Summary on Garbage collection in Java
Gag Wears Sports Gym Bag
1) Java Heap is divided into three generation for sake of garbage collection. These are young generation, tenured or old generation and Perm area.
2) New objects are created into young generation and subsequently moved to old generation.
3) String pool is created in Perm area of Heap, garbage collection can occur in perm space but depends upon JVM to JVM.
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4) Minor garbage collection is used to move object from Eden space to Survivor 1 and Survivor 2 space and Major collection is used to move object from young to tenured generation.
5) Whenever Major garbage collection occurs application threads stops during that period which will reduce application’s performance and throughput.
6) There are few performance improvement has been applied in garbage collection in java 6 and we usually use JRE 1.6.20 for running our application.
7) JVM command line options –Xmx and -Xms is used to setup starting and max size for Java Heap. Ideal ratio of this parameter is either 1:1 or 1:1.5 based upon my experience for example you can have either both –Xmx and –Xms as 1GB or –Xms 1.2 GB and 1.8 GB.
8) There is no manual way of doing garbage collection in Java.
1. Java Heap Memory is part of Memory allocated to JVM by Operating System.
2. Whenever we create objects they are created inside Heap in Java.
3. Java Heap space is divided into three regions or generation for sake of garbage collection called New Generation, Old or tenured Generation or Perm Space.
4. You can increase or change size of Java Heap space by using JVM command line option -Xms, -Xmx and -Xmn. don't forget to add word "M" or "G" after specifying size to indicate Mega or Giga. for example you can set java heap size to 258MB by executing following command :
java -Xmx256m HelloWord.
5. You can use either JConsole or Runtime.maxMemory(), Runtime.totalMemory(), Runtime.freeMemory() to query about Heap size programmatic in Java.
6. You can use command "jmap" to take Heap dump in Java and "jhat" to analyze that heap dump.
7. Java Heap space is different than Stack which is used to store call hierarchy and local variables.
8. Java Garbage collector is responsible for reclaiming memory from dead object and returning to Java Heap space.
9. Don’t panic when you get java.lang.outofmemoryerror, sometimes its just matter of increasing heap size but if it’s recurrent then look for memory leak in Java.
10. Use Profiler and Heap dump Analyzer tool to understand Java Heap space and how much memory is allocated to each object.
Some useful references:
http://middlewaremagic.com/weblogic/?tag=eden-space
http://middlewaremagic.com/weblogic/?p=4464
http://java.sun.com/docs/books/jvms/second_edition/html/Overview.doc.html
http://www.yourkit.com/docs/kb/sizes.jsp
Gag Wears Sports Gym Bag
1) Java Heap is divided into three generation for sake of garbage collection. These are young generation, tenured or old generation and Perm area.
2) New objects are created into young generation and subsequently moved to old generation.
3) String pool is created in Perm area of Heap, garbage collection can occur in perm space but depends upon JVM to JVM.
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4) Minor garbage collection is used to move object from Eden space to Survivor 1 and Survivor 2 space and Major collection is used to move object from young to tenured generation.
5) Whenever Major garbage collection occurs application threads stops during that period which will reduce application’s performance and throughput.
6) There are few performance improvement has been applied in garbage collection in java 6 and we usually use JRE 1.6.20 for running our application.
7) JVM command line options –Xmx and -Xms is used to setup starting and max size for Java Heap. Ideal ratio of this parameter is either 1:1 or 1:1.5 based upon my experience for example you can have either both –Xmx and –Xms as 1GB or –Xms 1.2 GB and 1.8 GB.
8) There is no manual way of doing garbage collection in Java.
10 Points about Java Heap Space
1. Java Heap Memory is part of Memory allocated to JVM by Operating System.
2. Whenever we create objects they are created inside Heap in Java.
3. Java Heap space is divided into three regions or generation for sake of garbage collection called New Generation, Old or tenured Generation or Perm Space.
4. You can increase or change size of Java Heap space by using JVM command line option -Xms, -Xmx and -Xmn. don't forget to add word "M" or "G" after specifying size to indicate Mega or Giga. for example you can set java heap size to 258MB by executing following command :
java -Xmx256m HelloWord.
5. You can use either JConsole or Runtime.maxMemory(), Runtime.totalMemory(), Runtime.freeMemory() to query about Heap size programmatic in Java.
6. You can use command "jmap" to take Heap dump in Java and "jhat" to analyze that heap dump.
7. Java Heap space is different than Stack which is used to store call hierarchy and local variables.
8. Java Garbage collector is responsible for reclaiming memory from dead object and returning to Java Heap space.
9. Don’t panic when you get java.lang.outofmemoryerror, sometimes its just matter of increasing heap size but if it’s recurrent then look for memory leak in Java.
10. Use Profiler and Heap dump Analyzer tool to understand Java Heap space and how much memory is allocated to each object.
Some useful references:
http://middlewaremagic.com/weblogic/?tag=eden-space
http://middlewaremagic.com/weblogic/?p=4464
http://java.sun.com/docs/books/jvms/second_edition/html/Overview.doc.html
http://www.yourkit.com/docs/kb/sizes.jsp
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