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本文主要研究一下jvm的mapped buffer的统计

示例

private void writeDirectBuffer() {// 分配一个256MB的直接缓冲区ByteBuffer buffer = ByteBuffer.allocateDirect(256 * 1024 * 1024);// 填充数据Random random = new Random();while (buffer.remaining() >= 4) {buffer.putInt(random.nextInt());}System.out.println("Allocated direct buffer with capacity " + buffer.capacity());
}private void writeMappedBuffer() throws IOException {RandomAccessFile file = new RandomAccessFile("/tmp/test.txt", "rw");FileChannel channel = file.getChannel();MappedByteBuffer buffer = channel.map(FileChannel.MapMode.READ_WRITE, 0, 128 * 1024 * 1024); // 映射128MB的空间// 随机写入数据for (int i = 0; i < buffer.capacity(); i++) {buffer.put((byte) i);}
}

jvm.buffer.memory.used

http://localhost:8080/actuator/metrics/jvm.buffer.memory.used

{"name": "jvm.buffer.memory.used","description": "An estimate of the memory that the Java virtual machine is using for this buffer pool","baseUnit": "bytes","measurements": [{"statistic": "VALUE","value": 402685952}],"availableTags": [{"tag": "id","values": ["direct","mapped"]}]
}

jvm.buffer.memory.used分了direct和mapped两大类，一共用了384MB

mapped

http://localhost:8080/actuator/metrics/jvm.buffer.memory.used?tag=id:mapped

{"name": "jvm.buffer.memory.used","description": "An estimate of the memory that the Java virtual machine is using for this buffer pool","baseUnit": "bytes","measurements": [{"statistic": "VALUE","value": 134217728}],"availableTags": []
}

可以看到这里mapped用了128MB

direct

http://localhost:8080/actuator/metrics/jvm.buffer.memory.used?tag=id:direct

{"name": "jvm.buffer.memory.used","description": "An estimate of the memory that the Java virtual machine is using for this buffer pool","baseUnit": "bytes","measurements": [{"statistic": "VALUE","value": 268500992}],"availableTags": []
}

可以看到这里direct用了256MB

Native Memory Tracking

开启

java -XX:+UnlockDiagnosticVMOptions -XX:NativeMemoryTracking=summary -jar target/app.jar

监控

jcmd 2315 VM.native_memory summary scale=MB
2315:Native Memory Tracking:Total: reserved=5882MB, committed=1100MB
-                 Java Heap (reserved=4096MB, committed=577MB)(mmap: reserved=4096MB, committed=577MB)-                     Class (reserved=1066MB, committed=46MB)(classes #7035)(malloc=10MB #10793)(mmap: reserved=1056MB, committed=37MB)-                    Thread (reserved=36MB, committed=36MB)(thread #37)(stack: reserved=36MB, committed=36MB)-                      Code (reserved=246MB, committed=16MB)(malloc=2MB #3971)(mmap: reserved=244MB, committed=13MB)-                        GC (reserved=160MB, committed=148MB)(malloc=10MB #219)(mmap: reserved=150MB, committed=138MB)-                  Internal (reserved=266MB, committed=266MB)(malloc=266MB #10067)-                    Symbol (reserved=9MB, committed=9MB)(malloc=8MB #74334)(arena=2MB #1)-    Native Memory Tracking (reserved=2MB, committed=2MB)(tracking overhead=2MB)

NTM的Internal包含了direct buffer的统计，不包含mapped buffer的占用，而BufferPoolMXBean包含了direct和mapped。
另外通过调用System.loadLibrary()加载的共享库分配的内存NTM没办法追踪到，MXBean貌似也没办法统计到。

小结

• 聊聊jvm的direct buffer统计
• Memory Types in JVM
• Native Memory Tracking in JVM
• How to monitor JVM native memory usage by enabling native memory tracking
• Troubleshooting Problems With Native (Off-Heap) Memory in Java Applications
• Can a Java Application Use More Memory Than the Heap Size?
• java-non-heap-memory-analyzes