[GitHub] [hbase] ragarkar commented on a diff in pull request #4799: HBASE-27389 Add cost function in balancer to consider the cost of bui…

["ragarkar (via GitHub)" <gi...@apache.org>]

ragarkar commented on code in PR #4799:
URL: https://github.com/apache/hbase/pull/4799#discussion_r1106697411


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hbase-balancer/src/main/java/org/apache/hadoop/hbase/master/balancer/PrefetchCacheCostFunction.java:
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@@ -0,0 +1,82 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.hadoop.hbase.master.balancer;
+
+import org.apache.hadoop.conf.Configuration;
+import org.apache.hadoop.hbase.HConstants;
+import org.apache.yetus.audience.InterfaceAudience;
+
+/**
+ * Compute the cost of a potential cluster configuration based on the number of HFile's already
+ * cached in the bucket cache
+ */
+@InterfaceAudience.Private
+public class PrefetchCacheCostFunction extends CostFunction {
+  private static final String PREFETCH_CACHE_COST_KEY =
+    "hbase.master.balancer.stochastic.prefetchCacheCost";
+  private static final float DEFAULT_PREFETCH_COST = 500;
+
+  private String prefetchedFileListPath;
+  private double prefetchRatio;
+  private float bestPrefetchRatio;
+
+  PrefetchCacheCostFunction(Configuration conf) {
+    prefetchedFileListPath = conf.get(HConstants.PREFETCH_PERSISTENCE_PATH_KEY);
+    // Disable the prefetch cache cost function if the prefetched file list persistence is not
+    // enabled
+    this.setMultiplier(prefetchedFileListPath == null
+      ? 0.0f
+      : conf.getFloat(PREFETCH_CACHE_COST_KEY, DEFAULT_PREFETCH_COST));
+    prefetchRatio = 0.0f;
+    bestPrefetchRatio = 0.0f;
+  }
+
+  @Override
+  void prepare(BalancerClusterState cluster) {
+    super.prepare(cluster);
+    prefetchRatio = 0.0f;
+    bestPrefetchRatio = 0.0f;
+
+    for (int region = 0; region < cluster.numRegions; region++) {
+      prefetchRatio +=
+        cluster.getOrComputeWeightedPrefetchRatio(region, cluster.regionIndexToServerIndex[region]);
+      bestPrefetchRatio += cluster.getOrComputeWeightedPrefetchRatio(region,
+        cluster.getOrComputeServerWithBestPrefetchRatio()[region]);
+    }
+    prefetchRatio = bestPrefetchRatio == 0.0f ? 1.0f : prefetchRatio / bestPrefetchRatio;
+  }
+
+  @Override
+  protected double cost() {
+    return 1 - prefetchRatio;
+  }
+
+  @Override
+  protected void regionMoved(int region, int oldServer, int newServer) {
+    float oldServerPrefetch = cluster.getOrComputeWeightedPrefetchRatio(region, oldServer);
+    float newServerPrefetch = cluster.getOrComputeWeightedPrefetchRatio(region, newServer);
+    float prefetchDelta = newServerPrefetch - oldServerPrefetch;
+    float normalizeDelta = bestPrefetchRatio == 0.0f ? 0.0f : prefetchDelta / bestPrefetchRatio;
+    prefetchRatio += normalizeDelta;
+  }

Review Comment:
   This code is triggered when the plan calculated by the balancer cannot be applied as it will increase the overall cost. In this case, the plan is cleaned up and all the updates done to the costs are reverted back through this method implemented on each cost function.



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