HBASE-26872 Load rate calculator for cost functions should be more precise (#4280)

Signed-off-by: Bryan Beaudreault <bbeaudreault@hubspot.com>
Signed-off-by: Viraj Jasani<virajjasani@apache.org>

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

@@ -1328,7 +1328,7 @@ public class StochasticLoadBalancer extends BaseLoadBalancer {
       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

@@ -358,6 +358,32 @@ public class TestStochasticLoadBalancer extends BalancerTestBase {
     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);
+      when(regionLoad.getStorefileSizeMB()).thenReturn(load);
+      regionLoads.add(regionLoad);
+    }
+
+    Configuration conf = HBaseConfiguration.create();
+    StochasticLoadBalancer.ReadRequestCostFunction readCostFunction =
+      new StochasticLoadBalancer.ReadRequestCostFunction(conf);
+    double rateResult = readCostFunction.getRegionLoadCost(regionLoads);
+    // read requests are treated as a rate, so here is the average rate
+    assertEquals(1.67, rateResult, 0.01);
+
+    StochasticLoadBalancer.StoreFileCostFunction storeFileCostFunction =
+      new StochasticLoadBalancer.StoreFileCostFunction(conf);
+    rateResult = storeFileCostFunction.getRegionLoadCost(regionLoads);
+    // storefile size cost is simply an average of it's value over time
+    assertEquals(2.0, rateResult, 0.01);
+  }
+
   @Test
   public void testLosingRs() throws Exception {
     int numNodes = 3;