Allow deviation of at least 1 document

Let content be well-balanced when there are few docs in a cluster

2 files changed, 23 insertions, 1 deletions

diff --git a/container-search/src/main/java/com/yahoo/search/dispatch/searchcluster/Group.java b/container-search/src/main/java/com/yahoo/search/dispatch/searchcluster/Group.java
index dca5892e0e7..7faad9d51cc 100644
--- a/container-search/src/main/java/com/yahoo/search/dispatch/searchcluster/Group.java
+++ b/container-search/src/main/java/com/yahoo/search/dispatch/searchcluster/Group.java
@@ -68,7 +68,7 @@ public class Group {
         if (numWorkingNodes > 0) {
             long average = activeDocs / numWorkingNodes;
             long deviation = nodes.stream().filter(node -> node.isWorking() == Boolean.TRUE).mapToLong(node -> Math.abs(node.getActiveDocuments() - average)).sum();
-            boolean isDeviationSmall = deviation <= (activeDocs * MAX_UNBALANCE);
+            boolean isDeviationSmall = deviation <= maxUnbalance(activeDocs);
             if ((!isContentWellBalanced.get() || isDeviationSmall != isContentWellBalanced.get()) && (activeDocs > 0)) {
                 log.info("Content is " + (isDeviationSmall ? "" : "not ") + "well balanced. Current deviation = " + deviation*100/activeDocs + " %" +
                          ". activeDocs = " + activeDocs + ", deviation = " + deviation + ", average = " + average);
@@ -79,6 +79,10 @@ public class Group {
         }
     }
 
+    double maxUnbalance(long activeDocs) {
+        return Math.max(1, activeDocs * MAX_UNBALANCE);
+    }
+
     /** Returns the active documents on this group. If unknown, 0 is returned. */
     long getActiveDocuments() { return activeDocuments.get(); }
 
diff --git a/container-search/src/test/java/com/yahoo/search/dispatch/searchcluster/SearchClusterCoverageTest.java b/container-search/src/test/java/com/yahoo/search/dispatch/searchcluster/SearchClusterCoverageTest.java
index 65e7173c4ee..c9f7469acbb 100644
--- a/container-search/src/test/java/com/yahoo/search/dispatch/searchcluster/SearchClusterCoverageTest.java
+++ b/container-search/src/test/java/com/yahoo/search/dispatch/searchcluster/SearchClusterCoverageTest.java
@@ -100,4 +100,22 @@ public class SearchClusterCoverageTest {
         assertTrue(tester.group(2).hasSufficientCoverage());
     }
 
+    @Test
+    public void one_group_few_docs_has_well_balanced_content() {
+        var tester = new SearchClusterTester(1, 2);
+
+        Node node0 = tester.group(0).nodes().get(0);
+        Node node1 = tester.group(0).nodes().get(1);
+
+        // 1 document
+        node0.setWorking(true);
+        node1.setWorking(true);
+
+        node0.setActiveDocuments(1);
+        node1.setActiveDocuments(0);
+
+        tester.pingIterationCompleted();
+        assertTrue(tester.group(0).isContentWellBalanced());
+    }
+
 }