9/10/2023 0 Comments Stack vs heap java que esThere is a JVM level option called “ MaxTenuringThreshold” to specify the object age threshold to promote the object to tenured space. When objects reach the max age threshold, then those objects are copied to the tenured space. The above process repeats for each minor GC. In the above diagram, the objects marked with red are eligible for GC, and the other objects will be copied from the eden and survivor spaces to the other survivor space, S1, and the objects age incrementally. The garbage collector removes the non-reachable objects and empties the eden space.įor the second minor GC, the garbage collector marks the non-reachable objects from “eden space” and the “To survivor space (S0)”, copies the GC roots to the other survivor space S1, and the reachable object's age will be incremented. The garbage collector won’t remove the GC roots. In the above diagram, the objects marked with a red color indicates that they are non-reachable. For this image's purpose I have made the “To Space” bigger than the allocated space. This process is called “ premature promotion”. The objects which are not fit into the survivor space will be moved to the tenured space. The JVM copies the reachable objects to “To Space”, S0, and increments the reachable object's age by 1. Let us say the JVM selected S0 as the “To Space”. The JVM selects one of the survivor spaces as “To Space”. During minor GC, the objects which are not reachable are marked to be collected. When the JVM is not able to get the memory from the eden space it initiates minor GC. Initially the survivor space and the tenured space is empty. The JVM performs GC as minor GC and major GC. As we said above when the application creates the object the JVM try to get the required memory from the eden space. The JVM uses a separate demon thread to do garbage collection. Now let us discuss the garbage collection process. The JIT compiled code will not be interpreted. The frequently accessed code blocks will be compiled to native code by the JIT and stored it in code cache. As part of JVM optimization, the Just In Time (JIT) compiler has been introduced. In that case, we may get out of memory errors.Ĭode Cache: JVM has an interpreter to interpret the byte code and convert it into hardware dependent machine code. We have JVM options to limit the Meta Space used by the JVM. This will have an adverse effect on application performance, as swapping the data from virtual memory to physical memory and vice versa is a costly operation. However, if it grows more than the available physical memory, then the operating system will use virtual memory. This is designed to grow in order to avoid 0ut of memory errors. This space is used to store the class definitions loaded by class loaders. In earlier versions of Java we called this “ Perm Gen Space". As per the document by default the meta space doesn’t have an upper limit. Meta Space: This memory is out of heap memory and part of the native memory. When we discuss the garbage collection process we will come to know how the above memory locations will be used. Tenured Space: The objects which reach to max tenured threshold during the minor GC or young GC, will be moved to “ Tenured Space” or “ Old Generation Space“. We have two equally divided survivor spaces called S0 and S1. Survivor Space: This contains the objects that have survived from the Young garbage collection or Minor garbage collection. Young Generation: The Young Generation or the New Space is divided into two portions called “ Eden Space” and “ Survivor Space".Įden Space:When we create an object, the memory will be allocated from the Eden Space. This memory is in turn split into two different areas called the “ Young Generation Space” and “ Tenured Space“. Heap Memory: JVM uses this memory to store objects. Let me explain the different memory blocks available for the JVM. How much memory the operating system allocates to the Java process depends on the operating system, processor, and the JRE. “ Native Memory” is the memory provided to the process by the operating system. Here, the JVM itself is a process, and the memory allocated to that process includes the Heap, Meta Space, JIT code cache, thread stacks, and shared libraries. Once we launch the JVM, the operating system allocates memory for the process. First let me depict the different memory areas available for Java processes. In this article I have used JDK8 Oracle Hot Spot 64 bit JVM. In this article we will try to understand the Java memory model and how garbage collection works.
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