JVM memory overflow

1、 Heap overflow

         Most memory overflows are heap overflows , The reason is that a large number of objects occupy heap space , And these objects hold strong references , It can't be recycled .-Xmx Parameter specifies when the heap space size is smaller than the object size , Overflow happens naturally .

    Error message ：java.lang.OutOfMemoryError: Java heap space
    To reduce heap overflow errors , On the one hand, you can use -Xmx Specify a larger heap space . In addition, you can use MAT perhaps VisualVM Tools such as , Find objects that take up a lot of heap space , And optimize the code reasonably .

2、 Direct memory overflow

         stay Java Of NIO in , Support direct memory usage , That is, through Java The code gets a block of off heap memory , This memory is directly requested from the operating system . Direct memory requests are generally slower than heap memory , But access speed is faster than heap memory , So for those spaces that can be reused and accessed frequently , Using direct memory can improve the performance of the system , But there is no memory JVM trusteeship , Improper use can easily trigger a direct memory overflow , Cause downtime .

        -XX:+PrintGCDetails Output GC details

        Error message ：java.lang.OutOfMemoryError: Direct buffer memory

        Direct memory does not necessarily trigger GC, Trigger direct memory usage to -XX:MaxDirectMemorySize Set the value of . Therefore, it is necessary to ensure that the memory does not overflow FullGC. Or set up a system reachable -XX:MaxDirectMemorySize value . Via displayed gc It can reclaim direct memory .

        Without using -XX:MaxDirectMemorySize When setting the maximum and most direct memory space , By default, the maximum available direct memory is equal to -Xmx That is, the size of the heap space . 

        Avoid direct memory overflow ：

                1、 reasonable GC

                2、 Set the system allowable -XX:MaxDirectMemorySize

                3、 Set small -Xmx

3、 Too many threads cause OOM

        The stack space of a thread is also allocated outside the heap , Very similar to direct memory , If you want the system to support more threads , Then you should use a smaller heap space . 

        Error message ： unable to create new native thread

        as a result of Java The process has reached the maximum available memory . Reduce heap space , Reduce the space occupied by threads -Xss Parameter can specify the stack space of the thread .

        -Xmx1g -Xss128k  Heap space is still 1g, But the thread stack space is reduced to 128k, The remaining available memory can accommodate more threads .

4、MetaspaceOOM

      Metaspace Meta space mainly stores metadata information of classes , for example ： type 、 attribute 、 Method 、 Access restrictions, etc . so Metaspace Space growth is due to reflection class loading 、 Class loading generated by dynamic proxy , in other words Metaspace The size of is related to the loaded data , The more classes you load metaspace The more memory it takes up .

        -XX:+TraceClassLoading -XX:+TraceClassUnloading You can record the loading and unloading of classes .

5、GC Inefficiency leads to OOM

      GC Is the key to memory reclamation , If GC Efficiency underground , Then the performance of the system will be seriously affected . If the heap space is too large ,GC It will take more time , And the objects released by recycling are less , Virtual machines think GC It is possible to throw OOM, When something happens , Throw out GC overhead limit exceeded.

        1、 Spend on GC More time than 98%

        2、 Whether the memory released by the older generation is less than 2%

        3、eden Whether the area free memory is less than 2%

        4、 Is it continuous 5 Time At the same time Above case.