If you encounter oom online, how to solve it?

OOM It means that there are loopholes in the program , It could be code or JVM Caused by parameter configuration . This article talks to readers ,Java Process triggered OOM How to check after

It is often said to maintain awe of the production environment , Quick problem solving is also a sign of awe

why OOM？

OOM Full name “Out Of Memory”, Indicates that memory is exhausted . When JVM Because there is not enough memory to allocate space for objects , And when the garbage collector has no space to recycle , Will throw this error

Why does it show up OOM, Generally caused by these problems

• Too little allocation ：JVM Small initialization memory , Business uses a lot of memory ; Or different JVM Area allocation memory is unreasonable

• Code vulnerability ： An object is frequently applied for , It's not used, but it's not released , Cause memory to run out

Memory leak ： The used memory has not been released , Cause the virtual machine can't use the memory again , At this point, the memory leaks . Because applicants don't have to , And can't be assigned to others by virtual machine

out of memory ： The requested memory exceeds JVM The memory size that can be provided , This is called overflow

Memory leaks persist , Finally, it will overflow , The two are causal

common OOM

More common OOM There are several types of

java.lang.OutOfMemoryError: PermGen space

Java7 Forever （ Method area ） overflow , It is used to store class information that has been loaded by the virtual machine 、 Constant 、 Static variables 、 Real time compiler compiled code and other data . Whenever a class is first loaded , Metadata will be stored in the permanent generation

It usually occurs in a large number of Class Object or JSP page , Or use CgLib Dynamic agent technology leads to

We can go through -XX：PermSize and -XX：MaxPermSize Modify the size of the method area

Java8 Change permanent generation to meta space , Report errors ： java.lang.OutOfMemoryError: Metadata space, The meta space memory is insufficient. Dynamic expansion is performed by default

java.lang.StackOverflowError

Virtual machine stack overflow , Generally, it is due to the existence of Dead loop or deep recursive call Caused by the . If the stack size is set too small, overflow will also occur , Can pass -Xss Set the stack size

The virtual machine throws a stack overflow error , You can locate the wrong class in the log 、 Method

java.lang.OutOfMemoryError: Java heap space

Java Heap memory overflow , The cause of overflow is generally due to JVM Memory leaks or improper heap settings

If it's a memory leak , You can view the leak object through the tool GC Roots Reference chain of . Master the type information of the leakage object and GC Roots Reference chain information , You can accurately locate the location of the leaked code

If there is no memory leak , It's that objects in memory really have to live , Then you should check the heap parameters of the virtual machine （-Xmx And -Xms）, See if you can increase the memory of the virtual machine

Summary ： Overflow scenarios of method area and virtual machine stack are not discussed too much in this article , The following mainly explains the common Java Pile space OOM Investigation thought

see JVM Distribution of memory

---

Suppose we Java application PID by 15162, Enter command view JVM Distribution of memory jmap -heap 15162

[[email protected] ~]# jmap -heap 15162 Attaching to process ID 15162, please wait... Debugger attached successfully. Server compiler detected. JVM version is 25.161-b12 using thread-local object allocation. Mark Sweep Compact GC Heap Configuration: MinHeapFreeRatio = 40 # Minimum heap usage ratio MaxHeapFreeRatio = 70 # Maximum heap usable ratio MaxHeapSize = 482344960 (460.0MB) # Maximum heap space size NewSize = 10485760 (10.0MB) # New generation allocation size MaxNewSize = 160759808 (153.3125MB) # Maximum Cenozoic allocable size OldSize = 20971520 (20.0MB) # Old age size NewRatio = 2 # Cenozoic proportion SurvivorRatio = 8 # The new generation and Survivor The proportion MetaspaceSize = 21807104 (20.796875MB) # Metaspace size CompressedClassSpaceSize = 1073741824 (1024.0MB) # Compressed Class Space Space size limit MaxMetaspaceSize = 17592186044415 MB # Maximum Metaspace size G1HeapRegionSize = 0 (0.0MB) # G1 Single Region size Heap Usage: # Heap usage New Generation (Eden + 1 Survivor Space): # The new generation capacity = 9502720 (9.0625MB) # The total capacity of the new generation used = 4995320 (4.763908386230469MB) # The new generation has used free = 4507400 (4.298591613769531MB) # Cenozoic residual capacity 52.56726495150862% used # Proportion of new generation use Eden Space: capacity = 8454144 (8.0625MB) # Eden Total area capacity used = 4029752 (3.8430709838867188MB) # Eden Zone used free = 4424392 (4.219429016113281MB) # Eden Area remaining capacity 47.665996699370154% used # Eden The proportion of district use is From Space: # One of them Survivor Area memory distribution capacity = 1048576 (1.0MB) used = 965568 (0.92083740234375MB) free = 83008 (0.07916259765625MB) 92.083740234375% used To Space: # the other one Survivor Area memory distribution capacity = 1048576 (1.0MB) used = 0 (0.0MB) free = 1048576 (1.0MB) 0.0% used tenured generation: # Old age capacity = 20971520 (20.0MB) used = 10611384 (10.119804382324219MB) free = 10360136 (9.880195617675781MB) 50.599021911621094% used 10730 interned Strings occupying 906232 bytes.

By looking at JVM Memory allocation and runtime usage , You can judge whether the memory allocation is reasonable

in addition , Can be in JVM View the most resource consuming objects at runtime ,jmap -histo:live 15162 | more

JVM The list of memory objects is sorted according to the memory occupied by the objects

• instances： Number of instances

• bytes： Company byte

• class name： Class name

​

Obviously see CustomObjTest Object instance and too much memory

It is a pity , The scheme has limitations , Because it can only check the problem that the object occupies too much memory

among “[” Representative array , for example “[C” representative Char Array ,"[B" representative Byte Array . If the array takes up too much memory , We don't know which objects hold it , So we need Dump Memory for offline analysis

jmap -histo:live Execute this command ,JVM Will trigger GC, Then statistics

Dump File analysis

Dump File is Java Memory image of process , Which mainly includes system information 、 Virtual machine properties 、 Full thread Dump、 Status of all classes and objects Etc

When the program has memory overflow or GC In case of abnormal situation , doubt JVM It happened. Memory leak , Then we can export Dump File analysis

JVM Start the parameter configuration and add the following parameters

• -XX:+HeapDumpOnOutOfMemoryError

• -XX:HeapDumpPath=./（ Parameter is Dump File generation path ）

When JVM happen OOM Exceptions are automatically exported Dump file , The default format of the file name is ：java_pid{pid}.hprof

The above configuration is thrown in the application OOM Automatically export after Dump, Or you can JVM Run time export Dump file

jmap -dump:file=[ File path ] [pid] # Example jmap -dump:file=./jvmdump.hprof 15162

Write a test code locally , To validate the OOM And analysis Dump file

Set up VM Parameters ：-Xms3m -Xmx3m -XX:+ HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=./

public static void main(String[] args) { List<Object> oomList = Lists.newArrayList(); // Create objects in an infinite loop while (true) { oomList.add(new Object()); } }

Learn from the error information ,java heap space Express OOM Occurs in the heap area , And created hprof Binary files are in the current folder

​

JvisualVM analysis

Dump There are many analysis tools , Relatively speaking JvisualVM、JProfiler、Eclipse Mat, Use more people . Let's say JvisualVM Give an example of Dump file

List two common functions , The first is to see the trigger OOM The thread stack of , Clearly know the cause of program overflow

The second is to view JVM Keep the largest object in memory , You can freely choose the number of troubleshooting

​

Clicking an object can also jump to the specific object reference details page

​

In this paper, Dump The file is relatively simple , There are a variety of reasons for mistakes in the formal environment , So it's not right to Dump Deep parsing of files

Be careful ：JvisualVM If the analysis is large Dump file , You may not be able to open it because of insufficient memory , Need to adjust the default memory

Summary and review

If you encounter JVM out of memory , You can check in the following steps

• jmap -heap Check whether the memory allocation is too small

• jmap -histo Check to see if there is an obvious over allocation of objects and no release

• jmap -dump export JVM Current memory snapshot , Use JDK Self contained or MAT And other tools to analyze snapshots

If the problem cannot be located above , Then you need to check whether the application is constantly creating resources , Such as network connection or thread , May lead to the depletion of system resources