Cache AndroidBlockGuardPolicy to avoid allocation.

Every incoming Binder call ends up triggering a BlockGuardPolicy
update, which would thrash between new AndroidBlockGuardPolicy
instances and BlockGuard.LAX_POLICY, causing GC churn.

This change avoids the extra allocations by recycling a single
AndroidBlockGuardPolicy in a ThreadLocal.  Worst-case thrashing case
is now 10% faster; from 7.46us to 6.65us.

Bug: 9424568
Change-Id: I9c3b1c097a2aecc9b1f109a824cf3ea319fb3393

2 files changed, 49 insertions, 5 deletions

diff --git a/core/java/android/os/StrictMode.java b/core/java/android/os/StrictMode.java
index 31da091..a9abdc5 100644
--- a/core/java/android/os/StrictMode.java
+++ b/core/java/android/os/StrictMode.java
@@ -783,13 +783,15 @@ public final class StrictMode {
             BlockGuard.setThreadPolicy(BlockGuard.LAX_POLICY);
             return;
         }
-        BlockGuard.Policy policy = BlockGuard.getThreadPolicy();
-        if (!(policy instanceof AndroidBlockGuardPolicy)) {
-            BlockGuard.setThreadPolicy(new AndroidBlockGuardPolicy(policyMask));
+        final BlockGuard.Policy policy = BlockGuard.getThreadPolicy();
+        final AndroidBlockGuardPolicy androidPolicy;
+        if (policy instanceof AndroidBlockGuardPolicy) {
+            androidPolicy = (AndroidBlockGuardPolicy) policy;
         } else {
-            AndroidBlockGuardPolicy androidPolicy = (AndroidBlockGuardPolicy) policy;
-            androidPolicy.setPolicyMask(policyMask);
+            androidPolicy = threadAndroidPolicy.get();
+            BlockGuard.setThreadPolicy(androidPolicy);
         }
+        androidPolicy.setPolicyMask(policyMask);
     }
 
     // Sets up CloseGuard in Dalvik/libcore
@@ -1060,6 +1062,14 @@ public final class StrictMode {
         }
     };
 
+    private static final ThreadLocal<AndroidBlockGuardPolicy>
+            threadAndroidPolicy = new ThreadLocal<AndroidBlockGuardPolicy>() {
+        @Override
+        protected AndroidBlockGuardPolicy initialValue() {
+            return new AndroidBlockGuardPolicy(0);
+        }
+    };
+
     private static boolean tooManyViolationsThisLoop() {
         return violationsBeingTimed.get().size() >= MAX_OFFENSES_PER_LOOP;
     }
diff --git a/core/tests/benchmarks/src/android/os/StrictModeBenchmark.java b/core/tests/benchmarks/src/android/os/StrictModeBenchmark.java
new file mode 100644
index 0000000..41af3820
--- /dev/null
+++ b/core/tests/benchmarks/src/android/os/StrictModeBenchmark.java
@@ -0,0 +1,34 @@
+/*
+ * Copyright (C) 2013 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package android.os;
+
+import android.os.StrictMode.ThreadPolicy;
+
+import com.google.caliper.SimpleBenchmark;
+
+public class StrictModeBenchmark extends SimpleBenchmark {
+
+    private ThreadPolicy mOff = new ThreadPolicy.Builder().build();
+    private ThreadPolicy mOn = new ThreadPolicy.Builder().detectAll().build();
+
+    public void timeToggleThreadPolicy(int reps) {
+        for (int i = 0; i < reps; i++) {
+            StrictMode.setThreadPolicy(mOn);
+            StrictMode.setThreadPolicy(mOff);
+        }
+    }
+}