Adjust AsyncTask.THREAD_POOL_EXECUTOR config

Remove the queue, reduce core pool size but no longer let it time out.

Reduce the timeout for additional threads.

If necessary, use a special executor, with an unbounded queue, to run
overflow tasks.

Bug: 123762797
Test: AOSP boots, also with MAXIMUM_POOL_SIZE = 1.
Change-Id: I4bc9593a044d1773ff1878684e2397a7c2a9a87a

diff --git a/core/java/android/os/AsyncTask.java b/core/java/android/os/AsyncTask.java
index 1f33693..a851e04 100644 (file)
--- a/core/java/android/os/AsyncTask.java
+++ b/core/java/android/os/AsyncTask.java
@@ -21,13 +21,14 @@ import android.annotation.Nullable;
 import android.annotation.WorkerThread;
 
 import java.util.ArrayDeque;
-import java.util.concurrent.BlockingQueue;
 import java.util.concurrent.Callable;
 import java.util.concurrent.CancellationException;
 import java.util.concurrent.ExecutionException;
 import java.util.concurrent.Executor;
 import java.util.concurrent.FutureTask;
 import java.util.concurrent.LinkedBlockingQueue;
+import java.util.concurrent.RejectedExecutionHandler;
+import java.util.concurrent.SynchronousQueue;
 import java.util.concurrent.ThreadFactory;
 import java.util.concurrent.ThreadPoolExecutor;
 import java.util.concurrent.TimeUnit;
@@ -190,13 +191,19 @@ import java.util.concurrent.atomic.AtomicInteger;
 public abstract class AsyncTask<Params, Progress, Result> {
     private static final String LOG_TAG = "AsyncTask";
 
-    private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();
-    // We want at least 2 threads and at most 4 threads in the core pool,
-    // preferring to have 1 less than the CPU count to avoid saturating
-    // the CPU with background work
-    private static final int CORE_POOL_SIZE = Math.max(2, Math.min(CPU_COUNT - 1, 4));
-    private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;
-    private static final int KEEP_ALIVE_SECONDS = 30;
+    // We keep only a single pool thread around all the time.
+    // We let the pool grow to a fairly large number of threads if necessary,
+    // but let them time out quickly. In the unlikely case that we run out of threads,
+    // we fall back to a simple unbounded-queue executor.
+    // This combination ensures that:
+    // 1. We normally keep few threads (1) around.
+    // 2. We queue only after launching a significantly larger, but still bounded, set of threads.
+    // 3. We keep the total number of threads bounded, but still allow an unbounded set
+    //    of tasks to be queued.
+    private static final int CORE_POOL_SIZE = 1;
+    private static final int MAXIMUM_POOL_SIZE = 20;
+    private static final int BACKUP_POOL_SIZE = 5;
+    private static final int KEEP_ALIVE_SECONDS = 3;
 
     private static final ThreadFactory sThreadFactory = new ThreadFactory() {
         private final AtomicInteger mCount = new AtomicInteger(1);
@@ -206,8 +213,29 @@ public abstract class AsyncTask<Params, Progress, Result> {
         }
     };
 
-    private static final BlockingQueue<Runnable> sPoolWorkQueue =
-            new LinkedBlockingQueue<Runnable>(128);
+    // Used only for rejected executions.
+    // Initialization protected by sRunOnSerialPolicy lock.
+    private static ThreadPoolExecutor sBackupExecutor;
+    private static LinkedBlockingQueue<Runnable> sBackupExecutorQueue;
+
+    private static final RejectedExecutionHandler sRunOnSerialPolicy =
+            new RejectedExecutionHandler() {
+        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
+            android.util.Log.w(LOG_TAG, "Exceeded ThreadPoolExecutor pool size");
+            // As a last ditch fallback, run it on an executor with an unbounded queue.
+            // Create this executor lazily, hopefully almost never.
+            synchronized (this) {
+                if (sBackupExecutor == null) {
+                    sBackupExecutorQueue = new LinkedBlockingQueue<Runnable>();
+                    sBackupExecutor = new ThreadPoolExecutor(
+                            BACKUP_POOL_SIZE, BACKUP_POOL_SIZE, KEEP_ALIVE_SECONDS,
+                            TimeUnit.SECONDS, sBackupExecutorQueue, sThreadFactory);
+                    sBackupExecutor.allowCoreThreadTimeOut(true);
+                }
+            }
+            sBackupExecutor.execute(r);
+        }
+    };
 
     /**
      * An {@link Executor} that can be used to execute tasks in parallel.
@@ -217,8 +245,8 @@ public abstract class AsyncTask<Params, Progress, Result> {
     static {
         ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(
                 CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE_SECONDS, TimeUnit.SECONDS,
-                sPoolWorkQueue, sThreadFactory);
-        threadPoolExecutor.allowCoreThreadTimeOut(true);
+                new SynchronousQueue<Runnable>(), sThreadFactory);
+        threadPoolExecutor.setRejectedExecutionHandler(sRunOnSerialPolicy);
         THREAD_POOL_EXECUTOR = threadPoolExecutor;
     }