8212084: G1: Implement UseGCOverheadLimit

src/hotspot/share/gc/g1/g1CollectedHeap.cpp

@@ -467,8 +467,20 @@ HeapWord* G1CollectedHeap::attempt_allocation_slow(uint node_index, size_t word_
     log_trace(gc, alloc)("%s: Unsuccessfully scheduled collection allocating %zu words",
                          Thread::current()->name(), word_size);
 
+    if (is_shutting_down()) {
+      stall_for_vm_shutdown();
+      return nullptr;
+    }
+
+    // Has the gc overhead limit been reached in the meantime? If so, this mutator
+    // should receive null even when unsuccessfully scheduling a collection as well
+    // for global consistency.
+    if (gc_overhead_limit_exceeded()) {
+      return nullptr;
+    }
+
     // We can reach here if we were unsuccessful in scheduling a collection (because
-    // another thread beat us to it). In this case immeditealy retry the allocation
+    // another thread beat us to it). In this case immediately retry the allocation
     // attempt because another thread successfully performed a collection and possibly
     // reclaimed enough space. The first attempt (without holding the Heap_lock) is
     // here and the follow-on attempt will be at the start of the next loop
@@ -485,11 +497,6 @@ HeapWord* G1CollectedHeap::attempt_allocation_slow(uint node_index, size_t word_
       log_warning(gc, alloc)("%s:  Retried allocation %u times for %zu words",
                              Thread::current()->name(), try_count, word_size);
     }
-
-    if (is_shutting_down()) {
-      stall_for_vm_shutdown();
-      return nullptr;
-    }
   }
 
   ShouldNotReachHere();
@@ -714,6 +721,18 @@ HeapWord* G1CollectedHeap::attempt_allocation_humongous(size_t word_size) {
     log_trace(gc, alloc)("%s: Unsuccessfully scheduled collection allocating %zu",
                          Thread::current()->name(), word_size);
 
+    if (is_shutting_down()) {
+      stall_for_vm_shutdown();
+      return nullptr;
+    }
+
+    // Has the gc overhead limit been reached in the meantime? If so, this mutator
+    // should receive null even when unsuccessfully scheduling a collection as well
+    // for global consistency.
+    if (gc_overhead_limit_exceeded()) {
+      return nullptr;
+    }
+
     // We can reach here if we were unsuccessful in scheduling a collection (because
     // another thread beat us to it).
     // Humongous object allocation always needs a lock, so we wait for the retry
@@ -725,11 +744,6 @@ HeapWord* G1CollectedHeap::attempt_allocation_humongous(size_t word_size) {
       log_warning(gc, alloc)("%s: Retried allocation %u times for %zu words",
                              Thread::current()->name(), try_count, word_size);
     }
-
-    if (is_shutting_down()) {
-      stall_for_vm_shutdown();
-      return nullptr;
-    }
   }
 
   ShouldNotReachHere();
@@ -955,25 +969,62 @@ void G1CollectedHeap::resize_heap_after_young_collection(size_t allocation_word_
   phase_times()->record_resize_heap_time((Ticks::now() - start).seconds() * 1000.0);
 }
 
+void G1CollectedHeap::update_gc_overhead_counter() {
+  assert(SafepointSynchronize::is_at_safepoint(), "precondition");
+
+  if (!UseGCOverheadLimit) {
+    return;
+  }
+
+  bool gc_time_over_limit = (_policy->analytics()->long_term_gc_time_ratio() * 100) >= GCTimeLimit;
+  double free_space_percent = percent_of(num_available_regions() * G1HeapRegion::GrainBytes, max_capacity());
+  bool free_space_below_limit = free_space_percent < GCHeapFreeLimit;
+
+  log_debug(gc)("GC Overhead Limit: GC Time %f Free Space %f Counter %zu",
+                (_policy->analytics()->long_term_gc_time_ratio() * 100),
+                free_space_percent,
+                _gc_overhead_counter);
+
+  if (gc_time_over_limit && free_space_below_limit) {
+    _gc_overhead_counter++;
+  } else {
+    _gc_overhead_counter = 0;
+  }
+}
+
+bool G1CollectedHeap::gc_overhead_limit_exceeded() {
+  return _gc_overhead_counter >= GCOverheadLimitThreshold;
+}
+
 HeapWord* G1CollectedHeap::satisfy_failed_allocation_helper(size_t word_size,
                                                             bool do_gc,
                                                             bool maximal_compaction,
                                                             bool expect_null_mutator_alloc_region) {
-  // Let's attempt the allocation first.
-  HeapWord* result =
-    attempt_allocation_at_safepoint(word_size,
-                                    expect_null_mutator_alloc_region);
-  if (result != nullptr) {
-    return result;
-  }
+  // Skip allocation if GC overhead limit has been exceeded to let the mutator run
+  // into an OOME. It can either exit "gracefully" or try to free up memory asap.
+  // For the latter situation, keep running GCs. If the mutator frees up enough
+  // memory quickly enough, the overhead(s) will go below the threshold(s) again
+  // and the VM may continue running.
+  // If we did not continue garbage collections, the (gc overhead) limit may decrease
+  // enough by itself to not count as exceeding the limit any more, in the worst
+  // case bouncing back-and-forth all the time.
+  if (!gc_overhead_limit_exceeded()) {
+    // Let's attempt the allocation first.
+    HeapWord* result =
+      attempt_allocation_at_safepoint(word_size,
+                                      expect_null_mutator_alloc_region);
+    if (result != nullptr) {
+      return result;
+    }
 
-  // In a G1 heap, we're supposed to keep allocation from failing by
-  // incremental pauses.  Therefore, at least for now, we'll favor
-  // expansion over collection.  (This might change in the future if we can
-  // do something smarter than full collection to satisfy a failed alloc.)
-  result = expand_and_allocate(word_size);
-  if (result != nullptr) {
-    return result;
+    // In a G1 heap, we're supposed to keep allocation from failing by
+    // incremental pauses.  Therefore, at least for now, we'll favor
+    // expansion over collection.  (This might change in the future if we can
+    // do something smarter than full collection to satisfy a failed alloc.)
+    result = expand_and_allocate(word_size);
+    if (result != nullptr) {
+      return result;
+    }
   }
 
   if (do_gc) {
@@ -997,6 +1048,10 @@ HeapWord* G1CollectedHeap::satisfy_failed_allocation_helper(size_t word_size,
 HeapWord* G1CollectedHeap::satisfy_failed_allocation(size_t word_size) {
   assert_at_safepoint_on_vm_thread();
 
+  // Update GC overhead limits after the initial garbage collection leading to this
+  // allocation attempt.
+  update_gc_overhead_counter();
+
   // Attempts to allocate followed by Full GC.
   HeapWord* result =
     satisfy_failed_allocation_helper(word_size,
@@ -1028,6 +1083,10 @@ HeapWord* G1CollectedHeap::satisfy_failed_allocation(size_t word_size) {
     return result;
   }
 
+  if (gc_overhead_limit_exceeded()) {
+    log_info(gc)("GC Overhead Limit exceeded too often (%zu).", GCOverheadLimitThreshold);
+  }
+
   // What else?  We might try synchronous finalization later.  If the total
   // space available is large enough for the allocation, then a more
   // complete compaction phase than we've tried so far might be
@@ -1209,6 +1268,7 @@ class HumongousRegionSetChecker : public G1HeapRegionSetChecker {
 
 G1CollectedHeap::G1CollectedHeap() :
   CollectedHeap(),
+  _gc_overhead_counter(0),
   _service_thread(nullptr),
   _periodic_gc_task(nullptr),
   _free_arena_memory_task(nullptr),

src/hotspot/share/gc/g1/g1CollectedHeap.hpp

@@ -169,6 +169,17 @@ class G1CollectedHeap : public CollectedHeap {
   friend class G1CheckRegionAttrTableClosure;
 
 private:
+  // GC Overhead Limit functionality related members.
+  //
+  // The goal is to return null for allocations prematurely (before really going
+  // OOME) in case both GC CPU usage (>= GCTimeLimit) and not much available free
+  // memory (<= GCHeapFreeLimit) so that applications can exit gracefully or try
+  // to keep running by easing off memory.
+  uintx _gc_overhead_counter;        // The number of consecutive garbage collections we were over the limits.
+
+  void update_gc_overhead_counter();
+  bool gc_overhead_limit_exceeded();
+
   G1ServiceThread* _service_thread;
   G1ServiceTask* _periodic_gc_task;
   G1MonotonicArenaFreeMemoryTask* _free_arena_memory_task;

src/hotspot/share/gc/parallel/parallelArguments.cpp

@@ -66,11 +66,6 @@ void ParallelArguments::initialize() {
     }
   }
 
-  // True in product build, since tests using debug build often stress GC
-  if (FLAG_IS_DEFAULT(UseGCOverheadLimit)) {
-    FLAG_SET_DEFAULT(UseGCOverheadLimit, trueInProduct);
-  }
-
   if (InitialSurvivorRatio < MinSurvivorRatio) {
     if (FLAG_IS_CMDLINE(InitialSurvivorRatio)) {
       if (FLAG_IS_CMDLINE(MinSurvivorRatio)) {

src/hotspot/share/gc/parallel/parallelScavengeHeap.cpp

@@ -374,6 +374,13 @@ bool ParallelScavengeHeap::check_gc_overhead_limit() {
     bool little_mutator_time = _size_policy->mutator_time_percent() * 100 < (100 - GCTimeLimit);
     bool little_free_space = check_gc_heap_free_limit(_young_gen->free_in_bytes(), _young_gen->capacity_in_bytes())
                           && check_gc_heap_free_limit(  _old_gen->free_in_bytes(),   _old_gen->capacity_in_bytes());
+
+    log_debug(gc)("GC Overhead Limit: GC Time %f Free Space Young %f Old %f Counter %zu",
+                  (100 - _size_policy->mutator_time_percent()),
+                  percent_of(_young_gen->free_in_bytes(), _young_gen->capacity_in_bytes()),
+                  percent_of(_old_gen->free_in_bytes(), _young_gen->capacity_in_bytes()),
+                  _gc_overhead_counter);
+
     if (little_mutator_time && little_free_space) {
       _gc_overhead_counter++;
       if (_gc_overhead_counter >= GCOverheadLimitThreshold) {
@@ -426,7 +433,7 @@ HeapWord* ParallelScavengeHeap::satisfy_failed_allocation(size_t size, bool is_t
   }
 
   if (check_gc_overhead_limit()) {
-    log_info(gc)("GCOverheadLimitThreshold %zu reached.", GCOverheadLimitThreshold);
+    log_info(gc)("GC Overhead Limit exceeded too often (%zu).", GCOverheadLimitThreshold);
     return nullptr;
   }
 

src/hotspot/share/gc/parallel/parallelScavengeHeap.hpp

@@ -88,7 +88,7 @@ class ParallelScavengeHeap : public CollectedHeap {
 
   WorkerThreads _workers;
 
-  uint _gc_overhead_counter;
+  uintx _gc_overhead_counter;
 
   bool _is_heap_almost_full;
 

src/hotspot/share/gc/shared/gc_globals.hpp

@@ -357,7 +357,7 @@
           "Initial ratio of young generation/survivor space size")          \
           range(3, max_uintx)                                               \
                                                                             \
-  product(bool, UseGCOverheadLimit, true,                                   \
+  product(bool, UseGCOverheadLimit, falseInDebug,                           \
           "Use policy to limit of proportion of time spent in GC "          \
           "before an OutOfMemory error is thrown")                          \
                                                                             \

test/hotspot/jtreg/gc/TestUseGCOverheadLimit.java

@@ -0,0 +1,98 @@
+/*
+ * Copyright (c) 2025, Oracle and/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.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+package gc;
+
+/*
+ * @test id=Parallel
+ * @requires vm.gc.Parallel
+ * @requires !vm.debug
+ * @summary Verifies that the UseGCOverheadLimit functionality works in Parallel GC.
+ * @library /test/lib
+ * @run driver gc.TestUseGCOverheadLimit Parallel
+ */
+
+/*
+ * @test id=G1
+ * @requires vm.gc.G1
+ * @requires !vm.debug
+ * @summary Verifies that the UseGCOverheadLimit functionality works in G1 GC.
+ * @library /test/lib
+ * @run driver gc.TestUseGCOverheadLimit G1
+ */
+
+import java.util.Arrays;
+import java.util.stream.Stream;
+
+import jdk.test.lib.process.OutputAnalyzer;
+import jdk.test.lib.process.ProcessTools;
+
+public class TestUseGCOverheadLimit {
+  public static void main(String args[]) throws Exception {
+    String[] parallelArgs = {
+      "-XX:+UseParallelGC",
+      "-XX:NewSize=122m",
+      "-XX:SurvivorRatio=99",
+      "-XX:GCHeapFreeLimit=10"
+    };
+    String[] g1Args = {
+      "-XX:+UseG1GC",
+      "-XX:GCHeapFreeLimit=5"
+    };
+
+    String[] selectedArgs = args[0].equals("G1") ? g1Args : parallelArgs;
+
+    final String[] commonArgs = {
+      "-XX:-UseCompactObjectHeaders", // Object sizes are calculated such that the heap is tight.
+      "-XX:ParallelGCThreads=1",      // Make GCs take longer.
+      "-XX:+UseGCOverheadLimit",
+      "-Xlog:gc=debug",
+      "-XX:GCTimeLimit=90",           // Ease the CPU requirement a little.
+      "-Xmx128m",
+      Allocating.class.getName()
+    };
+
+    String[] vmArgs = Stream.concat(Arrays.stream(selectedArgs), Arrays.stream(commonArgs)).toArray(String[]::new);
+    OutputAnalyzer output = ProcessTools.executeLimitedTestJava(vmArgs);
+    output.shouldNotHaveExitValue(0);
+
+    System.out.println(output.getStdout());
+
+    output.stdoutShouldContain("GC Overhead Limit exceeded too often (5).");
+  }
+
+  static class Allocating {
+    public static void main(String[] args) {
+      Object[] cache = new Object[1024 * 1024 * 2];
+
+      // Allocate random objects, keeping around data, causing garbage
+      // collections.
+      for (int i = 0; i < 1024* 1024 * 30; i++) {
+        Object[] obj = new Object[10];
+        cache[i % cache.length] = obj;
+      }
+
+      System.out.println(cache);
+    }
+  }
+}