Causes and solutions of oom (memory overflow)

One 、 Concept

out of memory (Out Of Memory, abbreviation OOM) It means that there is too much memory in the application system that cannot be recycled or used , Finally, the memory used for program running is larger than the maximum memory provided .

Two 、 Cause

2.1、 Memory leak

Due to the long-term reference of some resources , The garbage collector cannot recycle it , Thus, this resource cannot be released in time , Also known as memory leak . So try not to use all references as strong references , Weak references and soft references can be used where appropriate .

2.2、 Super large objects

Saving multiple consumed memory is too large or when loading a single oversized object , The size of this object exceeds the currently available memory space . For example, query the data in the database , Too many queries at one time , Directly lead to memory overflow . Therefore, if there is too much data in the query database, try to use paging query .

2.3、 All sorts of other reasons

For example, whether there is an dead cycle , Large loops repeat to produce objects , Whether there is a collection object after it is used , Still quoted , Cause unable to clear , Whether improper data structures are used , It leads to too much space and so on .

3、 ... and 、 Solution

3.1、 modify JVM Launch parameters , Add memory directly

JVM By default, the available memory is 64M,Tomcat By default, the available memory is 128MB, For a slightly more complex system, it will not be enough . In a project , Just because the default value used by the startup parameter , Regular report “Out Of Memory” error . therefore ,-Xms,-Xmx（ Minimum and maximum size of heap memory ） Parameters must not forget to add .

3.2、 Find out where a memory overflow might occur , And solve

Check the code for dead loops or recursive calls .

Check if there is a large loop that repeatedly generates new object entities .

Check the database query , Is there a query to get all the data .

Check List、Map Wait until the collection object is used , Problems not cleared .

Use the memory view tool to dynamically view memory usage .

Four 、Xmn、Xms、Xmx、Xss Parameter difference - Meaning and understanding

First ,-Xmn、-Xms、-Xmx、-Xss All are JVM Configuration parameters for memory , We can modify these parameters according to different needs , To achieve the best effect of running the program .

4.1、-Xms、-Xmx

-Xms、-Xmx Allocation is used to set the minimum and maximum size of the process heap memory .

4.2、-Xmn

-Xmn Used to set the size of the Cenozoic in the heap . Through this value, we can also get the size of the old generation ：-Xmx subtract -Xmn

4.3、-Xss

-Xss Set the amount of memory each thread can use .

In the same physical memory , Reducing this value can generate more threads . Of course, the operating system has a limit on the number of threads in a process , Can't generate... Indefinitely , Experience is in 3000~5000 about .

In addition to these configurations ,JVM There are a lot of configurations , Common ones are as follows ：

-XX:PermSize、-XX:MaxPermSize
Allocation is used to set the minimum and maximum size of the permanent generation .Java 8 Later, the method area was removed , Instead, the local meta space Metaspace, Size by -XX:MetaspaceSize and -XX:MaxMetaspaceSize Adjust the .

-XX:MaxTenuringThreshold
Set the survival times of the old generation . If it is 0, Then directly skip the new generation and enter the old generation .

-XX:NewRatio
Set the ratio of the old generation to the new generation , For example, the value is 3, It means that the ratio of Cenozoic generation to old generation is 1:3.