HBASE-26872 Load rate calculator for cost functions should be more precise (#4253)

Signed-off-by: Bryan Beaudreault <[email protected]>
Signed-off-by: Viraj Jasani<[email protected]>

Author: sunhelly

hbase-server/src/main/java/org/apache/hadoop/hbase/master/balancer/CostFromRegionLoadAsRateFunction.java

@@ -42,9 +42,9 @@ protected double getRegionLoadCost(Collection<BalancerRegionLoad> regionLoadList
     double cost = 0;
     do {
       double current = getCostFromRl(iter.next());
-      cost += current - previous;
+      cost += current >= previous ? current - previous : current;
       previous = current;
     } while (iter.hasNext());
     return Math.max(0, cost / (regionLoadList.size() - 1));
   }
-}
+}

hbase-server/src/test/java/org/apache/hadoop/hbase/master/balancer/TestStochasticLoadBalancer.java

@@ -23,7 +23,6 @@
 import static org.junit.Assert.assertTrue;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.when;
-
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
@@ -50,7 +49,6 @@
 import org.junit.ClassRule;
 import org.junit.Test;
 import org.junit.experimental.categories.Category;
-
 import org.apache.hbase.thirdparty.org.apache.commons.collections4.CollectionUtils;
 
 @Category({ MasterTests.class, MediumTests.class })
@@ -449,6 +447,25 @@ public void testRegionLoadCost() {
     assertEquals(2.5, result, 0.01);
   }
 
+  @Test
+  public void testRegionLoadCostWhenDecrease() {
+    List<BalancerRegionLoad> regionLoads = new ArrayList<>();
+    // test region loads of [1,2,1,4]
+    for (int i = 1; i < 5; i++) {
+      int load = i == 3 ? 1 : i;
+      BalancerRegionLoad regionLoad = mock(BalancerRegionLoad.class);
+      when(regionLoad.getReadRequestsCount()).thenReturn((long)load);
+      regionLoads.add(regionLoad);
+    }
+
+    Configuration conf = HBaseConfiguration.create();
+    ReadRequestCostFunction readCostFunction =
+      new ReadRequestCostFunction(conf);
+    double rateResult = readCostFunction.getRegionLoadCost(regionLoads);
+    // read requests are treated as a rate so the average rate here is simply 1
+    assertEquals(1.67, rateResult, 0.01);
+  }
+
   @Test
   public void testLosingRs() throws Exception {
     int numNodes = 3;