to allow old generation marking to be punctuated by young collections
+          // (even if they have degenerated). If this is a global cycle, we'd have cancelled
+          // the entire old gc before coming into this switch. Note that cancel_marking on
+          // the generation does NOT abandon incomplete SATB buffers as cancel_concurrent_mark does.
+          // We need to separate out the old pointers which is done below.
+          _generation->cancel_marking();
+        }
+
+        if (heap->is_concurrent_mark_in_progress()) {
+          // If either old or young marking is in progress, the SATB barrier will be enabled.
+          // The SATB buffer may hold a mix of old and young pointers. The old pointers need to be
+          // transferred to the old generation mark queues and the young pointers are NOT part
+          // of this snapshot, so they must be dropped here. It is safe to drop them here because
+          // we will rescan the roots on this safepoint.
+          heap->transfer_old_pointers_from_satb();
+        }
       }
 
-      // Note that we can only do this for "outside-cycle" degens, otherwise we would risk
-      // changing the cycle parameters mid-cycle during concurrent -> degenerated handover.
-      heap->set_unload_classes(heap->heuristics()->can_unload_classes());
+      if (_degen_point == ShenandoahDegenPoint::_degenerated_roots) {
+        // We only need this if the concurrent cycle has already swapped the card tables.
+        // Marking will use the 'read' table, but interesting pointers may have been
+        // recorded in the 'write' table in the time between the cancelled concurrent cycle
+        // and this degenerated cycle. These pointers need to be included the 'read' table
+        // used to scan the remembered set during the STW mark which follows here.
+        _generation->merge_write_table();
+      }
 
       op_reset();
 
       // STW mark
       op_mark();
@@ -167,11 +235,10 @@
         // it, we fail degeneration right away and slide into Full GC to recover.
 
         {
           heap->sync_pinned_region_status();
           heap->collection_set()->clear_current_index();
-
           ShenandoahHeapRegion* r;
           while ((r = heap->collection_set()->next()) != nullptr) {
             if (r->is_pinned()) {
               heap->cancel_gc(GCCause::_shenandoah_upgrade_to_full_gc);
               op_degenerated_fail();
@@ -186,15 +253,22 @@
           op_degenerated_fail();
           return;
         }
       }
 
+      // Update collector state regardless of whether or not there are forwarded objects
+      heap->set_evacuation_in_progress(false);
+      heap->set_concurrent_weak_root_in_progress(false);
+      heap->set_concurrent_strong_root_in_progress(false);
+
       // If heuristics thinks we should do the cycle, this flag would be set,
       // and we need to do update-refs. Otherwise, it would be the shortcut cycle.
       if (heap->has_forwarded_objects()) {
         op_init_updaterefs();
         assert(!heap->cancelled_gc(), "STW reference update can not OOM");
+      } else {
+        _abbreviated = true;
       }
 
     case _degenerated_updaterefs:
       if (heap->has_forwarded_objects()) {
         op_updaterefs();
@@ -204,16 +278,42 @@
 
       // Disarm nmethods that armed in concurrent cycle.
       // In above case, update roots should disarm them
       ShenandoahCodeRoots::disarm_nmethods();
 
+      if (heap->mode()->is_generational() && heap->is_concurrent_old_mark_in_progress()) {
+        // This is still necessary for degenerated cycles because the degeneration point may occur
+        // after final mark of the young generation. See ShenandoahConcurrentGC::op_final_updaterefs for
+        // a more detailed explanation.
+        heap->transfer_old_pointers_from_satb();
+      }
+
       op_cleanup_complete();
+      // We defer generation resizing actions until after cset regions have been recycled.
+      if (heap->mode()->is_generational()) {
+        auto result = ShenandoahGenerationalHeap::heap()->balance_generations();
+        LogTarget(Info, gc, ergo) lt;
+        if (lt.is_enabled()) {
+          LogStream ls(lt);
+          result.print_on("Degenerated GC", &ls);
+        }
+      }
       break;
     default:
       ShouldNotReachHere();
   }
 
+  if (heap->mode()->is_generational()) {
+    // In case degeneration interrupted concurrent evacuation or update references, we need to clean up transient state.
+    // Otherwise, these actions have no effect.
+    ShenandoahGenerationalHeap::heap()->reset_generation_reserves();
+
+    if (!ShenandoahGenerationalHeap::heap()->old_generation()->is_parseable()) {
+      op_global_coalesce_and_fill();
+    }
+  }
+
   if (ShenandoahVerify) {
     heap->verifier()->verify_after_degenerated();
   }
 
   if (VerifyAfterGC) {
@@ -228,27 +328,28 @@
     heap->notify_gc_no_progress();
     heap->cancel_gc(GCCause::_shenandoah_upgrade_to_full_gc);
     op_degenerated_futile();
   } else {
     heap->notify_gc_progress();
+    heap->shenandoah_policy()->record_success_degenerated(_generation->is_young(), _abbreviated);
+    _generation->heuristics()->record_success_degenerated();
   }
 }
 
 void ShenandoahDegenGC::op_reset() {
-  ShenandoahHeap::heap()->prepare_gc();
+  _generation->prepare_gc();
 }
 
 void ShenandoahDegenGC::op_mark() {
-  assert(!ShenandoahHeap::heap()->is_concurrent_mark_in_progress(), "Should be reset");
+  assert(!_generation->is_concurrent_mark_in_progress(), "Should be reset");
   ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_stw_mark);
-  ShenandoahSTWMark mark(false /*full gc*/);
-  mark.clear();
+  ShenandoahSTWMark mark(_generation, false /*full gc*/);
   mark.mark();
 }
 
 void ShenandoahDegenGC::op_finish_mark() {
-  ShenandoahConcurrentMark mark;
+  ShenandoahConcurrentMark mark(_generation);
   mark.finish_mark();
 }
 
 void ShenandoahDegenGC::op_prepare_evacuation() {
   ShenandoahHeap* const heap = ShenandoahHeap::heap();
@@ -256,12 +357,13 @@
     heap->verifier()->verify_roots_no_forwarded();
   }
 
   // STW cleanup weak roots and unload classes
   heap->parallel_cleaning(false /*full gc*/);
+
   // Prepare regions and collection set
-  heap->prepare_regions_and_collection_set(false /*concurrent*/);
+  _generation->prepare_regions_and_collection_set(false /*concurrent*/);
 
   // Retire the TLABs, which will force threads to reacquire their TLABs after the pause.
   // This is needed for two reasons. Strong one: new allocations would be with new freeset,
   // which would be outside the collection set, so no cset writes would happen there.
   // Weaker one: new allocations would happen past update watermark, and so less work would
@@ -269,17 +371,25 @@
   if (UseTLAB) {
     ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_final_manage_labs);
     heap->tlabs_retire(false);
   }
 
-  if (!heap->collection_set()->is_empty()) {
+  if (!heap->collection_set()->is_empty() || heap->old_generation()->has_in_place_promotions()) {
+    // Even if the collection set is empty, we need to do evacuation if there are regions to be promoted in place.
+    // Degenerated evacuation takes responsibility for registering objects and setting the remembered set cards to dirty.
+
+    if (ShenandoahVerify) {
+      heap->verifier()->verify_before_evacuation();
+    }
+
     heap->set_evacuation_in_progress(true);
-    heap->set_has_forwarded_objects(true);
 
     if(ShenandoahVerify) {
       heap->verifier()->verify_during_evacuation();
     }
+
+    heap->set_has_forwarded_objects(!heap->collection_set()->is_empty());
   } else {
     if (ShenandoahVerify) {
       heap->verifier()->verify_after_concmark();
     }
 
@@ -291,22 +401,23 @@
 
 void ShenandoahDegenGC::op_cleanup_early() {
   ShenandoahHeap::heap()->recycle_trash();
 }
 
+void ShenandoahDegenGC::op_global_coalesce_and_fill() {
+  ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_coalesce_and_fill);
+  ShenandoahGenerationalHeap::heap()->coalesce_and_fill_old_regions(false);
+}
+
 void ShenandoahDegenGC::op_evacuate() {
   ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_stw_evac);
   ShenandoahHeap::heap()->evacuate_collection_set(false /* concurrent*/);
 }
 
 void ShenandoahDegenGC::op_init_updaterefs() {
   // Evacuation has completed
   ShenandoahHeap* const heap = ShenandoahHeap::heap();
-  heap->set_evacuation_in_progress(false);
-  heap->set_concurrent_weak_root_in_progress(false);
-  heap->set_concurrent_strong_root_in_progress(false);
-
   heap->prepare_update_heap_references(false /*concurrent*/);
   heap->set_update_refs_in_progress(true);
 }
 
 void ShenandoahDegenGC::op_updaterefs() {
@@ -341,35 +452,38 @@
   ShenandoahGCPhase phase(ShenandoahPhaseTimings::degen_gc_cleanup_complete);
   ShenandoahHeap::heap()->recycle_trash();
 }
 
 void ShenandoahDegenGC::op_degenerated_fail() {
-  log_info(gc)("Cannot finish degeneration, upgrading to Full GC");
-  ShenandoahHeap::heap()->shenandoah_policy()->record_degenerated_upgrade_to_full();
-
-  ShenandoahFullGC full_gc;
-  full_gc.op_full(GCCause::_shenandoah_upgrade_to_full_gc);
+  upgrade_to_full();
 }
 
 void ShenandoahDegenGC::op_degenerated_futile() {
-  ShenandoahHeap::heap()->shenandoah_policy()->record_degenerated_upgrade_to_full();
-  ShenandoahFullGC full_gc;
-  full_gc.op_full(GCCause::_shenandoah_upgrade_to_full_gc);
+  upgrade_to_full();
 }
 
 const char* ShenandoahDegenGC::degen_event_message(ShenandoahDegenPoint point) const {
   switch (point) {
     case _degenerated_unset:
-      return "Pause Degenerated GC (<UNSET>)";
+      SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (<UNSET>)");
     case _degenerated_outside_cycle:
-      return "Pause Degenerated GC (Outside of Cycle)";
+      SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (Outside of Cycle)");
+    case _degenerated_roots:
+      SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (Roots)");
     case _degenerated_mark:
-      return "Pause Degenerated GC (Mark)";
+      SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (Mark)");
     case _degenerated_evac:
-      return "Pause Degenerated GC (Evacuation)";
+      SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (Evacuation)");
     case _degenerated_updaterefs:
-      return "Pause Degenerated GC (Update Refs)";
+      SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (Update Refs)");
     default:
       ShouldNotReachHere();
-      return "ERROR";
+      SHENANDOAH_RETURN_EVENT_MESSAGE(_generation->type(), "Pause Degenerated GC", " (?)");
   }
 }
+
+void ShenandoahDegenGC::upgrade_to_full() {
+  log_info(gc)("Degenerated GC upgrading to Full GC");
+  ShenandoahHeap::heap()->shenandoah_policy()->record_degenerated_upgrade_to_full();
+  ShenandoahFullGC full_gc;
+  full_gc.op_full(GCCause::_shenandoah_upgrade_to_full_gc);
+}
