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Based on the documentation : https://docs.oracle.com/javase/7/docs/api/java/lang/management/MemoryUsage.html
committed - represents the amount of memory (in bytes) that is guaranteed to be available for use by the Java virtual machine. The amount of committed memory may change over time (increase or decrease). The Java virtual machine may release memory to the system and committed could be less than init. committed will always be greater than or equal to used.
but the question is how JVM calculate the committed memory?
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Use this code: // Get current size of heap in bytes long heapSize = Runtime. getRuntime(). totalMemory(); // Get maximum size of heap in bytes.
You can verify that the JVM is using the increased Java heap space: Open a terminal window. Review the command output. The argument beginning with "-Xmx" will give you the value of the current Java heap space.
The initial value of (1024 X 1024) is the 1MB initial size of the heap memory pool. Adaptive Server reserves a small amount of memory for internal structures.
From the javadocs, committed is: The amount of memory (in bytes) that is guaranteed to be available for use by the Java virtual machine. The amount of committed memory may change over time (increase or decrease). The Java virtual machine may release memory to the system and committed could be less than init.
This cycle repeats until the committed heap size matches the maximum heap size, the maximum space allocatable. The JVM heap usage usually follows a "sawtooth" pattern. The memory in use can grow high and then be recollected, which is the expected behavior and does not indicate a memory leak.
Let’s start with what most people are already familiar with — Heap memory. Broadly speaking, the JVM heap consists of Objects (and arrays). Once the JVM starts up, the heap is created with an initial size and a maximum size it can grow to.
The JVM runtime environment uses a large memory pool called the heap for object allocation. The JVM automatically invokes garbage collections in order to clean up the heap of unreferenced or dead objects. In contrast, memory management in legacy programming languages like C++ was left to the programmer.
The heap size may grow a bit to accommodate for new objects: The used space is the amount of memory that is currently occupied by Java objects. It's always less than or equal to the max size. 4. Committed Size The committed size is the amount of memory guaranteed to be available for use by the Java virtual machine.
Here you can find a little bit more detail, but it does not explain the exact way how committed heap space is increased:
There is also a committed heap size which acts as a "high water mark", moving up once the JVM cannot free up space even on old collection / young collection to make room. In this case, the committed heap size is increased. This cycle repeats until the committed heap size matches the maximum heap size, the maximum space allocatable.
https://support.mulesoft.com/s/article/Java-JVM-memory-allocation-pattern-explained
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