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.
Table of Contents
- 1 Java (JVM) Memory Model
- 2 Memory Management in Java – Young Generation
- 3 Memory Management in Java – Old Generation
- 3.1 Stop the World Event
- 3.2 Java Memory Model – Permanent Generation
- 3.3 Java Memory Model – Method Area
- 3.4 Java Memory Model – Memory Pool
- 3.5 Java Memory Model – Runtime Constant Pool
- 3.6 Java Memory Model – Java Stack Memory
- 3.7 Memory Management in Java – Java Heap Memory Switches
- 3.8 Memory Management in Java – Java Garbage Collection
- 3.9 Memory Management in Java – Java Garbage Collection Types
- 3.10 Memory Management in Java – Java Garbage Collection Monitoring
- 3.11 Java Garbage Collection Tuning
Java (JVM) Memory Model
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.
Memory Management in Java – Young 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:
- Most of the newly created objects are located in the Eden memory space.
- When Eden space is filled with objects, Minor GC is performed and all the survivor objects are moved to one of the survivor spaces.
- Minor GC also checks the survivor objects and move them to the other survivor space. So at a time, one of the survivor space is always empty.
- Objects that are survived after many cycles of GC, are moved to the Old generation memory space. Usually, it’s done by setting a threshold for the age of the young generation objects before they become eligible to promote to Old generation.
Memory Management in Java – Old Generation
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.
Stop the World Event
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.
Java Memory Model – Permanent Generation
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.
Java Memory Model – Method Area
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.
Java Memory Model – Memory Pool
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.
Java Memory Model – Runtime Constant Pool
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 Memory Model – Java Stack Memory
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.
Memory Management in Java – Java Heap Memory Switches
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.
Memory Management in Java – Java Garbage Collection
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:
- Marking: This is the first step where garbage collector identifies which objects are in use and which ones are not in use.
- Normal Deletion: Garbage Collector removes the unused objects and reclaim the free space to be allocated to other objects.
- Deletion with Compacting: For better performance, after deleting unused objects, all the survived objects can be moved to be together. This will increase the performance of allocation of memory to newer objects.
There are two problems with a simple mark and delete approach.
- First one is that it’s not efficient because most of the newly created objects will become unused
- Secondly objects that are in-use for multiple garbage collection cycle are most likely to be in-use for future cycles too.
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.
Memory Management in Java – Java Garbage Collection Types
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.
- Serial GC (-XX:+UseSerialGC): Serial GC uses the simple mark-sweep-compact approach for young and old generations garbage collection i.e Minor and Major GC.Serial GC is useful in client machines such as our simple stand-alone applications and machines with smaller CPU. It is good for small applications with low memory footprint.
- Parallel GC (-XX:+UseParallelGC): Parallel GC is same as Serial GC except that is spawns N threads for young generation garbage collection where N is the number of CPU cores in the system. We can control the number of threads using
-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. - Parallel Old GC (-XX:+UseParallelOldGC): This is same as Parallel GC except that it uses multiple threads for both Young Generation and Old Generation garbage collection.
- Concurrent Mark Sweep (CMS) Collector (-XX:+UseConcMarkSweepGC): CMS Collector is also referred as concurrent low pause collector. It does the garbage collection for the Old generation. CMS collector tries to minimize the pauses due to garbage collection by doing most of the garbage collection work concurrently with the application threads.CMS collector on the young generation uses the same algorithm as that of the parallel collector. This garbage collector is suitable for responsive applications where we can’t afford longer pause times. We can limit the number of threads in CMS collector using
-XX:ParallelCMSThreads=nJVM option. - G1 Garbage Collector (-XX:+UseG1GC): The Garbage First or G1 garbage collector is available from Java 7 and its long term goal is to replace the CMS collector. The G1 collector is a parallel, concurrent, and incrementally compacting low-pause garbage collector.Garbage First Collector doesn’t work like other collectors and there is no concept of Young and Old generation space. It divides the heap space into multiple equal-sized heap regions. When a garbage collection is invoked, it first collects the region with lesser live data, hence “Garbage First”. You can find more details about it at Garbage-First Collector Oracle Documentation.
Memory Management in Java – Java Garbage Collection Monitoring
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
jstat
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.
- S0C and S1C: This column shows the current size of the Survivor0 and Survivor1 areas in KB.
- S0U and S1U: This column shows the current usage of the Survivor0 and Survivor1 areas in KB. Notice that one of the survivor areas are empty all the time.
- EC and EU: These columns show the current size and usage of Eden space in KB. Note that EU size is increasing and as soon as it crosses the EC, Minor GC is called and EU size is decreased.
- OC and OU: These columns show the current size and current usage of Old generation in KB.
- PC and PU: These columns show the current size and current usage of Perm Gen in KB.
- YGC and YGCT: YGC column displays the number of GC event occurred in young generation. YGCT column displays the accumulated time for GC operations for Young generation. Notice that both of them are increased in the same row where EU value is dropped because of minor GC.
- FGC and FGCT: FGC column displays the number of Full GC event occurred. FGCT column displays the accumulated time for Full GC operations. Notice that Full GC time is too high when compared to young generation GC timings.
- GCT: This column displays the total accumulated time for GC operations. Notice that it’s sum of YGCT and FGCT column values.
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.
Java VisualVM with Visual GC
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
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.
Method Area is part of heap. Not part of Perm Generation. For Confirmation see Oracle Documentation
before jdk8,method area is part of perm generation,but since java8 the method area has becomed a part of heap.
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!
thank you some much
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.
Yes in java 8 it is removed
Helped a lot. Keep it up!!
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
I thought Permanent Generation memory space is removed in Java 8? So -XX:PermGen and -XX:MaxPermSize are no longer valid options in Java 8? Perhaps clarifying this to avoid confusion for people who uses Java 8?
Yeap, you’re are right. PermGen is removed from Java 8. JV8 use an alternative called Meta Space which is not a part of Heap. Meta Space use directly memory of your computer.
So –XX:PermGen and XX:MaxPermSize are no longer valid for Java 8.
Thank you for that article. It helps to summarize all the knowledge. I have one question though. You write that G1 is not generational, and in the (https://docs.oracle.com/javase/8/docs/technotes/guides/vm/gctuning/g1_gc.html) docs they say: “G1 is generational in a logical sense. A set of empty regions is designated as the logical young generation”. So i winder if it is or isn’t ..:) Could you please say what you ment when writing it is not generational?:)
Thanks again
Great article. Thanks for posting.
One of the headers says “jsat” instead of “jstat”.
Thanks for noticing the typo, I have fixed it.
best explanation.. thank you so much for this article 🙂
If I create a custom immutable object..where in Java Memory it will be placed? in Memory Pool or in Heap? and why?
Hi Pankaj,
Can you please explain about these two problems in more details?
“There are two problems with simple mark and delete approach.
First one is that it’s not efficient because most of the newly created objects will become unused
Secondly objects that are in-use for multiple garbage collection cycle are most likely to be in-use for future cycles too.”
Thank you.
Perhaps what it means is:
First one is that it’s not efficient because most of the newly created objects will become unused – Most objects that are marked in current cycle as active will be “unmarked” in following. So marking it for now and then unmarking in next cycle is kind of redundant work.
Secondly objects that are in-use for multiple garbage collection cycle are most likely to be in-use for future cycles too. – If an object has survived multiple minor GCs, chances are it will survive longer, so it is redundant to check it for “use” and mark it again.
Above two explanations are pure guess work so use with caution.
Very Good explanation. However, I noted one point where we executed the following program:
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 used the UseSerialGC as Garbage Collector Algorithm. Whereas when we executed the following 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
As noted above the Garbage Collector algorithm is -XX:+UseG1GC. Does the JVM suppressed the provided -XX:+UseSerialGC jvm option or some thing other force to change the algorithm?
Crisp and excellent coverage.
Thank you so much for sharing such a valuable information. Keep posting great articles and tutorials.!!
SO GREAT~~~~
great article! thank you, dude
Very helpful. Thank you very much
Very informative article, thanks a lot Pankaj. Could you please provide the link for JDK8 article where PermGen is removed and metaspace is added.
Worth of reading indeed. could you please tell me since Scala is also using JVM are they have same Memory management technique?
Very informative.Thanks.
Very well explained article.Thanks.
Good read, but needs few editing.
First line:
Understanding JVM Memory Model, Java Memory Management are very important if you want to understand the working of Java Garbage Collection
Instead it should be:
Understanding working of Java Garbage Collection is very important if you want to understand the JVM Memory Model, Java Memory Management
The memory model depends on the garbage collector not the other way.
This is one of the fantastic and crisp article.
Hi Pankaj,
It was very basic and up to the point described
Thanks a lot.
It has improved my knowledge base.
Hi Pankaj
This article is really helpful.
Thanks
Avirup Das
This is definitely the most awesome and valuable article describing java memory management ever!!!!
Thanks
The article is too good, and all the required information related to memory maangement is provided.
Nice Article
Great article, I’ll definitely come back later for more 🙂
Thanks
Nice Article. Worth a read. Can you please share the new memory model used in Java 8 as Perm Gem has been removed.
Very much worth-full contents….
Thank for posting nice articles.
thanks for such a nice tutorial
is this the model existent even on java 8 ?
did you missed native memory
Amazing article 🙂
For the article of 2016 it would be nice to see any comments regarding removed PermGen/added Metaspace in Java8, for example
This is really Helpful 🙂
Amazing tutorials Pankaj..Keep up the good work..Have been a fan of all your articles.Crisply written as well
Excellent article, most useful, thank-you very much 🙂
I agree that this should stay as is and that a new revised article for Java 8 should be produced.
You wrote :- Objects that are survived after many cycles of GC, are moved to the Old generation memory space.
Please clarify, Minor GC is responsible for this ? or Major GC?
When we talk about stack and heap memory, In that concern, JVM memory is dividend into 3 parts, Young, old and Perm(mentioned in this page). And Heap Memory = Young Gen + Old Generation, Could you please confirm where stack memory remains in memory? I understand its in Perm Generation.
good article
Very nice , Great work and expatiation is extraordinary
O~wesome thank you so~ much!
really appreciating work…great work dude..and thanks a lot..
Great article, thanks!
Thanks a lot. !
Nice article , Thank you
Very Nice Comprehension
AWESOME EXPLANATION
very nice article, I have been trying to understand this for months and this article just made it for me.
Thanks a lot
Very Nice Comprehension.. Thanks for such a complete article.
Since the Perm Gen area has completely been remove, can we now change this tutorial.
I am saying this because of my high regards for you.
You have done an excellent job by doing all what you have done.
Also please come up with a tutorial with all new features that have been added in JDK 1.8
Thanks
I think author should keep the as is. This article can be be useful for versions below Java 8. Instead of rewriting this article, author should create new article with Java 8 memory model comparing with this one.
Thanks for the inputs, I will write a new one with Java 8 changes.
hi Pankaj, just read this article and got to be the best explanation of memory management I’ve seen yet!
have you got around to writing the Java 8 one yet?
Really nice article. An easy insight into the JVM memory model to begin with.
Thanks Imran, appreciate your comment.
have question around this “Perm Gen objects are garbage collected in a full garbage collection.”
Full GC = Major GC? or Full GC means complete restart of JVM?
Bcz I read somewhere Perm Gen never garbage collected
Hi Sunil,
Full GC is not Major GC. Full GC cleans Young Gen, Old Gen and Perm Gen.
Thanks for pitching in and answering it for me. 🙂
thanks! nice write up with full of information 🙂
you are welcome Sunil.
Hi,
Thanks for the article.
Can anyone help me with this:
– what’s the point of having survivor spaces ?
– if the eden space is bigger than survivor spaces and assuming that “at a time, one of the survivor space is always empty”, how could objects be moved to the survivor space when eden is full ?
Hi JVMLover,
1. what’s the point of having survivor spaces ?
Answer: There are two problems with simple mark and delete approach.
First one is that it’s not efficient because most of the newly created objects will become unused.
So survivor spaces is aim to solve the problem. So JVM use Minor GC, which will do garbage collection when eden is full. In this way, it will avoid to collect most of the newly created objects which are unused.
After doing Minor GC, there are some objects that still in used, so these objects are moved to survivors.
Two survivors is aim to compact survivor objects on each Minor GC, to free more memory.
“if the eden space is bigger than survivor spaces and assuming that “at a time, one of the survivor space is always empty”, how could objects be moved to the survivor space when eden is full ?”
Answer: Eden is probably bigger than survivor spaces. Survivor objects are moved to survivor space by JVM. We don’t need to know how to move them.
You have added some great information, thanks friend.
thanx you so much
Thank you. Very informative article especially for people who is not too technical.
Informative and clear write up. Thank you so much for putting this together.
Thank you for this scholarly yet clear information.
Thanks Pankaj! Really precise information on all the factors about GC at one place.
Hello Pankaj,
For a 64 bit Java 7 application experiencing high number of minor GCs, taking over two seconds, and one time over 15 seconds, what would you suggest is the issue ?
Further, noticed a pattern of a minor GC taking between 4-16 seconds, then shortly followed by a full GC taking almost about 3 seconds.
Any suggestions ?
Thank you
I have few questions on GC,
1, what exactly per gen does?
2, which scenario , the memory leakage occurs?
Best article i found on the web which such clear explanation
Please keep up the good work
HI Pankaj,
Can you please let me clear ?
Should we use —
-XX:PermSize
&
-XX:MaxPermSize
OR
-XX:PermGen
-XX:MaxPermGen
in case of perm generation
Thanks
Narendra
It’s
-XX:PermSize
&
-XX:MaxPermSize
not
-XX:PermGen
-XX:MaxPermGen
Very Nice Article…
Hi Pankaj,
I like your website so much and learned a lot from here. Thanks.
How can I know the garbage collection type of my JVM, or how to choose the garbage collection types on my JVM? Thanks.
Nice article.
I have one question
When we call Runtime.getRuntime().gc() then which garbage collector gets called, is it Miner GC or Major GC
hello article is good
The Best article I found on net regarding java memory pool….thank you for sharing it
Thanks Pankaj…Its really a good piece of documentation…
This is best ever tutorial I have read on internet, Very clearly covering everything from memory allocation to Garbage Collection. Kudos.
That is NOT about JMM, it’s about memory management and garbage collection. Java Memory Model is about how to make memory consistent for threads and what threads interactions are allowed and what optimizations and code reorderings are fine to make memory system seem consistent to us, programmers.
See William Pugh’s pages on this, or Wikipedia (Memory Model or Consistency Model) or works by Sarita Adve or JSR-133 FAQ.
Yes it’s memory model from garbage collection perspective.
Hi Pankaj,
Thanks for the wonderful document.
Just one thing I was trying to understand is, how does the Survivor space(S0 and S1) gets filled up? Is there any mechanism for space getting occupied between Eden, S0 and S1 ?
Thanks,
Amishi
As explained above, Minor GC moves objects from Eden to Survivor spaces and also collects unreferenced objects from survivor spaces and move them to another one. I would suggest to run Java VisualVM and see it in action for better understanding.
Thanks for this useful article. Easy to understand.
Good write up .
Whether this above explanation about Memory model is applicable to all JVM vendors ?
JVM Specification defines Heap memory but it doesn’t say anything about the Young Gen/Old Gen etc. Above memory model is applicable for all Hotspot VM implementations but I am not sure about other implementations.
Waited for this topic, thanks.