[222] memory overflow and location

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Answer section

1. brief introduction

         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 . At this point, the program will not run , The system will prompt for memory overflow , Sometimes the software closes automatically , After restarting the computer or software, release part of the memory and the software can run normally , It is configured by the system 、 Data flow 、 Memory overflow error caused by user code and other reasons , Even if the user re performs the task, it is still unavoidable .

         Memory leak （Memory Leak） It refers to the heap memory that has been dynamically allocated in the program. For some reason, the program is not released or cannot be released , Cause the waste of system memory , Causes the procedure to run the speed to slow down even the system crashes and so on the serious consequence .memory leak Will eventually lead to out of memory！

2. Memory overflow

• tps（ The number of messages processed per second Transaction Per Second） There's a lot of volatility , And slowly lower , Even reduced to 0, The response time fluctuates , Slowly rise

• adopt jstat（JVMStatisticsMonitoringTool Virtual machine statistics monitoring tool ） Command to see ,Jvm in Old The number of districts is increasing ,FullGC Very often , Corresponding FullGC The time consumed is also increasing . Reference resources https://blog.csdn.net/do_finsh/article/details/122609021

  notes ：jstat yes JDK A lightweight gadget that comes with you . Full name “Java Virtual Machine statistics monitoring tool”, It is located in java Of bin Under the table of contents , Mainly used JVM Built in instructions for Java Real time command line monitoring of application resources and performance , Yes, yes. Heap size And garbage collection monitoring .

• adopt jconsole/jvisualvm You can see , The heap memory curve keeps rising , Close to the upper limit , Into a straight line

• Log error java.lang.OutOfMemoryError: Java heap space  Memory gradually rises

3. Location method of memory overflow

• adopt jmap command ：jmap -histo pid | head -20, View the number of instances in the current heap memory and the first instance that occupies the most memory 20 Objects

• adopt jvisualvm, Remote heap dump, And then put dump Download the file , use jvisualvm Open for analysis , You can see more intuitive jvm Information of objects in

4. see JVM Common commands for running status

-gc  Garbage collection statistics 
-gcnew  New generation garbage collection statistics 
-gcold  Old age garbage collection statistics 
-gcutil  Garbage collection statistics ( percentage )


-gccapacity  Heap memory statistics 
-gcmetacapacity  Metadata spatial statistics 
-gcnewcapacity  New generation memory statistics 
-gcoldcapacity  Memory statistics in the old days 
-compiler  Compilation statistics 
-printcompilation JVM Compilation method statistics 




jstat -gc 12141 1000 40   Print gc Information   every other 1000ms  Print 40 Time 
jstat -gcutil 12141 1000 40

a. monitor jvm Of GC situation

#  Check the process number first 
ps -ef|grep java
#  monitor GC situation 
jstat -gcutil pid 1000  100

b. View heap configuration information

#  see java Heap configuration information of the process , Space size and configuration information of each area 
jmap -head pid

c. see jvm Occupation of classes and objects in

jmap -histo pid | head -20

d. Heap file dump

#  The heap memory is modified dump, Save it as a file 
jmap -dump:format=b,file= file name .hdump  pid

5、 The solution can be considered from the following aspects

• increase MaxPermSize Value

• Reduce the number of classes needed by the system

• Use ClassLoader Load each class reasonably , And recycle it regularly

Reference resources ：

https://blog.csdn.net/weixin_46361114/article/details/123175638

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