Xms Xmx permsize maxpermsize Difference

1. The meaning of the parameter-vmargs-xms128m-xmx512m-xx:permsize=64m-xx:maxpermsize=128m-vmargs that the parameters of the vm are followed, so the following are actually the parameters of the JVM- Xms128m JVM initially allocates heap memory-xmx512m JVM maximum allowable allocated heap memory, on demand-xx:permsize=64m JVM Initial allocation of non-heap memory-xx:maxpermsize=128m JVM maximum allowable allocated non-heap memory, On Demand

Let's first look at the mechanism of JVM memory management and then explain what each parameter represents.

1) 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 allocated heap memory of the JVM is specified by-XMS, and the default is the maximum allocated heap memory for 1/64;JVM of physical memory 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. Note: If-XMX is not specified or is small, the application may cause a java.lang.OutOfMemory error, which is from the JVM, not throwable, and cannot be captured with Try...catch.

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. (There is also a saying: the maxpermsize default is related to the-server-client option, the default 64m,-client under the-server option is maxpermsize option is 32m.) I don't have an experiment with this. The full name of PermGen space in the above error message is permanent Generation space, which is the permanent storage area of memory. It is not clear whether PermGen space is a non-heap memory, or is not a heap of memory, but at least it belongs to.  Xx:maxpermsize set too small will cause java.lang.OutOfMemoryError:PermGen space is memory benefits.  Say Why Memory Benefits: (1) This portion of memory is used to store class and meta information, and class is placed in the PermGen space area when it is loaded, unlike the heap area where the instance is stored.   (2) GC (garbage Collection) does not clean up permgen space during the main program run time, so if your app will load many classes, PermGen space error is likely to occur. This error is common when the Web server pre-compile the JSP.

2) JVM memory limit (max)

First, the JVM memory is limited to the actual maximum physical memory, 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?   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 the VM parameter causes the program not 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)-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. Speaking of actual physical memory here is a point,   if your memory is 1024MB, but the actual system used is not likely to be 1024MB, because a part of the hardware is occupied.

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 is written like this:-VM C:/java/jre1.6.0/bin/javaw.exe-vmargs- xms128m-xmx512m-xx:permsize=64m-xx:maxpermsize=128m actual running results can be found through the "help" in Eclipse-the "About Eclipse SDK" window Configuration Details button to view it. 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 connections) the meaning of the-xx:maxpermsize parameter is basically the same, I think the only difference is that the former is the Eclipse.exe set parameters when starting, 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:-XX:+USEPARALLELGC allows the GC to execute faster. (only new parameters added to the GC in JDK 5)

Add: If you have a large number of third-party jars under your web app that are larger than the default size of the server JVM, there will be a memory benefit problem. Workaround: Set the MaxPermSize size can be selected in Myelipse the appropriate server such as TOMCAT5, expand the JDK subkey page inside to increase the server startup JVM parameter settings:-xms128m-xmx256m-xx:permsize= 128m-xx:maxnewsize=256m-xx:maxpermsize=256m or manually set the MaxPermSize size, such as Tomcat, modify Tomcat_home/bin/catalina.bat, in echo " Using catalina_base: $CATALINA _base "Add the following line above: java_opts="-server-xx:permsize=64m-xx:maxpermsize=128m

Recommendation: Move the same third-party jar files to the Tomcat/shared/lib directory, which reduces the memory consumption of the jar document

Xms Xmx permsize maxpermsize Difference