Java Memory Model

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Abhishek Khare edited this page Jan 10, 2017 · 4 revisions

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.
Young generation is the place where all the new objects are created. When 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.
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.
Usually garbage collection is performed in Old Generation memory when it’s full. Old Generation Garbage Collection is called Major GC and usually takes 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 longer 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.