MEMORY MANAGEMENT HEAP, STACK AND GARBAGE COLLECTION

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1 MEMORY MANAGEMENT HEAP, STACK AND GARBAGE COLLECTION 2 1. What is the Heap Size: 2 2. What is Garbage Collection: 3 3. How are Java objects stored in memory? 3 4. What is the difference between stack and heap in Java? 3 5. Is there a command in Linux to find Java Heap size and mrmory used 7 6. Can you guarantee the garbage collection process? 7 7. How do you find memory usage from Java program? How much percent of the heap is used? 7 8. Mark and sweep algorithm and garbage collection in Java- MetLife 7 1

2 Memory Management Heap, Stack and Garbage Collection Last Updated on June 23rd, 2016 by Crunchify 6 Comments 1. What is the Heap Size: When a Java program starts, Java Virtual Machine gets some memory from Operating System. Java Virtual Machine or JVM uses this memory for all its need and part of this memory is call java heap memory. Whenever we create object using new operator or by any another means object is allocated memory from Heap and When object dies or garbage collected, memory goes back to Heap space in Java.. It is good practice for big production projects to set the minimum -Xms and maximum -Xmx heap sizes to the same value. For efficient garbage collection, the -Xmn value should be lower than the -Xmx value. Heap size does not determine the amount of memory your process uses. If you monitor your java process with an OS tool like top or taskmanager, you may see the amount of memory you use exceed the amount you have specified for -Xmx. -Xmx limits the java heap size, java will allocate memory for other things, including a stack for each thread. It is not unusual for the total memory consumption of the VM to exceed the value of -Xmx. maximum heap memory you can assign to a 32-bit JVM is 2^32 which is 4GB but practically the limit is much smaller. It also varies between operating systems e.g. form 1.5GB in Windows to almost 3GB in Solaris. maximum heap memory you can assign to 64-bit JVM allows you to specify larger heap size, theoretically 2^64 which is quite large but practically you can specify heap space up to 100GBs. There are even JVM e.g. Azul where heap space of 1000 gigs is also possible. 2

3 2. What is Garbage Collection: There are essentially two GC threads running. One is a very lightweight thread which does little collections primarily on the Young generation of the heap. The other is the Full GC thread which traverses the entire heap when there is not enough memory left to allocate space for objects which get promoted from the Young to the older generation(s). If there is a memory leak or inadequate heap allocated, eventually the older generation will start to run out of room causing the Full GC thread to run (nearly) continuously. Since this process stops the world, Java application won t be able to respond to requests and they ll start to back up or OOM. The amount allocated for the Young(Eden) generation is the value specified with -Xmn. The amount allocated for the older generation is the value of -Xmx minus the -Xmn. Generally, you don t want the Eden to be too big or it will take too long for the GC to look through it for space that can be reclaimed. 3. How are Java objects stored in memory? In Java, all objects are dynamically allocated on Heap In Java, when we only declare a variable of a class type, only a reference is created (memory is not allocated for the object). To allocate memory to an object, we must use new(). So the object is always allocated memory on heap. 4. What is the difference between stack and heap in Java? stack hold method frames local variables much smaller than heap memory NOT Shared between multiple threads heap objects arrays, etc shared among all threads in JVM. Heap in Java generally located at bottom of address space and move upwards. whenever we create object using new operator or by any another means object is allocated memory from Heap and When object dies or garbage collected -Xms denotes starting size of Heap while -Xmx denotes maximum size of Heap in Java. 3

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7 5. Is there a command in Linux to find Java Heap size and mrmory used Each Java process has a pid, which you first need to find with the jps command. Once you have the pid, you can use jstat -gc [insert-pid-here] to find statistics of the behavior of the garbage collected heap. jstat -gccapacity [insert-pid-here] will present information about memory pool generation and space capabilities. jstat -gcutil [insert-pid-here] will present the utilization of each generation as a percentage of its capacity. Useful to get an at a glance view of usage. 6. Can you guarantee the garbage collection process? No, you cannot guarantee the garbage collection, though you can make a request using System.gc() or Runtime.gc() method. 7. How do you find memory usage from Java program? How much percent of the heap is used? You can use memory related methods from java.lang.runtime class to get the free memory, total memory and maximum heap memory in Java. By using these methods, you can find out how many percents of the heap is used and how much heap space is remaining. Runtime.freeMemory() return amount of free memory in bytes, Runtime.totalMemory() returns total memory in bytes and Runtime.maxMemory() returns maximum memory in bytes. 8. Mark and sweep algorithm and garbage collection in Java- MetLife All the objects which are created dynamically (using new in C++ and Java) are allocated memory in the heap. If we go on creating objects we might get Out Of Memory error, since it is not possible to allocate heap memory to objects. So we need to clear heap memory by releasing memory for all those objects which are no longer referenced by the program (or the unreachable objects) so that the space is made available for subsequent new objects. This memory can be released by the programmer itself but it seems to be an overhead for the programmer, here garbage collection comes to our rescue, and it automatically releases the heap memory for all the unreferenced objects. There are many garbage collection algorithms which run in the background. One of them is mark and sweep. Any garbage collection algorithm must perform 2 basic operations. One, it should be able to detect all the unreachable objects and secondly, it must reclaim the heap space used by the garbage objects and make the space available again to the program. The above operations are performed by Mark and Sweep Algorithm in two phases: 1) Mark phase 2) Sweep phase Mark phase: Mark(root) If markedbit(root) = false then markedbit(root) = true For each v referenced by root Mark(v) Note: If we have more than one root, then we simply have to call Mark() for all the root variables. Sweep Phase 7

8 As the name suggests it sweeps the unreachable objects i.e. it clears the heap memory for all the unreachable objects. All those objects whose marked value is set to false are cleared from the heap memory, for all other objects (reachable objects) the marked bit is set to false. Now the mark value for all the reachable objects is set to false, since we will run the algorithm (if required) and again we will go through the mark phase to mark all the reachable objects. Algorithm Sweep Phase Sweep() For each object p in heap If markedbit(p) = true then else markedbit(p) = false heap.release(p) The mark-and-sweep algorithm is called a tracing garbage collector because is traces out the entire collection of objects that are directly or indirectly accessible by the program. 8