JVM third talk -- JVM performance tuning practice and high-frequency interview question record

1.JVM Parameters

The basic parameters

java -version/-help

-XX Parameters
Tuning usage , For example, set the heap memory size

-XX:MaxHeapSize=100M
# Parameter abbreviation 
-Xms100M = -XX:InitialHeapSize=100M

1.1 Print JVM Parameter list in

add to JVM Parameters -XX:+PrintFlagsFinal

1.2 Common commands

# View the current Java process 
jps
# View the current JVM Parameters 
jinfo -falgs PID
# See older generation 、 The proportion of the new generation   The default is 2:1
jinfo -flag NewRatio 12228
-XX:NewRatio=2
# see Eden and S The proportion of   Default 8:1:1
jinfo -flag SurvivorRatio 12228
-XX:SurvivorRatio=8
# Check the number of classes loaded   You can print GC Details  -verbose:gc -XX:+PrintGCDetails
jstat -class 12228
Loaded  Bytes  Unloaded  Bytes     Time
16331 30183.4        0     0.0      21.23
# View the current process GC Distribution  
jstat -gc 10076
 S0C    S1C    S0U    S1U      EC       EU        OC         OU       MC     MU    CCSC   CCSU   YGC     YGCT    FGC    FGCT     GCT
32768.0 24064.0  0.0   24041.6 406528.0 100342.3  195584.0   47452.5   83504.0 77530.6 11568.0 10542.4     13    0.530   3      0.474    1.004
# View the stack thread at this time   If there is a deadlock, the final log will also appear 
jstack PID  
# View the details of the current heap memory 
jmap -heap 10076
Attaching to process ID 10076, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.151-b12

using thread-local object allocation.
Parallel GC with 4 thread(s)

Heap Configuration:
   MinHeapFreeRatio         = 0
   MaxHeapFreeRatio         = 100
   MaxHeapSize              = 4206886912 (4012.0MB)
   NewSize                  = 88080384 (84.0MB)
   MaxNewSize               = 1401946112 (1337.0MB)
   OldSize                  = 176160768 (168.0MB)
   NewRatio                 = 2
   SurvivorRatio            = 8
   MetaspaceSize            = 21807104 (20.796875MB)
   CompressedClassSpaceSize = 1073741824 (1024.0MB)
   MaxMetaspaceSize         = 17592186044415 MB
   G1HeapRegionSize         = 0 (0.0MB)

Heap Usage:
PS Young Generation
# You can see Eden Area and S Area did not reach 8:1:1, because JVM It will be dynamically adjusted according to the object size of the current business code Eden、s0、s1 Size 
Eden Space:
   capacity = 416284672 (397.0MB)
   used     = 105738816 (100.84039306640625MB)
   free     = 310545856 (296.15960693359375MB)
   25.40060278750787% used
From Space:
   capacity = 24641536 (23.5MB)
   used     = 24618584 (23.478111267089844MB)
   free     = 22952 (0.02188873291015625MB)
   99.90685645570146% used
To Space:
   capacity = 33554432 (32.0MB)
   used     = 0 (0.0MB)
   free     = 33554432 (32.0MB)
   0.0% used
PS Old Generation
   capacity = 200278016 (191.0MB)
   used     = 48591400 (46.340370178222656MB)
   free     = 151686616 (144.65962982177734MB)
   24.26197391529982% used

You can also directly Java In order to get ManagementFactory.getMemoryPoolMXBeans() Get the above usage

1.3 JVM List of common parameters

Parameters	meaning	explain
-XX: CICompilerCount	Maximum number of parallel compilations	If the setting is greater than 1, Although the compilation speed will increase , But it affects the stability of the system , Will increase JVM Possibility of collapse
-XX:InitialHeapSize=100M	Initial heap size	Abbreviation -Xms 100M
-XX:MaxHeapSize=100M	Maximum heap size	Abbreviation -Xmx 100M
-XX:NewSize=20M	Set the size of the younger generation	The official website suggests not to easily modify the size of the younger generation
-XX:MaxNewSize=20M	Set the maximum size of the younger generation	The official website suggests not to easily modify the size of the younger generation
-XX:OldSize=20M	Set the size of the old age
-XX:MetaspaceSize=20M	Set the size of the method area
-XX:MaxMetaspaceSize=20M	Set the size of the maximum method area
-XX:UseParallelGC	Use ParallelGC	The new generation 、 Throughput priority
-XX:UseParallelOldGC	Use ParallelOldGC	Old age 、 Throughput priority
-XX:UseConcMarkSweepGC	Use CMS	Old age 、 Pause time first
-XX:UseG1GC	Use G1GC	New and old age 、 Pause time first
-XX:NewRatio	The ratio of the new generation to the old generation	for example –XX:NewRatio=3, Then the old age ： The new generation =3:1
-XX:SurvivorRatio	S Area and Eden Area ratio	Default -XX:SurvivorRatio=8,Eden And two S Area is 8:1:1
-XX:HeapDumpOnOutOfMemoryError	Start heap memory overflow printing	When JVM When heap memory overflows , That is to say OOM, Automatic generation dump file
-XX:HeapDumpPath	Specify the print path of heap memory overflow	Generate a in the specified directory heap.hprof file
-XX:PrintGCDetails -XX:PrintGCTimeStamps -XX:PrintGCDateStamps -Xloggc:gc.log	Print GC journal	Compare different garbage collectors to see
-Xss128k	Set the stack size for each thread
-XX:MaxTenuringThreshold=6	Set the maximum critical value of the old age	Generational age defaults 15
-XX:InitiatingHeapOccupancyPercent	Start concurrency GC The proportion of memory used in the cycle	Default 45%,0 Has been performed GC loop
-XX:G1HeapWastePercent	The proportion of waste space in operation	Default 10%, If the mark recyclable space is less than 10% Then set out immediately MixedGC
-XX:MaxGCPauseMillis=300ms	G1 Maximum pause time	The pause time cannot be too small , Otherwise, it will lead to GC Can't keep up with the speed of garbage , Final Full GC

1.4 happen OOM Save OOM journal

# Save heap memory information 
jmap -dump:file=heap.hprot 10076
# View heap memory information   Will start a 7000 port   View directly in the browser 
jhat heap.hprot

The above is instant , Now it needs to happen OOM Save when

# When a service occurs OOM Save the heap memory snapshot file when   You can use tools to view details 
-XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=heap.hprof

2.JVM Heap memory analysis tool

2.1 java bin In the catalog

• jconsole see java Information about the process
  

• jvisualvm Than jconsole More comprehensive display , In tools - Plug in Visual GC, The last article has detailed use , This article does not introduce

• jmc Similar to the above

2.2 GitHub

• arthas Tools developed by Alibaba GitHub

2.3 It's downloaded from the Internet

• MAT be based on heap.hprof File analysis heap GC root Information
• PerfMa online ,https://heaphero.io
• console.perfma.com ditto

3.JVM GC Log analysis tool

# By adding JVM Parameters   take GC Print out the log , The default collector is used Parallel
-XX:MaxHeapSize=100M -XX:+PrintGC -XX:+PrintGCDetails -XX:+PrintGCTimeStamps -XX:+PrintGCDateStamps -Xloggc:gc.log

Log content analysis

# start-up young GC, Reason allocation object failed   Total heap memory 29696K Already used 22897K  After triggering recycling 4086K 98304K Total memory 
2022-07-04T16:58:44.281+0800: 0.714: [GC (Allocation Failure) [PSYoungGen: 22897K->4086K(29696K)] 22897K->15737K(98304K), 0.0290035 secs] [Times: user=0.03 sys=0.02, real=0.03 secs] 

Analysis log tool

• gcviewer Down to the local jar package Analyzing log files
• gceasy online
• Understanding GC Collector log
• Mixed GC : young Areas and parts Old Of GC G1gc Random clearing
• G1GC The log collector focuses on parameters
  • Throughput throughput
  • Min Pause Minimum pause time
  • Max Pause Maximum pause time
  • GC frequency

3.JVM Performance optimization summary

Increase throughput as much as possible , Reduce pause time . You can choose an appropriate way to increase heap memory , Adjust the proportion of new and old age

3.1 Code latitude

• Repeating the declaration of variables will affect CPU The size of the instruction , for example for Variables in the loop
• Try to specify the size of the collection array , Expansion will waste useless space
• Try to lazy load member variables
• Useless variables can only be referenced as null Indicates to be recycled

3.2 JVM Parameter tuning

Commonly used JVM Common parameter tuning official website

1. Increase heap memory

• advantage ： Throughput increases ,GC Fewer times
• shortcoming ： The pause time increases

2. Yes, there is G1GC The pause time set by the collector is special

• advantage ： The pause time is reduced
• shortcoming ： Because of the characteristics of random clearing ,GC More times

3. Start concurrency GC when , Set the memory ratio ( The default is to achieve 45% Usage rate , Just recycle InitiatingHeapOccupancyPercent)
4. G1GC Tips on the official website

• Do not adjust manually young Area size , It will destroy the pause time
• Pause time can be set , But don't be too strict
• Setting parameter description
  • Proportion of heap memory InitiatingHeapOccupancyPercent
  • The percentage of heap memory waste allowed G1HeapWastePercent
  • MixedGC Survival rate G1MixedGCLiveThresholdPercent
  • Number of garbage collection tag threads ConcGCThreads=n

3.3 In the case of large concurrency

OOM The tuning can be completed by the proportion of new and old age
Other common ways

• Application level current limiting
• Load balancing
• Asynchronous message middleware
• Verification Code

3.4 Garbage collector

Choose the right garbage collector , Two dimensions Throughput and pause time

1. CMS

• Mark clear , There will be space debris , Full scan is very time-consuming

2. G1

• Mark clearing is divided into multiple Region District , Give priority to areas with more garbage
• Multi core memory is greater than 6G You can only use
• Pause time can be set manually , There will be no space debris

3. Garbage collector comparison

Collector type	Applicable area	Garbage collection algorithm	stop the world	type	JVM Parameters
Serial	New and old age	Cenozoic logo reproduction Old age mark arrangement	Garbage collection process	Serial ： Suitable for small memory , Single core CPU	-XX:+UseSerialGC
Parallel	New and old age	Cenozoic logo reproduction Old age mark arrangement	Garbage collection process	parallel ： Applicable to scientific calculation , Focus on throughput	-XX:+UseParallelGC
CMS	Old age	Mark clear	Marking and final marking	Concurrent , Applicable to application-side interaction , Pay attention to the pause time	-XX:+UseConcMarkSweepGC
G1	New and old age	Cenozoic logo reproduction Old age mark arrangement	Initial marker , Finally mark , eliminate	Concurrent , Than CMS Less space debris and support the desired pause time	-XX:+UseG1GC

3.5 The difference between memory leak and memory overflow

Memory leak ： There is no way to recycle in time , Non recyclability leads to memory leakage
out of memory ： Non recyclable garbage objects are slowly getting right , Bursting memory leads to memory leakage

3.6 Method area recovery

The method area stores class information , Constants and static variables

• There is no reference to this class in the heap
• Load the ClassLoader It's been recycled

3.7 Are unreachable objects bound to be recycled

Objects that have been finally marked must be recycled , Objects that pass only the initial tag , It may be used again after self rescue , No longer a garbage object

4.JVM Interview record

Frequent GC What do I do ？
YoungGC: Whether it is young There's not enough space in the zone , If it is enough, is it Eden Area and s The area proportion is unevenly distributed
Old GC Space is not enough , The threshold of the elderly generation is too low ,metaspace Is there not enough space ( Stack frame depth ( recursive ))

1. Print GC journal , Take a look with tools gc frequency ,cpu utilization
2. Appropriately increase the space of heap memory
3. Choose the right garbage collector
4. If you are using G1GC Set the appropriate pause time
5. Properly adjust the proportion of triggering heap memory reclamation
6. colony , Asynchronous caching ,MQ Time delay

technological process , surface , Sequence diagram . Difficulties and points needing attention

That's all for this chapter .

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