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JVM – Basic Operations

A Java application is launched with the java command which starts a JVM and runs the application. As an Application support engineer, there are few basic operations that you will typically perform on the JVM. Here are they:

Starting JVM:

Starting the JVM simply means starting the Java application. A Java application is started using the java command.

For example:

java HelloWorld 

java is the command

HelloWorld is the classfile without the .class extension

Note: Don’t use the .class extension in the java command. If you do, you will get an error

[ec2-user@java]$ java HelloWorld.class
Error: Could not find or load main class HelloWorld.class

There are numerous command line options for the java command. Here are some of the main ones you need to know.

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JVM: Basic Framework

As an Application Support Engineer, you will invariably end up managing Java based applications. JVM (Java Virtual Machine) is the all-important process that runs the Java Application you are going to manage (which can be either custom developed by your development group, or a commercial off the shelf product). It is imperative that you have a very good understanding of how JVM works.

Note: Some of the material in this section will be very basic that experienced Administrators can choose to skip.

At a high level:

  1. Java is a high level programming language (this is the code). A java program is a text file(s) that is/are human readable.
  2. Javac is a compiler program that compiles the human readable java code in to machine readable byte code.
  3. JVM (Java virtual machine) is a process that runs the java byte code. It is an implementation of specification created by Sun/Oracle.

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Java Heap is a labyrinth. You can spend hours or even days analyzing a heap dump from a production Java application. Heap is where all the objects created by your application live. And even in a relatively simple application, there could be hundreds of thousands of objects created. When dealing with Java memory leak, you know you are looking for one the following (or both):

  1. Excessive number of objects of certain class
  2. Objects with unusually large size

Fortunately, Appdynamics provides a feature using which you can easily reveal both of the above cases. Let’s get right to it.

  • In AppDynamics Controller UI, locate the Node that you want to troubleshoot and click on ‘Memory’ tab

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In this part, let’s explore AppDynamics facilities for diagnosing Memory Leaks. AppDynamics provides two powerful tools to hunt down memory leaks.

  1. Automatic Leak Detection
  2. Objet Instance Tracking

Automatic Leak Detection

With Automatic Leak Detection enabled, Appdyanmics can capture objects that live longer than usual period – i.e long lived Collection objects that simply won’t get garbage collected. If your application is leaking memory, it is definitely one of these long-living, never-dying objects that cause the leak. Use the following procedure to make use of this tool.

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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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Ten things you need to know about Java memory leak

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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How to instrument a Java application with AppDynamics?

Agent is a vital part of Appdynamics framework. It is the agent that acts like a workhorse to pull metrics from the Application and push it to the Controller. Agent is a piece of software that is installed ON your application. The only function of the agent is to pull the monitoring metrics and send them to the AppDynamics Controller where the data is crunched and made available via the Controller UI. Note that there is NO need of a code change in your application. There is however a setup/configuration change required depending on the platform. In this article, I will explain how to instrument a Java Application.

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Introduction to APM: Benefits of APM

So, what can an APM tool buy you? Setting aside the hypothetical ‘Peace of mind’ marketing pitch, let me show you how exactly an APM tool can help you support your Application effectively

1. Historic Monitoring of Key Metrics

APM tool can record the monitoring metrics which are invaluable in troubleshooting. For example, take a look at the ‘response time’ graph of a particular application. You can readily see that the application suffers during business hours.


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Introduction to APM (Application Performance Management)

Back in the 90s when I was working as a Solaris/HP-UX Administrator, all I needed was two or three commands to figure out what was wrong with a particular Server or Application. I will just glance at ‘vmstat’, ‘iostat’ and ‘top’ for a minute or two and the problem will reveal itself clearly. While those command still prove valuable at a certain level, in order to answer ‘Why is the application slow’ you need much more than just few OS commands.

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Troubleshooting GC: Test your knowledge – 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.

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