diff --git a/core/src/main/java/org/elasticsearch/index/shard/IndexShard.java b/core/src/main/java/org/elasticsearch/index/shard/IndexShard.java
index 292ef01b8d1..fee033feef3 100644
--- a/core/src/main/java/org/elasticsearch/index/shard/IndexShard.java
+++ b/core/src/main/java/org/elasticsearch/index/shard/IndexShard.java
@@ -189,6 +189,11 @@ public class IndexShard extends AbstractIndexShardComponent implements IndexSett
 
     private final IndexSearcherWrapper searcherWrapper;
 
+    /** True if this shard is still indexing (recently) and false if we've been idle for long enough (as periodically checked by {@link
+     *  IndexingMemoryController}). */
+    private final AtomicBoolean active = new AtomicBoolean();
+
+    private volatile long lastWriteNS;
     private final IndexingMemoryController indexingMemoryController;
 
     @Inject
@@ -450,6 +455,7 @@ public class IndexShard extends AbstractIndexShardComponent implements IndexSett
      */
     public boolean index(Engine.Index index) {
         ensureWriteAllowed(index);
+        markLastWrite(index);
         index = indexingService.preIndex(index);
         final boolean created;
         try {
@@ -474,6 +480,7 @@ public class IndexShard extends AbstractIndexShardComponent implements IndexSett
 
     public void delete(Engine.Delete delete) {
         ensureWriteAllowed(delete);
+        markLastWrite(delete);
         delete = indexingService.preDelete(delete);
         try {
             if (logger.isTraceEnabled()) {
@@ -883,6 +890,22 @@ public class IndexShard extends AbstractIndexShardComponent implements IndexSett
         }
     }
 
+    /** Returns timestamp of last indexing operation */
+    public long getLastWriteNS() {
+        return lastWriteNS;
+    }
+
+    /** Records timestamp of the last write operation, possibly switching {@code active} to true if we were inactive. */
+    private void markLastWrite(Engine.Operation op) {
+        lastWriteNS = op.startTime();
+        if (active.getAndSet(true) == false) {
+            // We are currently inactive, but a new write operation just showed up, so we now notify IMC
+            // to wake up and fix our indexing buffer.  We could do this async instead, but cost should
+            // be low, and it's rare this happens.
+            indexingMemoryController.forceCheck();
+        }
+    }
+
     private void ensureWriteAllowed(Engine.Operation op) throws IllegalIndexShardStateException {
         Engine.Operation.Origin origin = op.origin();
         IndexShardState state = this.state; // one time volatile read
@@ -954,6 +977,24 @@ public class IndexShard extends AbstractIndexShardComponent implements IndexSett
         return engine.indexBufferRAMBytesUsed();
     }
 
+    /** Called by {@link IndexingMemoryController} to check whether more than {@code inactiveTimeNS} has passed since the last
+     *  indexing operation, and become inactive (reducing indexing and translog buffers to tiny values) if so. */
+    public void checkIdle(long inactiveTimeNS) {
+        if (System.nanoTime() - lastWriteNS >= inactiveTimeNS) {
+            boolean wasActive = active.getAndSet(false);
+            if (wasActive) {
+                logger.debug("shard is now inactive");
+                indicesLifecycle.onShardInactive(this);
+            }
+        }
+    }
+
+    /** Returns {@code true} if this shard is active (has seen indexing ops in the last {@link
+     *  IndexingMemoryController#SHARD_INACTIVE_TIME_SETTING} (default 5 minutes), else {@code false}. */
+    public boolean getActive() {
+        return active.get();
+    }
+
     public final boolean isFlushOnClose() {
         return flushOnClose;
     }
diff --git a/core/src/main/java/org/elasticsearch/indices/memory/IndexingMemoryController.java b/core/src/main/java/org/elasticsearch/indices/memory/IndexingMemoryController.java
index aebaeccf483..821311ad636 100644
--- a/core/src/main/java/org/elasticsearch/indices/memory/IndexingMemoryController.java
+++ b/core/src/main/java/org/elasticsearch/indices/memory/IndexingMemoryController.java
@@ -51,6 +51,9 @@ public class IndexingMemoryController extends AbstractLifecycleComponent<Indexin
     /** Only applies when <code>indices.memory.index_buffer_size</code> is a %, to set a ceiling on the actual size in bytes (default: not set). */
     public static final String MAX_INDEX_BUFFER_SIZE_SETTING = "indices.memory.max_index_buffer_size";
 
+    /** If we see no indexing operations after this much time for a given shard, we consider that shard inactive (default: 5 minutes). */
+    public static final String SHARD_INACTIVE_TIME_SETTING = "indices.memory.shard_inactive_time";
+
     /** How frequently we check indexing memory usage (default: 5 seconds). */
     public static final String SHARD_MEMORY_INTERVAL_TIME_SETTING = "indices.memory.interval";
 
@@ -62,6 +65,7 @@ public class IndexingMemoryController extends AbstractLifecycleComponent<Indexin
 
     private final ByteSizeValue indexingBuffer;
 
+    private final TimeValue inactiveTime;
     private final TimeValue interval;
 
     private volatile ScheduledFuture scheduler;
@@ -101,12 +105,16 @@ public class IndexingMemoryController extends AbstractLifecycleComponent<Indexin
         }
         this.indexingBuffer = indexingBuffer;
 
+        this.inactiveTime = this.settings.getAsTime(SHARD_INACTIVE_TIME_SETTING, TimeValue.timeValueMinutes(5));
         // we need to have this relatively small to free up heap quickly enough
         this.interval = this.settings.getAsTime(SHARD_MEMORY_INTERVAL_TIME_SETTING, TimeValue.timeValueSeconds(5));
 
         this.statusChecker = new ShardsIndicesStatusChecker();
 
-        logger.debug("using indexing buffer size [{}] with {} [{}]", this.indexingBuffer, SHARD_MEMORY_INTERVAL_TIME_SETTING, this.interval);
+        logger.debug("using indexing buffer size [{}] with {} [{}], {} [{}]",
+                     this.indexingBuffer,
+                     SHARD_INACTIVE_TIME_SETTING, this.inactiveTime,
+                     SHARD_MEMORY_INTERVAL_TIME_SETTING, this.interval);
     }
 
     @Override
@@ -175,6 +183,15 @@ public class IndexingMemoryController extends AbstractLifecycleComponent<Indexin
         return shard != null && shard.canIndex() && CAN_UPDATE_INDEX_BUFFER_STATES.contains(shard.state());
     }
 
+    /** ask this shard to check now whether it is inactive, and reduces its indexing and translog buffers if so.  returns Boolean.TRUE if
+     *  it did deactive, Boolean.FALSE if it did not, and null if the shard is unknown */
+    protected void checkIdle(ShardId shardId, long inactiveTimeNS) {
+        final IndexShard shard = getShard(shardId);
+        if (shard != null) {
+            shard.checkIdle(inactiveTimeNS);
+        }
+    }
+
     /** gets an {@link IndexShard} instance for the given shard. returns null if the shard doesn't exist */
     protected IndexShard getShard(ShardId shardId) {
         IndexService indexService = indicesService.indexService(shardId.index().name());
@@ -214,10 +231,11 @@ public class IndexingMemoryController extends AbstractLifecycleComponent<Indexin
             // Fast check to sum up how much heap all shards' indexing buffers are using now:
             long totalBytesUsed = 0;
             for (ShardId shardId : availableShards()) {
-                long shardBytesUsed = getIndexBufferRAMBytesUsed(shardId);
-                if (shardBytesUsed > 0) {
-                    totalBytesUsed += shardBytesUsed;
-                }
+
+                // Give shard a chance to transition to inactive so sync'd flush can happen:
+                checkIdle(shardId, inactiveTime.nanos());
+
+                totalBytesUsed += getIndexBufferRAMBytesUsed(shardId);
             }
 
             if (totalBytesUsed > indexingBuffer.bytes()) {
diff --git a/core/src/test/java/org/elasticsearch/index/engine/InternalEngineTests.java b/core/src/test/java/org/elasticsearch/index/engine/InternalEngineTests.java
index a224bc87fec..2b929f76ad3 100644
--- a/core/src/test/java/org/elasticsearch/index/engine/InternalEngineTests.java
+++ b/core/src/test/java/org/elasticsearch/index/engine/InternalEngineTests.java
@@ -91,6 +91,7 @@ import org.elasticsearch.index.store.Store;
 import org.elasticsearch.index.translog.Translog;
 import org.elasticsearch.index.translog.TranslogConfig;
 import org.elasticsearch.index.translog.TranslogTests;
+import org.elasticsearch.indices.memory.IndexingMemoryController;
 import org.elasticsearch.test.DummyShardLock;
 import org.elasticsearch.test.ESTestCase;
 import org.elasticsearch.threadpool.ThreadPool;
@@ -1564,7 +1565,8 @@ public class InternalEngineTests extends ESTestCase {
     public void testDeletesAloneCanTriggerRefresh() throws Exception {
         Settings settings = Settings.builder()
                               .put(defaultSettings)
-                              .put(EngineConfig.INDEX_BUFFER_SIZE_SETTING, "1kb").build();
+                              .put(EngineConfig.INDEX_BUFFER_SIZE_SETTING, "1kb")
+                              .put(IndexingMemoryController.SHARD_MEMORY_INTERVAL_TIME_SETTING, "100ms").build();
         try (Store store = createStore();
             Engine engine = new InternalEngine(config(settings, store, createTempDir(), new MergeSchedulerConfig(defaultSettings), newMergePolicy()), false)) {
             for (int i = 0; i < 100; i++) {
diff --git a/core/src/test/java/org/elasticsearch/index/shard/IndexShardTests.java b/core/src/test/java/org/elasticsearch/index/shard/IndexShardTests.java
index ce3e4e51aba..fd402130092 100644
--- a/core/src/test/java/org/elasticsearch/index/shard/IndexShardTests.java
+++ b/core/src/test/java/org/elasticsearch/index/shard/IndexShardTests.java
@@ -339,6 +339,8 @@ public class IndexShardTests extends ESSingleNodeTestCase {
         client().prepareIndex("test", "test").setSource("{}").get();
         ensureGreen("test");
         IndicesService indicesService = getInstanceFromNode(IndicesService.class);
+        // force the shard to become idle now:
+        indicesService.indexService("test").getShardOrNull(0).checkIdle(0);
         assertBusy(new Runnable() { // should be very very quick
             @Override
             public void run() {
diff --git a/core/src/test/java/org/elasticsearch/indices/memory/IndexingMemoryControllerTests.java b/core/src/test/java/org/elasticsearch/indices/memory/IndexingMemoryControllerTests.java
index 033d625a26d..2d54fb0a1fd 100644
--- a/core/src/test/java/org/elasticsearch/indices/memory/IndexingMemoryControllerTests.java
+++ b/core/src/test/java/org/elasticsearch/indices/memory/IndexingMemoryControllerTests.java
@@ -75,6 +75,10 @@ public class IndexingMemoryControllerTests extends ESTestCase {
             }
         }
 
+        @Override
+        protected void checkIdle(ShardId shardId, long inactiveTimeNS) {
+        }
+
         @Override
         public void refreshShardAsync(ShardId shardId) {
             indexBufferRAMBytesUsed.put(shardId, 0L);
@@ -103,7 +107,6 @@ public class IndexingMemoryControllerTests extends ESTestCase {
         final ShardId shard1 = new ShardId("test", 1);
         controller.simulateIndexing(shard1);
         controller.assertBuffer(shard1, new ByteSizeValue(1, ByteSizeUnit.MB));
-                                
 
         // add another shard
         final ShardId shard2 = new ShardId("test", 2);
@@ -144,7 +147,8 @@ public class IndexingMemoryControllerTests extends ESTestCase {
         controller.assertBuffer(shard1, new ByteSizeValue(2, ByteSizeUnit.MB));
         controller.assertBuffer(shard2, new ByteSizeValue(2, ByteSizeUnit.MB));
 
-        // index into one shard only, hits the 5mb limit, so shard1 is refreshed
+        // index into one shard only, crosses the 5mb limit, so shard1 is refreshed
+        controller.simulateIndexing(shard1);
         controller.simulateIndexing(shard1);
         controller.assertBuffer(shard1, new ByteSizeValue(0, ByteSizeUnit.MB));
         controller.assertBuffer(shard2, new ByteSizeValue(2, ByteSizeUnit.MB));
@@ -153,7 +157,8 @@ public class IndexingMemoryControllerTests extends ESTestCase {
         controller.simulateIndexing(shard2);
         controller.assertBuffer(shard2, new ByteSizeValue(4, ByteSizeUnit.MB));
         controller.simulateIndexing(shard2);
-        // shard2 used up the full 5 mb and is now cleared:
+        controller.simulateIndexing(shard2);
+        // shard2 crossed 5 mb and is now cleared:
         controller.assertBuffer(shard2, new ByteSizeValue(0, ByteSizeUnit.MB));
     }