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在命令行中使用Eclipse MAT工具

最近应用在测试中出现Out Of Memory的问题, 通过jmap查看,发现JVM heap全用满了。

有很多工具可以查看JVM堆的信息, 收费的比如JProfiler, YourKit,免费的如Oracle JDK自带的visualvm, jhat和Eclipse MAT。

这个应用安装在一台AWS上,没有图形界面, 内存也比较小,想通过VNC远程桌面启动visualvm或者MAT不可能,通过jhat分析dump出来的snapshot(大约4.3G)也很慢,半天没有分析完毕,这种办法也放弃。

最后通过MAT的命令行工具分析了dump出来的snapshot,查找到OOM的元凶。

使用MAT命令行工具

首先通过jstat或者jmap查看heap信息,比如通过jmap查看:

JVM version is 25.31 -b07

using thread-local object allocation.

Parallel GC with 4 thread(s)

Heap Configuration:

MinHeapFreeRatio = 0

MaxHeapFreeRatio = 100

MaxHeapSize = 4294967296 ( 4096.0 MB)

NewSize = 1431306240 ( 1365.0 MB)

MaxNewSize = 1431306240 ( 1365.0 MB)

OldSize = 2863661056 ( 2731.0 MB)

NewRatio = 2

SurvivorRatio = 8

MetaspaceSize = 21807104 ( 20.796875 MB)

CompressedClassSpaceSize = 1073741824 ( 1024.0 MB)

MaxMetaspaceSize = 17592186044415 MB

G1HeapRegionSize = 0 ( 0.0 MB)

Heap Usage:

PS Young Generation

Eden Space:

capacity = 482344960 ( 460.0 MB)

used = 468288384 ( 446.5946044921875 MB)

free = 14056576 ( 13.4053955078125 MB)

97.08578358525816 % used

From Space:

capacity = 278921216 ( 266.0 MB)

used = 0 ( 0.0 MB)

free = 278921216 ( 266.0 MB)

0.0 % used

To Space:

capacity = 477102080 ( 455.0 MB)

used = 0 ( 0.0 MB)

free = 477102080 ( 455.0 MB)

0.0 % used

PS Old Generation

capacity = 2863661056 ( 2731.0 MB)

used = 2863365080 ( 2730.7177352905273 MB)

free = 295976 ( 0.28226470947265625 MB)

99.98966441927965 % used

12340 interned Strings occupying 1051736 bytes.

最多的类的实例:

num #instances #bytes class name

----------------------------------------------

1: 21606534 1530253752 [C

2: 21606239 518549736 java.lang.String

3: 19198980 460775520 scala.collection.immutable.ListSet$Node

4: 4568546 109645104 scala.collection.immutable.HashSet$HashSetCollision1

5: 103739 63212992 [B

6: 1487034 53464560 [Lscala.collection.immutable.HashSet;

7: 1487034 35688816 scala.collection.immutable.HashSet$HashTrieSet

8: 1350368 32408832 scala.collection.immutable.$colon$colon

9: 1090897 26181528 scala.collection.immutable.HashSet$HashSet1

10: 200035 17603080 akka.actor.ActorCell

11: 100536 8042880 java.lang.reflect.Constructor

12: 500026 8000416 scala.runtime.ObjectRef

从分析来看猜测是akka actor mailbox里面的字符串消息太多了。

既然没有办法图形化启动visualvm和MAT,那么就使用MAT文件夹下的 ParseHeapDump.sh , 特别适合分析大堆的信息。

首先你需要修改MemoryAnalyzer.ini中的Xmx值,确保有充足的硬盘空间(至少dump文件的两倍)。

然后运行

./ParseHeapDump.sh heap.bin org.eclipse.mat.api:suspects org.eclipse.mat.api:overview org.eclipse.mat.api:top_components

会得到suspects, overview和top_components三个视图的信息。

在命令行中使用Eclipse MAT工具

可以看到 akka.dispatch.Dispatcher$$anon$1 一个实例占用了2.4GB的内存,这就是罪魁祸首。这其实是akka dispatcher的mailbox中的java.util.concurrent.ConcurrentLinkedQueue,每个Node占用了81M的内存,

消息体太大了。

编写程序得到所需信息

你也可以引用MAT的类,得到heap dump中的信息, 因为MAT使用Eclipse RCP框架, 基于osgi架构,使用起来不太方便,所以你可以别人抽取出来的MAT库,如 https://bitbucket.org/joebowbeer/andromat ,

然后实现一个命令行程序,比如下面的例子就是输出所有的字符串的值:

package com.colobu.mat;

import org.eclipse.mat.SnapshotException;

import org.eclipse.mat.parser.model.PrimitiveArrayImpl;

import org.eclipse.mat.snapshot.ISnapshot;

import org.eclipse.mat.parser.internal.SnapshotFactory;

import org.eclipse.mat.snapshot.model.IClass;

import org.eclipse.mat.snapshot.model.IObject;

import org.eclipse.mat.util.ConsoleProgressListener;

import org.eclipse.mat.util.IProgressListener;

import java.io.File;

import java.io.IOException;

import java.util.Collection;

import java.util.HashMap;

public class Main {

public static void main (String[] args) throws SnapshotException, IOException {

String arg = args[args.length - 1 ];

String fileName = arg;

IProgressListener listener = new ConsoleProgressListener(System.out);

SnapshotFactory sf = new SnapshotFactory();

ISnapshot snapshot = sf.openSnapshot( new File(fileName),

new HashMap<String, String>(), listener);

System.out.println(snapshot.getSnapshotInfo());

System.out.println();

String[] classNames = { "java.lang.String" };

for (String name : classNames) {

Collection<IClass> classes = snapshot.getClassesByName(name, false );

if (classes == null || classes.isEmpty()) {

System.out.println(String.format( "Cannot find class %s in heap dump" , name));

continue ;

}

assert classes.size() == 1 ;

IClass clazz = classes.iterator().next();

int [] objIds = clazz.getObjectIds();

long minRetainedSize = snapshot.getMinRetainedSize(objIds, listener);

System.out.println(String.format( "%s instances = %d, retained size >= %d" , clazz.getName(), objIds.length, minRetainedSize));

for ( int i = 0 ; i < objIds.length; i++) {

IObject str = snapshot.getObject(objIds[i]);

String address = Long.toHexString(snapshot.mapIdToAddress(objIds[i]));

PrimitiveArrayImpl chars = (PrimitiveArrayImpl) str.resolveValue( "value" );

String value = new String(( char []) chars.getValueArray());

System.out.println(String.format( "id=%d, address=%s, value=%s" , objIds[i], address, value));

}

}

}

}

基本上使用 ParseHeapDump.sh 已经得到了我所需要的结果,优化akka actor消息的内容解决了我的问题。

参考文档

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