JVM parameter Settings-vmargs-xms128m-xmx512m-xx:permsize=64m-xx:maxpermsize=128m

Source: Internet
Author: User
Tags xms


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 means that the JVM parameters are set, so the following are all the parameters of the JVM, so let's first look at the mechanism of the JVM's memory management and then explain what each parameter means. Heap (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. Simply put, the heap is Java code-readable memory, which is reserved for developers, and non-heap is the JVM's own use, so the method area, JVM internal processing or optimization of the required memory (such as Jit  compiled code cache), Each class structure, such as running a constant pool, fields, and method data, and the code for methods and construction methods are in non-heap memory. The heap memory allocation JVM initially allocates memory 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. The non-heap memory allocation JVM uses-xx:permsize to set the non-heap memory initial value, which defaults to 1/64 of the physical memory, and by xx:maxpermsize sets the maximum non-heap memory size, which defaults to 1/4 of the physical memory. JVM Memory Limit (max) First 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. Simply put, 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 (generally speaking, Windows system under the 1.5g-2g,linux system for 2G-3G), There is no limit to processors over 64bit.

2. Why do I have-xmx and-xx:maxpermsize set to 512M after eclipse can be started and some machines fail to boot?  
through the above introduction to JVM memory management we have learned that JVM memory consists of two kinds: 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 not to start: The value of-XMS in the
1) parameter is greater than-XMX, or the value of-xx:permsize is greater than-xx:maxpermsize;
2)-xmx value and-xx: The sum of the maxpermsize exceeds the maximum JVM memory limit, such as the current operating system maximum memory limit, 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.

 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 do not comply with the Eclipse.ini file setting rules: The parameter shape, such as "key value" in this form, there are spaces in the middle of the need to write a newline, if the value of a space in the 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:-VM C:/java/jre1.6.0/bin/javaw.exe according to the above mentioned, The last parameter can be written as this in Eclipse.ini: The result of the-vmargs-xms128m-xmx512m-xx:permsize=64m-xx:maxpermsize=128m actually runs through the eclipse "help" -View the "Configuration Details" button in the "About Eclipse SDK" window.
It is also necessary to note that the contents of the Eclipse.ini file included in the Eclipse compact package are as follows:-showsplash org.eclipse.platform--launcher. Xxmaxpermsize 256m-vmargs-xms40m-xmx256m which –launcher. Xxmaxpermsize (Note that the front is two cable) the meaning of the-xx:maxpermsize parameter is basically the same, I think the only The 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.

 4. Other start-up parameters. If you have a dual-core CPU, you might be able to try this parameter:
Allows the GC to execute faster. (only new parameters added to the GC in JDK 5)

JVM parameter Settings-vmargs-xms128m-xmx512m-xx:permsize=64m-xx:maxpermsize=128m

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