This closes #1519

runners/core-java/src/main/java/org/apache/beam/runners/core/GroupAlsoByWindowViaWindowSetDoFn.java

@@ -26,7 +26,6 @@
 import org.apache.beam.sdk.transforms.windowing.BoundedWindow;
 import org.apache.beam.sdk.util.SystemDoFnInternal;
 import org.apache.beam.sdk.util.TimerInternals;
-import org.apache.beam.sdk.util.TimerInternals.TimerData;
 import org.apache.beam.sdk.util.WindowingStrategy;
 import org.apache.beam.sdk.util.state.StateInternals;
 import org.apache.beam.sdk.util.state.StateInternalsFactory;
@@ -72,9 +71,9 @@ private GroupAlsoByWindowViaWindowSetDoFn(
 
   @Override
   public void processElement(ProcessContext c) throws Exception {
-    KeyedWorkItem<K, InputT> element = c.element();
+    KeyedWorkItem<K, InputT> keyedWorkItem = c.element();
 
-    K key = c.element().key();
+    K key = keyedWorkItem.key();
     TimerInternals timerInternals = c.windowingInternals().timerInternals();
     StateInternals<K> stateInternals = stateInternalsFactory.stateInternalsForKey(key);
 
@@ -92,10 +91,8 @@ public void processElement(ProcessContext c) throws Exception {
             reduceFn,
             c.getPipelineOptions());
 
-    reduceFnRunner.processElements(element.elementsIterable());
-    for (TimerData timer : element.timersIterable()) {
-      reduceFnRunner.onTimer(timer);
-    }
+    reduceFnRunner.processElements(keyedWorkItem.elementsIterable());
+    reduceFnRunner.onTimers(keyedWorkItem.timersIterable());
     reduceFnRunner.persist();
   }
 

runners/core-java/src/main/java/org/apache/beam/runners/core/GroupAlsoByWindowsViaOutputBufferDoFn.java

@@ -18,17 +18,18 @@
 package org.apache.beam.runners.core;
 
 import com.google.common.collect.Iterables;
+import java.util.ArrayList;
 import java.util.List;
 import org.apache.beam.runners.core.triggers.ExecutableTriggerStateMachine;
 import org.apache.beam.runners.core.triggers.TriggerStateMachines;
 import org.apache.beam.sdk.transforms.windowing.BoundedWindow;
 import org.apache.beam.sdk.util.SystemDoFnInternal;
+import org.apache.beam.sdk.util.TimerInternals;
 import org.apache.beam.sdk.util.WindowedValue;
 import org.apache.beam.sdk.util.WindowingStrategy;
 import org.apache.beam.sdk.util.state.InMemoryTimerInternals;
 import org.apache.beam.sdk.util.state.StateInternals;
 import org.apache.beam.sdk.util.state.StateInternalsFactory;
-import org.apache.beam.sdk.util.state.TimerCallback;
 import org.joda.time.Instant;
 
 /**
@@ -59,9 +60,8 @@ public void processElement(ProcessContext c) throws Exception {
     // timer manager from the context because it doesn't exist. So we create one and emulate the
     // watermark, knowing that we have all data and it is in timestamp order.
     InMemoryTimerInternals timerInternals = new InMemoryTimerInternals();
-    timerInternals.advanceProcessingTime(TimerCallback.NO_OP, Instant.now());
-    timerInternals.advanceSynchronizedProcessingTime(
-        TimerCallback.NO_OP, BoundedWindow.TIMESTAMP_MAX_VALUE);
+    timerInternals.advanceProcessingTime(Instant.now());
+    timerInternals.advanceSynchronizedProcessingTime(Instant.now());
     StateInternals<K> stateInternals = stateInternalsFactory.stateInternalsForKey(key);
 
     ReduceFnRunner<K, InputT, OutputT, W> reduceFnRunner =
@@ -85,22 +85,50 @@ public void processElement(ProcessContext c) throws Exception {
       reduceFnRunner.processElements(chunk);
 
       // Then, since elements are sorted by their timestamp, advance the input watermark
-      // to the first element, and fire any timers that may have been scheduled.
-      timerInternals.advanceInputWatermark(reduceFnRunner, chunk.iterator().next().getTimestamp());
+      // to the first element.
+      timerInternals.advanceInputWatermark(chunk.iterator().next().getTimestamp());
+      // Advance the processing times.
+      timerInternals.advanceProcessingTime(Instant.now());
+      timerInternals.advanceSynchronizedProcessingTime(Instant.now());
 
-      // Fire any processing timers that need to fire
-      timerInternals.advanceProcessingTime(reduceFnRunner, Instant.now());
+      // Fire all the eligible timers.
+      fireEligibleTimers(timerInternals, reduceFnRunner);
 
       // Leave the output watermark undefined. Since there's no late data in batch mode
       // there's really no need to track it as we do for streaming.
     }
 
     // Finish any pending windows by advancing the input watermark to infinity.
-    timerInternals.advanceInputWatermark(reduceFnRunner, BoundedWindow.TIMESTAMP_MAX_VALUE);
+    timerInternals.advanceInputWatermark(BoundedWindow.TIMESTAMP_MAX_VALUE);
 
     // Finally, advance the processing time to infinity to fire any timers.
-    timerInternals.advanceProcessingTime(reduceFnRunner, BoundedWindow.TIMESTAMP_MAX_VALUE);
+    timerInternals.advanceProcessingTime(BoundedWindow.TIMESTAMP_MAX_VALUE);
+    timerInternals.advanceSynchronizedProcessingTime(BoundedWindow.TIMESTAMP_MAX_VALUE);
+
+    fireEligibleTimers(timerInternals, reduceFnRunner);
 
     reduceFnRunner.persist();
   }
+
+  private void fireEligibleTimers(InMemoryTimerInternals timerInternals,
+      ReduceFnRunner<K, InputT, OutputT, W> reduceFnRunner) throws Exception {
+    List<TimerInternals.TimerData> timers = new ArrayList<>();
+    while (true) {
+        TimerInternals.TimerData timer;
+        while ((timer = timerInternals.removeNextEventTimer()) != null) {
+          timers.add(timer);
+        }
+        while ((timer = timerInternals.removeNextProcessingTimer()) != null) {
+          timers.add(timer);
+        }
+        while ((timer = timerInternals.removeNextSynchronizedProcessingTimer()) != null) {
+          timers.add(timer);
+        }
+        if (timers.isEmpty()) {
+          break;
+        }
+        reduceFnRunner.onTimers(timers);
+        timers.clear();
+    }
+  }
 }

runners/core-java/src/main/java/org/apache/beam/runners/core/PaneInfoTracker.java

@@ -54,6 +54,10 @@ public void clear(StateAccessor<?> state) {
     state.access(PANE_INFO_TAG).clear();
   }
 
+  public void prefetchPaneInfo(ReduceFn<?, ?, ?, ?>.Context context) {
+    context.state().access(PaneInfoTracker.PANE_INFO_TAG).readLater();
+  }
+
   /**
    * Return a ({@link ReadableState} for) the pane info appropriate for {@code context}. The pane
    * info includes the timing for the pane, who's calculation is quite subtle.

runners/core-java/src/main/java/org/apache/beam/runners/core/ReduceFnContextFactory.java

@@ -37,7 +37,6 @@
 import org.apache.beam.sdk.util.Timers;
 import org.apache.beam.sdk.util.WindowingStrategy;
 import org.apache.beam.sdk.util.state.MergingStateAccessor;
-import org.apache.beam.sdk.util.state.ReadableState;
 import org.apache.beam.sdk.util.state.State;
 import org.apache.beam.sdk.util.state.StateAccessor;
 import org.apache.beam.sdk.util.state.StateContext;
@@ -117,7 +116,7 @@ public ReduceFn<K, InputT, OutputT, W>.ProcessValueContext forValue(
   }
 
   public ReduceFn<K, InputT, OutputT, W>.OnTriggerContext forTrigger(W window,
-      ReadableState<PaneInfo> pane, StateStyle style, OnTriggerCallbacks<OutputT> callbacks) {
+      PaneInfo pane, StateStyle style, OnTriggerCallbacks<OutputT> callbacks) {
     return new OnTriggerContextImpl(stateAccessor(window, style), pane, callbacks);
   }
 
@@ -389,11 +388,11 @@ public Timers timers() {
 
   private class OnTriggerContextImpl extends ReduceFn<K, InputT, OutputT, W>.OnTriggerContext {
     private final StateAccessorImpl<K, W> state;
-    private final ReadableState<PaneInfo> pane;
+    private final PaneInfo pane;
     private final OnTriggerCallbacks<OutputT> callbacks;
     private final TimersImpl timers;
 
-    private OnTriggerContextImpl(StateAccessorImpl<K, W> state, ReadableState<PaneInfo> pane,
+    private OnTriggerContextImpl(StateAccessorImpl<K, W> state, PaneInfo pane,
         OnTriggerCallbacks<OutputT> callbacks) {
       reduceFn.super();
       this.state = state;
@@ -424,7 +423,7 @@ public StateAccessor<K> state() {
 
     @Override
     public PaneInfo paneInfo() {
-      return pane.read();
+      return pane;
     }
 
     @Override