Add logging of top tablets to enable better hot spot detection

core/src/main/java/org/apache/accumulo/core/conf/Property.java

@@ -485,6 +485,9 @@ public enum Property {
       "The time between adjustments of the server thread pool."),
   TSERV_MAX_MESSAGE_SIZE("tserver.server.message.size.max", "1G", PropertyType.MEMORY,
       "The maximum size of a message that can be sent to a tablet server."),
+  TSERV_LOG_TOP_TABLETS_COUNT("tserver.log.top.tablets.count", "0", PropertyType.COUNT,
+      "Number of top tablets to log when saving tablet stats. If <= 0, logging "
+          + "of top tablets is disabled"),
   TSERV_HOLD_TIME_SUICIDE("tserver.hold.time.max", "5m", PropertyType.TIMEDURATION,
       "The maximum time for a tablet server to be in the \"memory full\" state."
           + " If the tablet server cannot write out memory in this much time, it will"

server/tserver/src/main/java/org/apache/accumulo/tserver/TabletServer.java

@@ -40,6 +40,7 @@
 import java.util.List;
 import java.util.Map;
 import java.util.Map.Entry;
+import java.util.PriorityQueue;
 import java.util.Random;
 import java.util.Set;
 import java.util.SortedMap;
@@ -358,17 +359,60 @@ public TabletServer(ServerConfigurationFactory confFactory, VolumeManager fs) th
     this.logSorter = new LogSorter(instance, fs, aconf);
     this.replWorker = new ReplicationWorker(this, fs);
     this.statsKeeper = new TabletStatsKeeper();
+    final int numTopTabletsToLog = aconf.getCount(Property.TSERV_LOG_TOP_TABLETS_COUNT);
+    final boolean logTopTablets = numTopTabletsToLog > 0;
     SimpleTimer.getInstance(aconf).schedule(new Runnable() {
       @Override
       public void run() {
+
+        Comparator<Pair<String,Long>> topTabletComparator = new Comparator<Pair<String,Long>>() {
+          @Override
+          public int compare(Pair<String,Long> first, Pair<String,Long> second) {
+            return second.getSecond().compareTo(first.getSecond());
+          }
+        };
+        PriorityQueue<Pair<String,Long>> topTabletsByIngestCount =
+            new PriorityQueue<>(numTopTabletsToLog, topTabletComparator);
+        PriorityQueue<Pair<String,Long>> topTabletsByQueryCount =
+            new PriorityQueue<>(topTabletsByIngestCount);
         synchronized (onlineTablets) {
           long now = System.currentTimeMillis();
+          topTabletsByIngestCount.clear();
+          topTabletsByQueryCount.clear();
           for (Tablet tablet : onlineTablets.values())
             try {
               tablet.updateRates(now);
+              if (logTopTablets) {
+                addToTopTablets(tablet.totalIngest(), topTabletsByIngestCount, numTopTabletsToLog);
+                addToTopTablets(tablet.totalQueries(), topTabletsByQueryCount, numTopTabletsToLog);
+              }
             } catch (Exception ex) {
               log.error("Error updating rates for {}", tablet.getExtent(), ex);
             }
+
+          if (logTopTablets) {
+            logTopTablets(topTabletsByIngestCount, "QUERY", numTopTabletsToLog);
+            logTopTablets(topTabletsByQueryCount, "INGEST", numTopTabletsToLog);
+          }
+        }
+      }
+
+      private void addToTopTablets(long count,
+          PriorityQueue<Pair<String,Long>> topTabletsByIngestCount, int numTopTabletsToLog) {
+        if (topTabletsByIngestCount.size() < numTopTabletsToLog
+            || topTabletsByIngestCount.peek().getSecond() < count) {
+          if (topTabletsByIngestCount.size() == numTopTabletsToLog) {
+            topTabletsByIngestCount.remove();
+          }
+        }
+      }
+
+      private void logTopTablets(PriorityQueue<Pair<String,Long>> topTabletsByIngestCount,
+          String label, int numTopTabletsToLog) {
+        for (int i = 0; i < numTopTabletsToLog; i++) {
+          Pair<String,Long> pair = topTabletsByIngestCount.poll();
+          log.debug("Top {} tablet by {} count -- extent: {} count: {}", i, label, pair.getFirst(),
+              pair.getSecond());
         }
       }
     }, 5000, 5000);

server/tserver/src/main/java/org/apache/accumulo/tserver/tablet/Tablet.java

@@ -2395,6 +2395,10 @@ public long totalQueries() {
     return this.queryCount;
   }
 
+  public long totalIngest() {
+    return this.ingestCount;
+  }
+
   // synchronized?
   public void updateRates(long now) {
     queryRate.update(now, queryCount);