[Rocksandra] set merge operands limit

Summary: to experiment performance difference

Test Plan: will manual test

Reviewers: wpc, #ig-cassandra, chenshen, svemuri

Reviewed By: chenshen, svemuri

Subscribers: trunkagent, fdeliege

Differential Revision: https://phabricator.intern.facebook.com/D6011752

Tasks: T21631952

Signature: 6011752:1507580817:364cec6b102924da72bab29d8bd1f12f9bf1ab93

src/java/org/apache/cassandra/rocksdb/RocksDBCF.java

@@ -26,7 +26,6 @@
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
-import java.util.Collections;
 import java.util.List;
 import java.util.UUID;
 
@@ -74,6 +73,7 @@
 import org.rocksdb.StatsLevel;
 import org.rocksdb.WriteOptions;
 
+import static org.apache.cassandra.rocksdb.RocksDBConfigs.MERGE_OPERANDS_LIMIT;
 import static org.apache.cassandra.rocksdb.RocksDBConfigs.ROCKSDB_DIR;
 
 /**
@@ -114,7 +114,7 @@ public RocksDBCF(ColumnFamilyStore cfs) throws RocksDBException
         int gcGraceSeconds = cfs.metadata.params.gcGraceSeconds;
         boolean purgeTtlOnExpiration = cfs.metadata.params.purgeTtlOnExpiration;
         compactionFilter = new CassandraCompactionFilter(purgeTtlOnExpiration, gcGraceSeconds);
-        mergeOperator = new CassandraValueMergeOperator(gcGraceSeconds);
+        mergeOperator = new CassandraValueMergeOperator(gcGraceSeconds, MERGE_OPERANDS_LIMIT);
 
         DBOptions dbOptions = new DBOptions();
         List<ColumnFamilyDescriptor> cfDescs = new ArrayList<>();

src/java/org/apache/cassandra/rocksdb/RocksDBConfigs.java

@@ -39,6 +39,9 @@ public class RocksDBConfigs
     // disable write to rocksdb commit log or not, default is false
     public static final boolean DISABLE_WRITE_TO_COMMITLOG = Boolean.getBoolean("cassandra.rocksdb.disable_write_to_commitlog");
 
+    // Limit the number of operands in merge operator, 0 means no limit
+    public static final int MERGE_OPERANDS_LIMIT = Integer.getInteger("cassandra.rocksdb.merge_operands_limit", 0);
+
     /**
      * Streaming configs
      */