Java Programming

Troubleshooting JVM memory issues can be daunting, but you if you don’t fix it, it can kill your application and possibly your face in front of your customers. There is good news though. With any modern APM tool, you virtually get an X-Ray vision to this labyrinth named Java Heap.

In this blog post, I’ll show you how to use AppDynamics to troubleshoot Java memory issues.

AppDynamics is one of the leading APM vendors in the market, providing tons of useful tools ranging from Hardware monitoring to End user experience monitoring to operational data analytics (Actually it can be quite overwhelming. If you don’t know what exactly you are looking for in the tool, you can go in circles. But don’t worry, I can help).

There are three major areas where AppDynamics can help in regards to Java memory troubleshooting.

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Java memory leaks can be deadly, and difficult to troubleshoot. Are you one of those shops where you restart your Application Servers at regular intervals (weekly, daily or more frequently)? It is pathetic, is it not? Wait a minute, gone are the days where we had 128 MB memory on servers. We have several giga bytes of memory on servers, don’t we? Why do we still run into memory issues? Good question. But sad truth is there are several reasons why Memory leak is not something that will go away. All you can do is to prepare yourself. And that’s what this article is about. Let’s dive into 10 things you need to know about Java memory leak.

1.  Java Heap Memory leak is different from Native Memory leak

Java heap is where the objects created by your application live. The maximum heap is determined by the –Xmx flag of the Java command line that starts the application. If you write code that leaks memory, there is where it will blow up.

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At times you may want to find out which jar files are loaded by your Application. It is especially useful if you are dealing with classes present in multiple jar files and causing ClassCast exceptions. It is also useful if you are receiving ClassNotFound error and want to ensure the jar file that has the class file is loaded by your application.

There are few ways to do this.

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No surprise with number 1. Number 9 may be a surprise to many.

TIOBE tracks the popularity of programming languages. Note that the rating is not about unveiling the ‘most superior’ programming language.

The rating is based on number of skilled engineers and search engine stats.

Number 1 is no surprise. It is Java. Note that Android’s success has got lot to with Java’s re-emergence (Java is the official language for Android development). So, I’m guessing it is tough for C to catch up to Java from now on.

Number 9 is Perl. Coming from Unix Administrator background, I see why this is the case. Countless tools and ad-hoc scripts are written in PERL in the Unix world. While it not the prettiest out there, PERL is not going away any time soon.

And by the way, if you have any doubts whether or not to learn python, you can stop doubting and start learning.

Here is the complete list:

Source: http://www.tiobe.com/index.php/content/paperinfo/tpci/index.html

Answers

1. The Java command line option to enable Verbose Garbage Collection is:

a. –DenableVerboseGC=true

b. –verboseGC=yes

c. –verbose:gc

d. –enable.verbose.gc

e. –XX:+VerboseGC

Answer: c

Explanation: -verbose:gc is the correct answer. All other options are invalid

2. Short lived and Long lived Java objects are stored in these regions of Heap, respectively

a. Tenured,OldGen

b. OldGen,Tenured

c. Tenured,newGen

d. YoungGen,OldGen

e. Tenured,nursery

Answer: d

Explanation: Short lived objects live in Young Generation (also known as nursery or new generation). When minor garbage collection cannot reclaim memory from objects that are still being used (referenced), they get moved to Old gen (via another hop at survivor space). Old gen is also known as tenured gen.

3. Your application uses lots of File Handles. The memory used to maintain these File Handles are stored in which part of JVM memory

a. PermGen

b. YoungGen

c. Tenured

d. OldGen

e. Native Memory

Answer: e

Explanation: Native memory is used for all Operating system level components (such as File handles, sockets etc). Native memory is also used for any native code (such as C libraries) that runs as part of your application. PermGen is used for Class objects (and in some versions, interned strings). Young,Tenured (oldgen) are used to store your application java objects.

4. You have just deployed a new Java application with ONLY out of the box tuning parameters. Upon using the application, users complain your application is extremely slow. By reviewing the verbose GC log file, you have identified that the frequency of GC is extremely high (once every few seconds). What is your best next step ?

a. Tune –Xms and –Xmx to provide reasonable amount of memory

b. Schedule regular automatic restarts of your application

c. Restart your application

d. Increase PermGen Space

e. Add CPU to your Host Server

Answer: a

Explanation: The default Max heap is not enough in most cases (this various by implementation, but typically 128 or 256 MB). So, the best action is to first increase the max heap (-Xmx). You may want to set –Xms (initial heap) to the same value as –Xmx if possible. Otherwise, you can go with half or ¾ th of the Max heap.

5. Your application just ran out memory (OutOfMemory Error) and it has produced a big heap dump file. What is the best tool to analyze this heap dump to find out what is filling up the memory

a. Verbose GC logs

b. Thread Dump analyzer

c. Eclipse MAT (Memory Analyzer)

d. IBM Pattern Modeling and Analysis Tool for Java garbage collector

e. Jstack

Answer: c

Explanation: Eclipse MAT (Memory Analyzer) is the tool to be used for analyzing Heap dumps. Verbose GC logs just show the GC activity in detail, Thread dump analyzer is for analyzing Thread dumps, IBM PMAT is for visualizing verbose GC logs and finally jstack is a command line tool that comes with JDK that can be used to take thread dumps on a running Java application.

Previous: Troubleshooting GC: Test your knowledge

Test your knowledge on Troubleshooting Java Garbage Collection

1. The Java command line option to enable Verbose Garbage Collection is:

a. –DenableVerboseGC=true

b. –verboseGC=yes

c. –verbose:gc

d. –enable.verbose.gc

e. –XX:+VerboseGC

2. Short lived and Long lived Java objects are stored in these regions of Heap, respectively

a. Tenured,OldGen

b. OldGen,Tenured

c. Tenured,newGen

d. YoungGen,OldGen

e. Tenured,nursery

3. Your application uses lots of File Handles. The memory used to maintain these File Handles are stored in which part of JVM memory

a. PermGen

b. YoungGen

c. Tenured

d. OldGen

e. Native Memory

4. You have just deployed a new Java application with ONLY out of the box tuning parameters. Upon using the application, users complain your application is extremely slow. By reviewing the verbose GC log file, you have identified that the frequency of GC is extremely high (once every few seconds). What is your best next step ?

a. Tune –Xms and –Xmx to provide reasonable amount of memory

b. Schedule regular automatic restarts of your application

c. Restart your application

d. Increase PermGen Space

e. Add CPU to your Host Server

5. Your application just ran out memory (OutOfMemory Error) and it has produced a big heap dump file. What is the best tool to analyze this heap dump to find out what is filling up the memory

a. Verbose GC logs

b. Thread Dump analyzer

c. Eclipse MAT (Memory Analyzer)

d. IBM Pattern Modeling and Analysis for Java garbage collector

e. Jstack

Coming up: Troubleshooting GC: Test your knowledge – Answers

In your Application support/Developer Journey, you will definitely come across a time where you will want to really dive into the Java heap and see what is filling up the memory. We are literally talking about all those java objects that are in the heap. Mind you this can run into hundreds of millions in numbers. How do you get an insight into what is in the Heap? By analyzing a Heap dump. And how do you analyze the heap dump? By using the all-powerful Eclipse Memory Analyzer (MAT)

Before we begin using MAT, how do you capture heap dump?

Now, this varies from application to application. For example, if you are running IBM WebSphere, you could use a wsadmin script to invoke heap dump. Earlier you saw visualvm can create a heap dump for you through the visualvm interface. You can also use the command ‘jmap’ to create heap dump. Further you can configure the Application server to automatically perform a heap dump when an ‘out of memory’ error is encountered (XX:-HeapDumpOnOutOfMemoryError). Note that this option is verified to work on Oracle Hotspot JVM only.

Downloading Eclipse MAT:

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IBM has given us a great tool that we can use to analyze verbose GC files literally in seconds. I’m not kidding. All you have to do is generate verbose GC logs for few hours and feed the file to this tool and Bingo; it analyzes the file and tells you what you need to know. Let’s see this tool in action.

Downloading IBM Pattern Modeling and Analysis Tool for Java Garbage Collector:

https://www.ibm.com/developerworks/community/groups/service/html/communityview?communityUuid=22d56091-3a7b-4497-b36e-634b51838e11

Click on the ‘download’ link at the bottom of the page. You will get a jar file named “ga<version>.jar”.

For example, ga456.jar.

Invoking IBM PMAT:

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VisualVM is a free monitoring tool for the JVM. It is much more feature rich than Jconsole. It comes built in on the Oracle JDKs. You can also download a standalone version from visualvm.java.net.

Invoking visualvm:

The tool ‘jvisualvm’ is located in bin directory under ‘JDK Home’.

Jvisualvm

Note, similar to jconsole, if you are running visualvm with the same user id as the application, the application will appear under ‘Local’ and you can begin monitoring right away. However, if that is notthe case, you must first enable JMX remote management by adding the following parameters to the Java command line.

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