base/allocator: Enable periodic decommit by default.

From the finch experiment (see linked bug), this saves .7-5MB of memory per
renderer on Windows and Android, at a cost of ~0.01% of idle time main thread
CPU.

As a consequence, enabling it by default.

Metrics note: The impact it not expected to be seen in the reported metrics, see
the bug for details. This may even be reported as a slight memory regression,
which is expected.

In this CL:
- Enable periodic decommit by default
- Update the unit tests accordingly.

The feature is not removed yet, to leave the possibility to disable it on the
M77 branch.

Bug: 942512
Change-Id: Ia673c55a62e0c273645dc783bbde2f9458a72d32
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1712563Reviewed-by: Benoit L <lizeb@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Benoit L <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#679933}

base/allocator/partition_allocator/memory_reclaimer.cc

@@ -17,8 +17,9 @@ namespace internal {
 
 const Feature kNoPartitionAllocDecommit{"NoPartitionAllocDecommit",
                                         FEATURE_DISABLED_BY_DEFAULT};
+// TODO(crbug.com/942512): Remove the feature after the M77 branch.
 const Feature kPartitionAllocPeriodicDecommit{"PartitionAllocPeriodicDecommit",
-                                              FEATURE_DISABLED_BY_DEFAULT};
+                                              FEATURE_ENABLED_BY_DEFAULT};
 
 }  // namespace internal
 
@@ -43,7 +44,6 @@ void PartitionAllocMemoryReclaimer::RegisterPartition(
     internal::PartitionRootBase* partition) {
   AutoLock lock(lock_);
   DCHECK(partition);
-  DCHECK(!timer_);
   auto it_and_whether_inserted = partitions_.insert(partition);
   DCHECK(it_and_whether_inserted.second);
 }

base/allocator/partition_allocator/memory_reclaimer_unittest.cc

@@ -73,15 +73,20 @@ TEST_F(PartitionAllocMemoryReclaimerTest, Simple) {
   EXPECT_TRUE(task_environment_.NextTaskIsDelayed());
 }
 
-TEST_F(PartitionAllocMemoryReclaimerTest, IsDisabledByDefault) {
+TEST_F(PartitionAllocMemoryReclaimerTest, IsEnabledByDefault) {
   StartReclaimer();
-  EXPECT_EQ(0u, task_environment_.GetPendingMainThreadTaskCount());
+  EXPECT_EQ(2u, task_environment_.GetPendingMainThreadTaskCount());
 }
 
-TEST_F(PartitionAllocMemoryReclaimerTest, FreesMemory) {
+TEST_F(PartitionAllocMemoryReclaimerTest, CanBeDisabled) {
   test::ScopedFeatureList scoped_feature_list;
-  scoped_feature_list.InitAndEnableFeature(
+  scoped_feature_list.InitAndDisableFeature(
       internal::kPartitionAllocPeriodicDecommit);
+  StartReclaimer();
+  EXPECT_EQ(0u, task_environment_.GetPendingMainThreadTaskCount());
+}
+
+TEST_F(PartitionAllocMemoryReclaimerTest, FreesMemory) {
   PartitionRootGeneric* root = allocator_->root();
 
   size_t committed_initially = root->total_size_of_committed_pages;
@@ -99,9 +104,6 @@ TEST_F(PartitionAllocMemoryReclaimerTest, FreesMemory) {
 }
 
 TEST_F(PartitionAllocMemoryReclaimerTest, Reclaim) {
-  StartReclaimer();
-  EXPECT_EQ(0u, task_environment_.GetPendingMainThreadTaskCount());
-
   PartitionRootGeneric* root = allocator_->root();
   size_t committed_initially = root->total_size_of_committed_pages;
 
@@ -116,45 +118,13 @@ TEST_F(PartitionAllocMemoryReclaimerTest, Reclaim) {
     EXPECT_LT(committed_after, committed_before);
     EXPECT_LE(committed_initially, committed_after);
   }
-
-  // |Reclaim()| is always enabled.
-  {
-    test::ScopedFeatureList scoped_feature_list;
-    scoped_feature_list.InitAndEnableFeature(
-        internal::kPartitionAllocPeriodicDecommit);
-
-    AllocateAndFree();
-
-    size_t committed_before = root->total_size_of_committed_pages;
-    EXPECT_GT(committed_before, committed_initially);
-    PartitionAllocMemoryReclaimer::Instance()->Reclaim();
-    size_t committed_after = root->total_size_of_committed_pages;
-
-    EXPECT_LT(committed_after, committed_before);
-    EXPECT_LE(committed_initially, committed_after);
-  }
 }
 
 TEST_F(PartitionAllocMemoryReclaimerTest, DeprecatedReclaim) {
-  StartReclaimer();
-  EXPECT_EQ(0u, task_environment_.GetPendingMainThreadTaskCount());
   PartitionRootGeneric* root = allocator_->root();
 
+  // Deprecated reclaim is disabled by default.
   {
-    // Deprecated reclaim enabled by default.
-    AllocateAndFree();
-    size_t committed_before = root->total_size_of_committed_pages;
-    PartitionAllocMemoryReclaimer::Instance()->DeprecatedReclaim();
-    size_t committed_after = root->total_size_of_committed_pages;
-    EXPECT_LT(committed_after, committed_before);
-  }
-
-  // Either of the features disables deprecated reclaim.
-  {
-    test::ScopedFeatureList scoped_feature_list;
-    scoped_feature_list.InitAndEnableFeature(
-        internal::kPartitionAllocPeriodicDecommit);
-
     AllocateAndFree();
     size_t committed_before = root->total_size_of_committed_pages;
     PartitionAllocMemoryReclaimer::Instance()->DeprecatedReclaim();
@@ -164,18 +134,16 @@ TEST_F(PartitionAllocMemoryReclaimerTest, DeprecatedReclaim) {
     PartitionAllocMemoryReclaimer::Instance()->Reclaim();
   }
 
+  // Deprecated reclaim works when periodic reclaim is disabled.
   {
     test::ScopedFeatureList scoped_feature_list;
-    scoped_feature_list.InitAndEnableFeature(
-        internal::kNoPartitionAllocDecommit);
-
+    scoped_feature_list.InitAndDisableFeature(
+        internal::kPartitionAllocPeriodicDecommit);
     AllocateAndFree();
     size_t committed_before = root->total_size_of_committed_pages;
     PartitionAllocMemoryReclaimer::Instance()->DeprecatedReclaim();
     size_t committed_after = root->total_size_of_committed_pages;
-    EXPECT_EQ(committed_after, committed_before);
-
-    PartitionAllocMemoryReclaimer::Instance()->Reclaim();
+    EXPECT_LT(committed_after, committed_before);
   }
 }
 
@@ -185,9 +153,6 @@ TEST_F(PartitionAllocMemoryReclaimerTest, StatsRecording) {
     return;
 
   HistogramTester histogram_tester;
-  test::ScopedFeatureList scoped_feature_list;
-  scoped_feature_list.InitAndEnableFeature(
-      internal::kPartitionAllocPeriodicDecommit);
   StartReclaimer();
   EXPECT_EQ(GetExpectedTasksCount(),
             task_environment_.GetPendingMainThreadTaskCount());