JVM memory settings in eclipse

Eclipse. ini memory settings
-Vmargs-xms128m-xmx512m-XX: permsize = 64 m
-XX: maxpermsize = 128 m

1. What are the meanings of each parameter?

In the parameters-vmargs indicates setting JVM parameters, so the following are actually JVM parameters. First, let's take a look at the JVM memory management mechanism, and then explain the meaning of each parameter.

Heap and non-heap memory
According to the official statement: "A Java virtual machine has a heap. The heap is the runtime data area, and the memory of all class instances and arrays is allocated from this place. Heap is in Java
Created when the VM is started ." "The memory outside the heap in JVM is called non-heap memory (
Memory )". JVM manages two types of memory: heap and non-heap. In short, heap is the memory available for Java code and is reserved for developers.
Self-use, so the memory required for processing or optimization in the Method Area and JVM (such as the code cache after JIT compilation), and each class structure (such as the runtime data pool, fields, and method data) and methods and constructor Methods
The code is in non-heap memory.

Heap Memory Allocation
The initial memory allocated by JVM is specified by-XMS. The default value is 1/64 of the physical memory. The maximum memory allocated by JVM is specified by-xmx. The default value is 1/4 of the physical memory. The default idle heap memory is smaller
At 40%, the JVM will increase the heap size until the maximum limit of-xmx. When the free heap memory is greater than 70%, the JVM will reduce the minimum limit of heap until-XMS. Therefore, the server is generally set to-XMS,
-Xmx is equal to avoid adjusting the heap size after each GC.
Non-heap memory allocation
JVM uses-XX: permsize to set the non-heap memory initial value. The default value is 1/64 of the physical memory. The maximum non-heap memory size is set by XX: maxpermsize. The default value is 1/4 of the physical memory.

JVM memory limit (maximum)
First, JVM memory is limited to the actual maximum physical memory (nonsense! If the physical memory is infinitely large, the maximum JVM memory has a great relationship with the operating system. Simply put, although the 32-bit Processor
Although the controllable memory space is 4 GB, the specific operating system will impose a limit, which is generally 2 GB-3 GB (generally 1.5 GB-
2G, 2g-3G in Linux), and 64-bit or higher processors will not have restrictions.

2.
Why can I enable eclipse after setting-xmx and-XX: maxpermsize to m, but cannot start some machines?

Through the introduction of JVM memory management, we have learned that JVM memory includes two types: heap memory and non-heap memory. In addition, the maximum JVM memory depends on the actual physical memory and operating system. Therefore, setting VM parameters causes program startup failure mainly due to the following reasons:

1) In the parameter, the value of-XMS is greater than-xmx, or the value of-XX: permsize is greater than-XX: maxpermsize;

2)
-Xmx value and-XX: The sum of maxpermsize exceeds the maximum JVM memory limit, such as the maximum memory limit of the current operating system or the actual physical memory. Speaking of physical
Memory here, it should be noted that if your memory is 1024 MB, but the actual usage in the system is not 1024 MB, because some of them are occupied by hardware.

3. Why didn't eclipse execute the corresponding settings when writing the above parameters to the eclipse. ini file?

Why is the same parameter valid in the shortcut or command line and invalid in the eclipse. ini file? This is because we have not followed the setting rules of the eclipse. ini file:

Parameters are in the form of "item value". If there is a space in the middle, you need to wrap it. If there is a space in the value, you need to include it in double quotation marks. For example, we use-VM
The C:/Java/jre1.6.0/bin/javaw.exe parameter sets the virtual machine, which must be written in the eclipse. ini file as follows:

-VM
C:/Java/jre1.6.0/bin/javaw.exe
As mentioned above, the final parameter can be written in eclipse. ini as follows:

-Vmargs
-Xms128m
-Xmx512m
-XX: permsize = 64 m
-XX: maxpermsize = 128 m
The actual running result can be found in "configuration" in the "help"-"about eclipse SDK" Window of Eclipse.
View Details.

Note that the eclipse. ini file in the eclipse compressed package contains the following content:

-Showsplash
Org. Eclipse. Platform
-- Launcher. xxmaxpermsize
256 m
-Vmargs
-Xms40m
-Xmx256m
Among them, the-launcher. xxmaxpermsize (note that there are two connection lines at the top) has the same meaning as the-XX: maxpermsize parameter. I think the only one
The prefix is the parameter set during startup of eclipse.exe, which is the JVM parameter used by eclipse. In fact, you can set one of the two, so you can set
-Launcher. xxmaxpermsize and use # In the next line to comment out.

3. Other startup parameters.
If you have a dual-core CPU, you can try this parameter:

-XX: + useparallelgc

Enables faster GC execution. (Only the newly added GC parameters in JDK 5)