Talk about JVM memory settings by MyEclipse memory shortage

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If not set, the use of myeclipse often appears as shown in insufficient memory hints.

The hint says very clearly: "MyEclipse has detected that less than 5% of the 64MB of Perm Gen (non-heap memory) space remains." This means that only less than 5% of the non-heap memory is currently idle. So we just have to set this value a bit larger.

The parameters that are set are given in the prompt:

    1. -vmargs-xms128m-xmx512m-xx:permsize=64m-xx:maxpermsize=128m

Here are a few questions:
1. What are the meanings of each parameter?
2. Why do some machines I set-xmx and-xx:maxpermsize to 512M after eclipse can be started, and some machines will not boot?
3. Why is the above parameter written to the Eclipse.ini file eclipse does not perform the corresponding settings?

Here's a reply.

1. What are the meanings of each parameter?

The-vmargs in the parameter is to set the JVM parameters, so the following is actually the JVM parameters, we first understand the JVM memory management mechanism, and then explain the meaning of each parameter represents.

  • Heap and non-heap (non-heap) memory
    According to the official statement, "Java virtual machines have a heap, the heap is a runtime data region, and all class instances and arrays of memory are allocated from here." The heap is created when the Java virtual machine is started. "" The memory outside the heap in the JVM is called non-heap (non-heap memory) ". You can see that the JVM primarily manages two types of memory: heap and non-heap. In a nutshell, a heap is a Java code-readable memory that is left to the developer, not a heap, which is left to itself by the JVM, so the method area, the JVM internally processes or optimizes the required memory (such as the JIT-compiled code cache), each class structure (such as running a constant pool, field, and method data) And the code for methods and construction methods are in non-heap memory.
  • Heap memory allocation
    The initial memory allocated by the JVM is specified by-XMS, the default is physical memory 1/64;JVM the maximum allocated memory is specified by-XMX, which defaults to 1/4 of the physical memory. When the default free heap memory is less than 40%, the JVM increases the heap until the maximum limit of-xmx, and when the free heap memory is greater than 70%, the JVM reduces the heap until the minimum limit of-XMS. So the server generally sets-xms,-xmx equal to avoid resizing the heap after each GC.
  • Non-heap memory allocation
    The JVM uses-xx:permsize to set the non-heap memory initial value, which defaults to 1/64 of the physical memory, and the maximum non-heap memory by Xx:maxpermsize, which by default is 1/4 of physical memory.
  • JVM Memory Limit (max)
    First, the JVM memory is limited to the actual maximum physical memory (nonsense!). hehe), assuming that the physical memory is infinitely large, the maximum value of the JVM memory is very much related to the operating system. In short, the 32-bit processor, although the controllable memory space has 4GB, but the specific operating system will give a limit, This limit is generally 2GB-3GB (typically under Windows systems for the 1.5g-2g,linux system 2g-3g), and processors over 64bit are not limited.

2. Why do some machines I set-xmx and-xx:maxpermsize to 512M after eclipse can be started, and some machines will not boot?

We have learned from the above introduction to JVM memory management that there are two types of JVM memory: heap memory and non-heap memory, and the JVM's maximum memory depends first on actual physical memory and operating system. So there are several reasons why setting a VM parameter causes the program to fail to start:

1) The value of-XMS in the parameter is greater than-XMX, or the value of-xx:permsize is greater than-xx:maxpermsize;

2) The sum of-XMX values and-xx:maxpermsize exceeds the maximum limit of JVM memory, such as the maximum memory limit of the current operating system, or actual physical memory, and so on. When it comes to physical memory, it is important to note that if your memory is 1024MB, the actual system is not likely to be 1024MB, because a portion of it is consumed by the hardware.

Note: If-XMX is not specified or is specified as small, the application may cause a java.lang.OutOfMemory error from the JVM that is not throwable and cannot be captured with Try...catch.

PermSize and MaxPermSize indicate that the virtual machine is permanently generating objects for Java (permanant
Generation) such as class objects, method objects, these reflective (reflective) objects allocate memory limits that are not included in the heap memory area.

3. Why is the above parameter written to the Eclipse.ini file eclipse does not perform the corresponding settings?

Why is the same argument valid on a shortcut or command line and is not valid in the Eclipse.ini file? This is because we have not complied with the setup rules for Eclipse.ini files:

Parameter shape such as "Item Value" In this form, there are spaces in the middle of the need to write a newline, if there are spaces in the value need to be enclosed in double quotation marks. For example, we use the-VM C:\Java\jre1.6.0\bin\javaw.exe parameter to set up the virtual machine, in the Eclipse.ini file to write this:

  1. -vm
  2. C:\Java\jre1.6.0\bin\javaw.exe

As stated above, the final parameter can be written in Eclipse.ini:

    1. -vmargs
    2. -xms128m
    3. -xmx512m
    4. -xx:permsize=64m
    5. -xx:maxpermsize=128m

The results of the actual operation can be viewed through the "Configuration Details" button in the "Help" – "About Eclipse SDK" window in Eclipse.

It is also necessary to note that the contents of the Eclipse.ini file included in the Eclipse compression package are as follows:

    1. -showsplash
    2. Org.eclipse.platform
    3. --launcher. Xxmaxpermsize
    4. 256m
    5. -vmargs
    6. -xms40m
    7. -xmx256m

Which--launcher. Xxmaxpermsize (Note that the front is two connectors) the meaning of the-xx:maxpermsize parameter is basically the same, I feel The only difference is that the former is the parameter set when the Eclipse.exe is started, and the latter is the parameter in the JVM used by Eclipse. In fact, the two set a can, so here can put--launcher. Xxmaxpermsize and Next line use # comment out.

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Talk about JVM memory settings by MyEclipse memory shortage

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