JVM Memory Model and GC

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2 JVM Memory Model and GC Developer Community Support Fairoz Matte Principle Member Of Technical Staff Java Platform Sustaining Engineering, Copyright 2015, Oracle and/or its affiliates. All rights reserved. Oracle Confidential Internal/Restricted/Highly Restricted

3 Agenda What is JVM? JVM Architecture Hotspot Memory Structure Garbage Collection Process Demo Application Garbage collection Types Copyright 2015, Oracle and/or its affiliates. All rights reserved. Oracle Confidential Internal/Restricted/Highly Restricted 3

4 What is JVM? The Java Virtual Machine (JVM) is an abstract computing machine. This way, Java programs are written to the same set of interfaces and libraries. Each JVM implementation for a specific operating system, translates the Java programming instructions into instructions and commands that run on the local operating system. Java programs achieve platform independence. The Java virtual machine knows nothing of the Java programming language, only of a particular binary format, the class file format. Copyright 2015, Oracle and/or its affiliates. All rights reserved. Oracle Confidential Internal/Restricted/Highly Restricted 4

JVM Architecture Copyright 2015, Oracle and/or its affiliates.

6 Hotspot Memory Structure Pre Java 8 Java 8 NEW GEN (Eden + Survivor) NEW GEN (Eden + Survivor) OLD GEN HEAP -XMx OLD GEN HEAP -XMx PERM GEN (-XX:MAXPERMSIZE) METASPACE (-XX:METASPACESIZE) Native Memory Native Memory JVM Process JVM Process Copyright 2015, Oracle and/or its affiliates. All rights reserved. Oracle Confidential Internal/Restricted/Highly Restricted 6

7 Hotspot Memory Structure - Terminologies The Young Generation is where all new objects are allocated and aged. When the young generation fills up, this causes a minor garbage collection. Minor collections can be optimized assuming a high object mortality rate. A young generation full of dead objects is collected very quickly. Some surviving objects are aged and eventually move to the old generation. Stop the World Event - All minor garbage collections are "Stop the World" events. This means that all application threads are stopped until the operation completes. Minor garbage collections are always Stop the World events. Copyright 2015, Oracle and/or its affiliates. All rights reserved. Oracle Confidential Internal/Restricted/Highly Restricted 7

8 Hotspot Memory Structure - Terminologies The Old Generation is used to store long surviving objects. Typically, a threshold is set for young generation object and when that age is met, the object gets moved to the old generation. Eventually the old generation needs to be collected. This event is called a major garbage collection. Major garbage collection are also Stop the World events. Often a major collection is much slower because it involves all live objects Metaspace : A native memory to store class meta-data information and that grows automatically & garbage collected. Native Memory: Native memory is the memory which is available to a process, e.g. the Java process. Native memory is controlled by the operating system and based on physical memory Copyright 2015, Oracle and/or its affiliates. All rights reserved. Oracle Confidential Internal/Restricted/Highly Restricted 8

9 Garbage Collection Process Automatic garbage collection is the process of looking at heap memory, identifying which objects are in use and which are not, and deleting the unused objects. Step 1: Marking Before Marking After Marking Live Object Unreferenced Object Copyright 2015, Oracle and/or its affiliates. All rights reserved. Oracle Confidential Internal/Restricted/Highly Restricted 9

10 Garbage Collection Process Step 2: Normal Deletion After Normal deletion Memory allocated holds a list of references to free spaces, and searches for free space whenever an allocation is required Copyright 2015, Oracle and/or its affiliates. All rights reserved. Oracle Confidential Internal/Restricted/Highly Restricted 10

11 Garbage Collection Process Step 2a: Deletion with Compaction After Normal deletion with Compaction Memory allocated holds the reference to the beginning of free space, and allocated memory sequentially Copyright 2015, Oracle and/or its affiliates. All rights reserved. Oracle Confidential Internal/Restricted/Highly Restricted 11

Generational Garbage Collection Process Heap is separated into different generations More and more objects are allocated, the list of objects grows and grows leading to longer and longer garbage

12 Generational Garbage Collection Process Heap is separated into different generations More and more objects are allocated, the list of objects grows and grows leading to longer and longer garbage collection time However, empirical analysis of applications has shown that most objects are short lived Copyright 2015, Oracle and/or its affiliates. All rights reserved. Oracle Confidential Internal/Restricted/Highly Restricted 12

13 Generational Garbage Collection Process Object Allocation First, any new objects are allocated to the eden space. Both survivor spaces start out empty. Created in Eden Just allocated Eden S0 survivor space 1 2 S1 survivor space Copyright 2015, Oracle and/or its affiliates. All rights reserved. Oracle Confidential Internal/Restricted/Highly Restricted 13

14 Generational Garbage Collection Process Filling the eden space When the eden space fills up, a minor garbage collection is triggered. Created in Eden Just allocated Eden S0 survivor space 1 2 S1 survivor space Copyright 2015, Oracle and/or its affiliates. All rights reserved. Oracle Confidential Internal/Restricted/Highly Restricted 14

15 Generational Garbage Collection Process Copying referenced objects Referenced objects are moved to the first survivor space. Unreferenced objects are deleted when the eden space is cleared.. Eden Unreferenced Object S0 survivor space S1 survivor space Referenced Object Copyright 2015, Oracle and/or its affiliates. All rights reserved. Oracle Confidential Internal/Restricted/Highly Restricted 15

16 Generational Garbage Collection Process Object Aging Unreferenced objects are deleted and referenced objects are moved to a survivor space. However, in this case, they are moved to the second survivor space (S1). In addition, objects from the last minor GC on the first survivor space (S0) have their age incremented and get moved to S1... Eden Unreferenced Object From survivor space To survivor space Referenced Object Copyright 2015, Oracle and/or its affiliates. All rights reserved. Oracle Confidential Internal/Restricted/Highly Restricted 16

17 Generational Garbage Collection Process Additional Aging At the next minor GC, the same process repeats. However this time the survivor spaces switch. Referenced objects are moved to S0. Surviving objects are aged. Eden and S1 are cleared. Eden Unreferenced Object To survivor space From survivor space Referenced Object Copyright 2015, Oracle and/or its affiliates. All rights reserved. Oracle Confidential Internal/Restricted/Highly Restricted 17

18 Generational Garbage Collection Process Promotion This slide demonstrates promotion. After a minor GC, when aged objects reach a certain age threshold (8 in this example) they are promoted from young generation to old generation. Eden Unreferenced Object To survivor space 3 1 From survivor space Referenced Object Old generation 9 9 Copyright 2015, Oracle and/or its affiliates. All rights reserved. Oracle Confidential Internal/Restricted/Highly Restricted 18

19 Generational Garbage Collection Process Promotion As minor GCs continue to occure objects will continue to be promoted to the old generation space. Allocation Yong Generation Promotion Old Generation Copyright 2015, Oracle and/or its affiliates. All rights reserved. Oracle Confidential Internal/Restricted/Highly Restricted 19

20 Generational Garbage Collection Process GC Process Summary Created in Eden Just allocated Eden S0 survivor space 1 2 S1 survivor space Old generation 9 9 Copyright 2015, Oracle and/or its affiliates. All rights reserved. Oracle Confidential Internal/Restricted/Highly Restricted 20

21 Generational Garbage Collection Process Demo You have seen the garbage collection process using a series of pictures. Now it is time to experience and explore the process live. In this activity, you will run a Java application and analyze the garbage collection process using Visual VM. The Visual VM program is included with the JDK and allows developers to monitor various aspects of a running JVM. Copyright 2015, Oracle and/or its affiliates. All rights reserved. Oracle Confidential Internal/Restricted/Highly Restricted 21

22 Garbage Collection Demo Application Start a Demo Application With the Java JDK and demos installed, you can now run demo application that will be analyzed in this activity. For this example, the demos are installed in D:\2015\jdk-8u101-windows-x64-demos The demo application is a 2D graphics demo. To execute it type: java - Xmx12m -Xms3m -Xmn1m XX:+UseSerialGC jar D:\2015\jdk-8u101- windows-x64-demos\jdk1.8.0_101\demo\jfc\java2d\java2demo.jar Copyright 2015, Oracle and/or its affiliates. All rights reserved. Oracle Confidential Internal/Restricted/Highly Restricted 22

Garbage Collection Demo Application Java 2D Demo Copyright 2015, Oracle and/or its

Garbage Collection Demo Application Start Transforms Copyright 2015, Oracle and/or its

Garbage Collection Demo Application Start VisualVM Copyright 2015, Oracle and/or its

26 Garbage Collection Demo Application Start Java2D demo application to visualize GC activities Copyright 2015, Oracle and/or its affiliates. All rights reserved. Oracle Confidential Internal/Restricted/Highly Restricted 26

Garbage Collection Demo Application Visual GC Copyright 2015, Oracle and/or its

28 Generational Garbage Collection Types Java Garbage Collectors You now know the basics of garbage collection and have observed the garbage collector in action on a sample application. In this section, you will learn about the garbage collectors available for Java and the command line switches you need to select them. Switch -Xms -Xmx Description Sets the initial heap size for when the JVM starts. Sets the maximum heap size. Copyright 2015, Oracle and/or its affiliates. All rights reserved. Oracle Confidential Internal/Restricted/Highly Restricted 28

29 Generational Garbage Collection Types Java Garbage Collectors Serial GC The serial collector is the default for client style machines in Java SE 5 and 6. With the serial collector, both minor and major garbage collections are done serially Usage cases The Serial GC is the garbage collector of choice for most applications that do not have low pause time requirements and run on client-style machines. Copyright 2015, Oracle and/or its affiliates. All rights reserved. Oracle Confidential Internal/Restricted/Highly Restricted 29

30 Generational Garbage Collection Types Java Garbage Collectors Serial GC Command Line Options: To enable the Serial Collector use: -XX:+UseSerialGC java -Xmx12m -Xms3m -Xmn1m -XX:+UseSerialGC -jar c:\javademos\demo\jfc\java2d\java2demo.jar. Copyright 2015, Oracle and/or its affiliates. All rights reserved. Oracle Confidential Internal/Restricted/Highly Restricted 30

31 Generational Garbage Collection Types Java Garbage Collectors Parallel GC (throughput collector) The parallel garbage collector uses multiple threads to perform the young genertion garbage collection. By default on a host with N CPUs, the parallel garbage collector uses N garbage collector threads in the collection. Usage cases It can use multiple CPUs to speed up application throughput. This collector should be used when a lot of work need to be done and long pauses are acceptable. For example, batch processing like printing reports or bills or performing a large number of database queries. Copyright 2015, Oracle and/or its affiliates. All rights reserved. Oracle Confidential Internal/Restricted/Highly Restricted 31

32 Generational Garbage Collection Types Java Garbage Collectors Parallel GC Command Line Options: To enable the Parallel Collector use: -XX:+UseParallelGC The number of garbage collector threads can be controlled with command-line options: -XX:ParallelGCThreads=<desired number> Copyright 2015, Oracle and/or its affiliates. All rights reserved. Oracle Confidential Internal/Restricted/Highly Restricted 32

33 Generational Garbage Collection Type Java Garbage Collectors CMS GC (Concurrent Mark Sweep) It collects the tenured generation. It attempts to minimize the pauses due to garbage collection by doing most of the garbage collection work concurrently with the application threads. Normally the concurrent low pause collector does not copy or compact the live objects. A garbage collection is done without moving the live objects. If fragmentation becomes a problem, allocate a larger heap. Usage cases The CMS collector should be used for applications that require low pause times and can share resources with the garbage collector. Examples include desktop UI application that respond to events. Copyright 2015, Oracle and/or its affiliates. All rights reserved. Oracle Confidential Internal/Restricted/Highly Restricted 33

34 Generational Garbage Collection Types Java Garbage Collectors CMS GC Command Line Options: To enable the CMS Collector use: -XX:+UseConcMarkSweepGC and to set the number of threads use: -XX:ParallelCMSThreads=<n>. Copyright 2015, Oracle and/or its affiliates. All rights reserved. Oracle Confidential Internal/Restricted/Highly Restricted 34

35 Generational Garbage Collection Types Java Garbage Collectors G1 GC (Generation 1) The G1 collector is a parallel, concurrent, and incrementally compacting low-pause garbage collector that has quite a different layout from the other garbage collectors described previously Command Line Options: To enable the G1 Collector use: To enable the G1 Collector use: -XX:+UseG1GC Copyright 2015, Oracle and/or its affiliates. All rights reserved. Oracle Confidential Internal/Restricted/Highly Restricted 35

Java Performance Tuning

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