diff --git a/judgels-backends/judgels-grader-app/src/main/java/judgels/gabriel/grading/GradingModule.java b/judgels-backends/judgels-grader-app/src/main/java/judgels/gabriel/grading/GradingModule.java
index e33b50846..e067e9c35 100644
--- a/judgels-backends/judgels-grader-app/src/main/java/judgels/gabriel/grading/GradingModule.java
+++ b/judgels-backends/judgels-grader-app/src/main/java/judgels/gabriel/grading/GradingModule.java
@@ -4,7 +4,9 @@
 import dagger.Provides;
 import io.dropwizard.lifecycle.setup.LifecycleEnvironment;
 import java.nio.file.Path;
+import java.util.concurrent.ArrayBlockingQueue;
 import java.util.concurrent.ExecutorService;
+import java.util.concurrent.ThreadPoolExecutor;
 import javax.inject.Named;
 import javax.inject.Provider;
 import javax.inject.Singleton;
@@ -41,10 +43,11 @@ GradingRequestPoller gradingRequestPoller(
         ExecutorService executorService = lifecycleEnv.executorService("grading-worker-%d")
                 .maxThreads(config.getNumWorkerThreads())
                 .minThreads(config.getNumWorkerThreads())
+                .workQueue(new ArrayBlockingQueue<>(config.getNumWorkerThreads()))
                 .build();
 
         return new GradingRequestPoller(
-                executorService,
+                (ThreadPoolExecutor) executorService,
                 config.getGradingRequestQueueName(),
                 messageClient,
                 workerFactory);
diff --git a/judgels-backends/judgels-grader-app/src/main/java/judgels/gabriel/grading/GradingRequestPoller.java b/judgels-backends/judgels-grader-app/src/main/java/judgels/gabriel/grading/GradingRequestPoller.java
index 06ceabfd8..980bbd701 100644
--- a/judgels-backends/judgels-grader-app/src/main/java/judgels/gabriel/grading/GradingRequestPoller.java
+++ b/judgels-backends/judgels-grader-app/src/main/java/judgels/gabriel/grading/GradingRequestPoller.java
@@ -1,9 +1,8 @@
 package judgels.gabriel.grading;
 
-import java.time.Duration;
 import java.util.Optional;
 import java.util.concurrent.CompletableFuture;
-import java.util.concurrent.ExecutorService;
+import java.util.concurrent.ThreadPoolExecutor;
 import javax.inject.Provider;
 import judgels.messaging.MessageClient;
 import judgels.messaging.api.Message;
@@ -13,15 +12,13 @@
 public class GradingRequestPoller implements Runnable {
     private static final Logger LOGGER = LoggerFactory.getLogger(GradingRequestPoller.class);
 
-    private static final Duration POLLING_DELAY = Duration.ofSeconds(2);
-
-    private final ExecutorService executorService;
+    private final ThreadPoolExecutor executorService;
     private final String queueName;
     private final MessageClient messageClient;
     private final Provider<GradingWorker> workerFactory;
 
     public GradingRequestPoller(
-            ExecutorService executorService,
+            ThreadPoolExecutor executorService,
             String queueName,
             MessageClient messageClient,
             Provider<GradingWorker> workerFactory) {
@@ -36,13 +33,14 @@ public GradingRequestPoller(
     public void run() {
         while (true) {
             try {
+                if (executorService.getQueue().remainingCapacity() == 0) {
+                    sleep(2 * 1000);
+                    continue;
+                }
+
                 Optional<Message> maybeMessage = messageClient.receiveMessage(queueName);
-                if (!maybeMessage.isPresent()) {
-                    try {
-                        Thread.sleep(POLLING_DELAY.toMillis());
-                    } catch (InterruptedException e) {
-                        Thread.currentThread().interrupt();
-                    }
+                if (maybeMessage.isEmpty()) {
+                    sleep(2 * 1000);
                     continue;
                 }
 
@@ -55,14 +53,18 @@ public void run() {
                             return null;
                         });
 
-                try {
-                    Thread.sleep((int) (Math.random() * 1000));
-                } catch (InterruptedException e) {
-                    Thread.currentThread().interrupt();
-                }
+                sleep((long) (Math.random() * 1000));
             } catch (Throwable e) {
                 LOGGER.error("Failed to run grading request poller", e);
             }
         }
     }
+
+    private void sleep(long milliseconds) {
+        try {
+            Thread.sleep(milliseconds);
+        } catch (InterruptedException e) {
+            Thread.currentThread().interrupt();
+        }
+    }
 }