keith-turner commented on a change in pull request #2422:
URL: https://github.com/apache/accumulo/pull/2422#discussion_r791032803



##########
File path: 
core/src/main/java/org/apache/accumulo/core/clientImpl/TabletServerBatchReaderIterator.java
##########
@@ -497,26 +499,44 @@ private void 
doLookups(Map<String,Map<KeyExtent,List<Range>>> binnedRanges,
     for (final String tsLocation : locations) {
 
       final Map<KeyExtent,List<Range>> tabletsRanges = 
binnedRanges.get(tsLocation);
-      if (maxTabletsPerRequest == Integer.MAX_VALUE || tabletsRanges.size() == 
1) {
-        QueryTask queryTask = new QueryTask(tsLocation, tabletsRanges, 
failures, receiver, columns);
-        queryTasks.add(queryTask);
+      if (options.isUseScanServer()) {
+        // Ignore the tablets location and find a scan server to use
+        ScanServerLocator ssl = context.getScanServerLocator();
+        tabletsRanges.forEach((k, v) -> {
+          try {
+            String location = ssl.reserveScanServer(new TabletIdImpl(k));

Review comment:
       >  I figured that if a ScanServer had many threads and performed more 
than one scan at a time, then we would potentially run into the same situation 
we have in in the TabletServer w/r/t memory usage.
   
   I can see the benefit of a single thread.  We could start off implementing 
the thread pool with syncQ with a hard coded thread of size of one and get this 
really fast busy exception behavior.  Later on, as we gain experience, we could 
refine the scan server config and make what causes busy exceptions on a scan 
server configurable.




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