Parallelize message persisters.

service/src/main/java/org/whispersystems/textsecuregcm/storage/MessagePersister.java

@@ -34,7 +34,7 @@ public class MessagePersister implements Managed {
 
     private final Duration        persistDelay;
 
-    private final    Thread  workerThread;
+    private final    Thread[]     workerThreads = new Thread[WORKER_THREAD_COUNT];
     private volatile boolean      running;
 
     private final MetricRegistry metricRegistry             = SharedMetricRegistries.getOrCreate(Constants.METRICS_NAME);
@@ -48,6 +48,8 @@ public class MessagePersister implements Managed {
     static final int QUEUE_BATCH_LIMIT   = 100;
     static final int MESSAGE_BATCH_LIMIT = 100;
 
+    private static final int WORKER_THREAD_COUNT = 4;
+
     private static final Logger logger = LoggerFactory.getLogger(MessagePersister.class);
 
     public MessagePersister(final MessagesCache messagesCache, final MessagesManager messagesManager, final AccountsManager accountsManager, final Duration persistDelay) {
@@ -56,7 +58,8 @@ public class MessagePersister implements Managed {
         this.accountsManager          = accountsManager;
         this.persistDelay             = persistDelay;
 
-        this.workerThread = new Thread(() -> {
+        for (int i = 0; i < workerThreads.length; i++) {
+            workerThreads[i] = new Thread(() -> {
                 while (running) {
                     try {
                         final int queuesPersisted = persistNextQueues(Instant.now());
@@ -69,9 +72,8 @@ public class MessagePersister implements Managed {
                         logger.warn("Failed to persist queues", t);
                     }
                 }
-        }, "MessagePersisterWorker");
-
-        metricRegistry.gauge(name(getClass(), "workerThreadRunning"), () -> () -> workerThread.isAlive() ? 1 : 0);
+            }, "MessagePersisterWorker-" + i);
+        }
     }
 
     @VisibleForTesting
@@ -82,19 +84,24 @@ public class MessagePersister implements Managed {
     @Override
     public void start() {
         running = true;
+
+        for (final Thread workerThread : workerThreads) {
             workerThread.start();
         }
+    }
 
     @Override
     public void stop() {
         running = false;
 
+        for (final Thread workerThread : workerThreads) {
             try {
                 workerThread.join();
             } catch (final InterruptedException e) {
                 logger.warn("Interrupted while waiting for worker thread to complete current operation");
             }
         }
+    }
 
     @VisibleForTesting
     int persistNextQueues(final Instant currentTime) {