Investigating Out of Memory/Memory Leak Problems
但现在这个网址已经无法访问，我在Metalink找到这篇文章并与大家分享。文章发表的较早，但是OOM发生的原理和解决的方法不变。文中提到的/3GB参数和垃圾回收日志分析的方法我在《JAVA性能优化—IBM JDK JVM参数设置》和《JAVA性能优化-GC日志分析》都提到过。如果英文的看起来比较麻烦，我google到台湾人做的一份翻译JavaAPsvr_A_200411_MemoryLeak，不过我看着“记忆体”觉得更别扭。
Problem Description
Out Of Memory (OOM) – An application displays Out of Memory errors due to memory exhaustion, either in java heap or native memory.
Memory Leak – Constant memory growth in either java heap or native memory, which will eventually end up in out of memory situation. The techniques to debug the memory leak situations are the same as the out of memory situations.
Problem Troubleshooting
Please note that not all of the following items would need to be done. Some issues can be solved by only following a few of the items.
Quick Links:
- Java heap, Native memory, Process size
- Difference between process address space and physical memory
- Why does the OOM problem occur and What does the JVM do in this situation?
- Steps to debug the problem
- HP JVM specific tools/tips>
- Jrockit specific features
- Bibliography
Java heap This is the memory that the JVM uses to allocate java objects. The maximum value of java heap memory is specified using �Xmx flag in the java command line. If the maximum heap size is not specified, then the limit is decided by the JVM considering factors like the amount of physical memory in the machine and the amount of free memory available at that moment. It is always recommended to specify the max java heap value.
Native memory This is the memory that the JVM uses for its own internal operations. The amount of native memory heap that will be used by the JVM depends on the amount of code generated, threads created, memory used during GC for keeping java object information and temporary space used during code generation, optimization etc.
If there is a third party native module, it could also use the native memory. For example, native JDBC drivers allocate native memory.
The max amount of native memory is limited by the virtual process size limitation on any given OS and the amount of memory already committed for the java heap with -Xmx flag. For example, if the application can allocate a total of 3 GB and if the max java heap is 1 GB, then the max possible native memory is approximately 2 GB.
Process size Process size will be the sum of the java heap, native memory and the memory occupied by the loaded executables and libraries. On 32 bit operating systems, the virtual address space of a process can go up to 4 GB. Out of this 4 GB, the OS kernel reserves some part for itself (typically 1 ~ 2 GB). The remaining is available for the application.
Windows ：By default, 2 GB is available for the application and 2 GB is reserved for Kernel’s use. However, on some variants of Windows, there is a /3GB switch which can be used to change this ratio such that the application gets 3 GB. More details on the /3GB switch can be found at:
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/memory/base/4gt_ram_tuning.asp
RH Linux AS 2.1 ： 3 GB is available for the application.
For other operating systems, please refer to the OS documentation for your configuration.
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