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Understanding JVM Memory Model, Java Memory Management are very important if you want to understand the working of Java Garbage Collection. Today we will look into memory management in Java, different parts of JVM memory and how to monitor and perform garbage collection tuning.
As you can see in the above image, JVM memory is divided into separate parts. At broad level, JVM Heap memory is physically divided into two parts - Young Generation and Old Generation.
The young generation is the place where all the new objects are created. When the young generation is filled, garbage collection is performed. This garbage collection is called Minor GC. Young Generation is divided into three parts - Eden Memory and two Survivor Memory spaces. Important Points about Young Generation Spaces:
Old Generation memory contains the objects that are long-lived and survived after many rounds of Minor GC. Usually, garbage collection is performed in Old Generation memory when it’s full. Old Generation Garbage Collection is called Major GC and usually takes a longer time.
All the Garbage Collections are “Stop the World” events because all application threads are stopped until the operation completes. Since Young generation keeps short-lived objects, Minor GC is very fast and the application doesn’t get affected by this. However, Major GC takes a long time because it checks all the live objects. Major GC should be minimized because it will make your application unresponsive for the garbage collection duration. So if you have a responsive application and there are a lot of Major Garbage Collection happening, you will notice timeout errors. The duration taken by garbage collector depends on the strategy used for garbage collection. That’s why it’s necessary to monitor and tune the garbage collector to avoid timeouts in the highly responsive applications.
Permanent Generation or “Perm Gen” contains the application metadata required by the JVM to describe the classes and methods used in the application. Note that Perm Gen is not part of Java Heap memory. Perm Gen is populated by JVM at runtime based on the classes used by the application. Perm Gen also contains Java SE library classes and methods. Perm Gen objects are garbage collected in a full garbage collection.
Method Area is part of space in the Perm Gen and used to store class structure (runtime constants and static variables) and code for methods and constructors.
Memory Pools are created by JVM memory managers to create a pool of immutable objects if the implementation supports it. String Pool is a good example of this kind of memory pool. Memory Pool can belong to Heap or Perm Gen, depending on the JVM memory manager implementation.
Runtime constant pool is per-class runtime representation of constant pool in a class. It contains class runtime constants and static methods. Runtime constant pool is part of the method area.
Java Stack memory is used for execution of a thread. They contain method specific values that are short-lived and references to other objects in the heap that is getting referred from the method. You should read Difference between Stack and Heap Memory.
Java provides a lot of memory switches that we can use to set the memory sizes and their ratios. Some of the commonly used memory switches are:
|VM Switch||VM Switch Description|
|-Xms||For setting the initial heap size when JVM starts|
|-Xmx||For setting the maximum heap size.|
|-Xmn||For setting the size of the Young Generation, rest of the space goes for Old Generation.|
|-XX:PermGen||For setting the initial size of the Permanent Generation memory|
|-XX:MaxPermGen||For setting the maximum size of Perm Gen|
|-XX:SurvivorRatio||For providing ratio of Eden space and Survivor Space, for example if Young Generation size is 10m and VM switch is -XX:SurvivorRatio=2 then 5m will be reserved for Eden Space and 2.5m each for both the Survivor spaces. The default value is 8.|
|-XX:NewRatio||For providing ratio of old/new generation sizes. The default value is 2.|
Most of the times, above options are sufficient, but if you want to check out other options too then please check JVM Options Official Page.
Java Garbage Collection is the process to identify and remove the unused objects from the memory and free space to be allocated to objects created in future processing. One of the best features of Java programming language is the automatic garbage collection, unlike other programming languages such as C where memory allocation and deallocation is a manual process. Garbage Collector is the program running in the background that looks into all the objects in the memory and find out objects that are not referenced by any part of the program. All these unreferenced objects are deleted and space is reclaimed for allocation to other objects. One of the basic ways of garbage collection involves three steps:
There are two problems with a simple mark and delete approach.
The above shortcomings with the simple approach is the reason that Java Garbage Collection is Generational and we have Young Generation and Old Generation spaces in the heap memory. I have already explained above how objects are scanned and moved from one generational space to another based on the Minor GC and Major GC.
There are five types of garbage collection types that we can use in our applications. We just need to use the JVM switch to enable the garbage collection strategy for the application. Let’s look at each of them one by one.
-XX:ParallelGCThreads=nJVM option.Parallel Garbage Collector is also called throughput collector because it uses multiple CPUs to speed up the GC performance. Parallel GC uses a single thread for Old Generation garbage collection.
We can use the Java command line as well as UI tools for monitoring garbage collection activities of an application. For my example, I am using one of the demo application provided by Java SE downloads. If you want to use the same application, go to Java SE Downloads page and download JDK 7 and JavaFX Demos and Samples. The sample application I am using is Java2Demo.jar and it’s present in
jdk1.7.0_55/demo/jfc/Java2D directory. However this is an optional step and you can run the GC monitoring commands for any java application. Command used by me to start the demo application is:
pankaj@Pankaj:~/Downloads/jdk1.7.0_55/demo/jfc/Java2D$ java -Xmx120m -Xms30m -Xmn10m -XX:PermSize=20m -XX:MaxPermSize=20m -XX:+UseSerialGC -jar Java2Demo.jar
We can use
jstat command line tool to monitor the JVM memory and garbage collection activities. It ships with standard JDK, so you don’t need to do anything else to get it. For executing
jstat you need to know the process id of the application, you can get it easily using
ps -eaf | grep java command.
pankaj@Pankaj:~$ ps -eaf | grep Java2Demo.jar 501 9582 11579 0 9:48PM ttys000 0:21.66 /usr/bin/java -Xmx120m -Xms30m -Xmn10m -XX:PermSize=20m -XX:MaxPermSize=20m -XX:+UseG1GC -jar Java2Demo.jar 501 14073 14045 0 9:48PM ttys002 0:00.00 grep Java2Demo.jar
So the process id for my java application is 9582. Now we can run jstat command as shown below.
pankaj@Pankaj:~$ jstat -gc 9582 1000 S0C S1C S0U S1U EC EU OC OU PC PU YGC YGCT FGC FGCT GCT 1024.0 1024.0 0.0 0.0 8192.0 7933.3 42108.0 23401.3 20480.0 19990.9 157 0.274 40 1.381 1.654 1024.0 1024.0 0.0 0.0 8192.0 8026.5 42108.0 23401.3 20480.0 19990.9 157 0.274 40 1.381 1.654 1024.0 1024.0 0.0 0.0 8192.0 8030.0 42108.0 23401.3 20480.0 19990.9 157 0.274 40 1.381 1.654 1024.0 1024.0 0.0 0.0 8192.0 8122.2 42108.0 23401.3 20480.0 19990.9 157 0.274 40 1.381 1.654 1024.0 1024.0 0.0 0.0 8192.0 8171.2 42108.0 23401.3 20480.0 19990.9 157 0.274 40 1.381 1.654 1024.0 1024.0 48.7 0.0 8192.0 106.7 42108.0 23401.3 20480.0 19990.9 158 0.275 40 1.381 1.656 1024.0 1024.0 48.7 0.0 8192.0 145.8 42108.0 23401.3 20480.0 19990.9 158 0.275 40 1.381 1.656
The last argument for jstat is the time interval between each output, so it will print memory and garbage collection data every 1 second. Let’s go through each of the columns one by one.
The advantage of jstat is that it can be executed in remote servers too where we don’t have GUI. Notice that the sum of S0C, S1C and EC is 10m as specified through
-Xmn10m JVM option.
If you want to see memory and GC operations in GUI, then you can use
jvisualvm tool. Java VisualVM is also part of JDK, so you don’t need to download it separately. Just run
jvisualvm command in the terminal to launch the Java VisualVM application. Once launched, you need to install Visual GC plugin from Tools -< Plugins option, as shown in below image. After installing Visual GC, just open the application from the left side column and head over to Visual GC section. You will get an image of JVM memory and garbage collection details as shown in below image.
Java Garbage Collection Tuning should be the last option you should use for increasing the throughput of your application and only when you see a drop in performance because of longer GC timings causing application timeout. If you see
java.lang.OutOfMemoryError: PermGen space errors in logs, then try to monitor and increase the Perm Gen memory space using -XX:PermGen and -XX:MaxPermGen JVM options. You might also try using
-XX:+CMSClassUnloadingEnabled and check how it’s performing with CMS Garbage collector. If you see a lot of Full GC operations, then you should try increasing Old generation memory space. Overall garbage collection tuning takes a lot of effort and time and there is no hard and fast rule for that. You would need to try different options and compare them to find out the best one suitable for your application. That’s all for Java Memory Model, Memory Management in Java and Garbage Collection, I hope it helps you in understanding JVM memory and garbage collection process.
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Method Area is part of heap. Not part of Perm Generation. For Confirmation see Oracle Documentation
Awesome article, as always! thanks
Good read Pankaj! Demos with jstat and jVisualVM were really what will set this article apart from other content on the internet. Kudos!
- Anshul Jindal
thank you some much
- mahesh marpudi
Very Helpful article :)
Nice article. Learned more about JVM.
Very nice article. Can you please update this article as PermGen memory space has been removed in Java 8.
- Bhavesh Vani
Helped a lot. Keep it up!!
- shubham kumar
Your article says that the permgen is NOT a part of the heap memory. But https://www.oracle.com/webfolder/technetwork/tutorials/obe/java/gc01/index.html says otherwise.
Pankaj Great explanation and very nearly explained. Thanks