JVM tuning (a) - the query parameters and troubleshooting

JVM parameter types

Standard parameters

• -help
• -server -client
• -version -showversion
• -cp -classpath

X-parameters

• -Xint: interpreted
• -Xcomp: for the first time compiled to native code
• -Xmixed: Mixed Mode, JVM to decide whether or not compiled to native code

XX parameter

Feature

• Non-standardized parameters
• Relatively unstable
• Mainly for tuning and JVM Debug

classification

• Boolean type

  格式：-XX:[+-]<name>表示启用或禁用name属性
  比如：
  -XX:+UseConcMarkSweepGC
  -XX:+UseG1GC

• Non-Boolean type

  格式：-XX:<name>=<value> 表示name属性的值是value
  比如：
  -XX:MaxGCPauseMilis=500
  XX:GCTimeRatio=19

Examples

• -Xms equivalent to -XX: InitialHeapSize // initialize heap size
• -Xms equivalent to -XX: MaxHeapSize // maximum heap size

JVM run-time parameters View

-XX:+PrintFlagsInitial   //查看初始值
-XX:+PrintFlagsFinal   //查看最终的值
-XX:+UnlockExperimentalVMOptions   //解锁实验参数
-XX:+UnlockDiagnosticVMOptions  //解锁诊断参数
-XX:+PrintCommandLineFlags  //打印命令行参数

PrintFlagsFinal

= Indicates Default

: = The value modified by a user or users JVM

jps

View java process, similar to the Linux ps

jinfo

jinfo -flag MaxHeapSize xxxx(进程号)   //查看最大堆内存

jinfo -flag UseConcMarkSweepGC xxx(进程号)     //查看是否使用了这个GC
jinfo -flag UseG1GC xxx(进程号)    //是否使用了G1回收器
jinfo -flag UseParallelGC xxx(进程号)  //是否使用了并行回收器 

jstat View Virtual Machine statistics

Format:

options:-class,-compiler,-gc,-printcompilation

Class Loader

View class loader information:

jstat -class xxx(进程号)

说明：
3176(进程号)
1000（1000毫秒，没隔1秒）
10（一共输出10次）
3176这个进程，没隔1s输出一次，一共输出10次

Garbage Collection

-gc/-gcutil/-gccause/-gcnew/-gccold

-gc output

S0C / S1C / S0U / S1U: S0 and S1 and the total amount of

EC / EU: total area and the amount of Eden

OC / OU: the total amount of the Old District

MC / MU: total amount region Metaspace

CCSC / CCSU: Compression of the total amount of space

YGC / YGCT: the number and time of youngGC

FGC / FGCT: the number and time of FullGC

GCT: The total GC time

use

jstat -gc xxx(进程号) 1000 10   //动态输出，没隔1s输出一次，一共输出10次

JIT compiler

View JIT compiler information

jstat -compiler PID
jstat -printcompilation PID

jmap + MAT combat memory overflow

jvm memory structure

How to export memory-mapped files

Memory overflow automatically exported

设置两个参数：
-XX:+HeapDumpOnOutOfMemoryError     //开启功能
-XX:HeapDumpPath=./     //导出到什么路径

Use the command to manually export jmap

jmap -dump:format=b,file=help.hprof
format=b 导出的格式是二进制的文件

MAT analysis of memory overflow

• Download MAT tool
• The exported image file into the MAT for analysis

jstack actual infinite loop with deadlock

State of the thread:

1、new

2、running

3、blocked

4、waiting

5、timed_waiting

6、terminated

Actual infinite loop resulting in high cpu Biao

• View cpu load:

top //查看laod average,发现cpu非常高；

• Find high cpu usage processes in the top list
• By jstack PID> xxx.txt, print to file xxx.txt through the process of information jstack
• Internal resources to process threads occupied for analysis:

top -p PID -H   #查看某个进程内部线程占用情况

Found five threads occupied cpu is very high:

• FIG out analysis on the PID (decimal) decimal conversion:

printf "%x" 8247
2037

• In documents before the step in the export of jstack information, search out the last step in the conversion of hexadecimal PID

  0x2037 represent a hexadecimal number: 2037

  After searching out, you can see the thread of the stack information

  

Deadlock cause actual cpu soar

• View project process ID

  ps -ef | grep tomcat

• By jstack PID> xxx.txt, print to file xxx.txt through the process of information jstack

• Pulling the end of the text

The analysis tools can accurately locate the project in the presence of deadlock and location