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Subject [GitHub] [hadoop] toddlipcon commented on a change in pull request #595: HDFS-14304: High lock contention on hdfsHashMutex in libhdfs
Date Tue, 19 Mar 2019 19:14:20 GMT
toddlipcon commented on a change in pull request #595: HDFS-14304: High lock contention on
hdfsHashMutex in libhdfs
URL: https://github.com/apache/hadoop/pull/595#discussion_r267052910
 
 

 ##########
 File path: hadoop-hdfs-project/hadoop-hdfs-native-client/src/main/native/libhdfs/jclasses.h
 ##########
 @@ -0,0 +1,117 @@
+/**
+ * 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.
+ */
+
+#ifndef LIBHDFS_JCLASSES_H
+#define LIBHDFS_JCLASSES_H
+
+#include <jni.h>
+
+/**
+ * Encapsulates logic to cache jclass objects so they can re-used across
+ * calls to FindClass. Creating jclass objects every time libhdfs has to
+ * invoke a method can hurt performance. By cacheing jclass objects we avoid
+ * this overhead.
+ *
+ * We use the term "cached" here loosely; jclasses are not truly cached,
+ * instead they are created once during JVM load and are kept alive until the
+ * process shutdowns. There is no eviction of jclass objects.
+ *
+ * @see https://www.ibm.com/developerworks/library/j-jni/index.html#notc
+ */
+
+/**
+ * Each enum value represents one jclass that is cached. Enum values should
+ * be passed to getJclass or getName to get the jclass object or class name
+ * represented by the enum value.
+ */
+typedef enum {
+    JC_CONFIGURATION,
+    JC_PATH,
+    JC_FILE_SYSTEM,
+    JC_FS_STATUS,
+    JC_FILE_UTIL,
+    JC_BLOCK_LOCATION,
+    JC_DFS_HEDGED_READ_METRICS,
+    JC_DISTRIBUTED_FILE_SYSTEM,
+    JC_FS_DATA_INPUT_STREAM,
+    JC_FS_DATA_OUTPUT_STREAM,
+    JC_FILE_STATUS,
+    JC_FS_PERMISSION,
+    JC_READ_STATISTICS,
+    JC_HDFS_DATA_INPUT_STREAM,
+    JC_DOMAIN_SOCKET,
+    JC_URI,
+    JC_BYTE_BUFFER,
+    JC_ENUM_SET,
+    JC_EXCEPTION_UTILS,
+    // A special marker enum that counts the number of cached jclasses
+    NUM_CACHED_CLASSES
+} CachedJavaClass;
+
+/**
+ * Whether initCachedClasses has been called or not. Protected by the mutex
+ * jclassInitMutex.
+ */
+extern int jclassesInitialized;
+
+/**
+ * Internally initializes all jclass objects listed in the CachedJavaClass enum.
+ */
+jthrowable initCachedClasses(JNIEnv* env);
 
 Review comment:
   per above, I think we should document that this is idempotent and threadsafe (safe to call
multiple times)

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