JVM Runtime parameters

1.JVM Parameter options

Official website address ：https://docs.oracle.com/javase/8/docs/technotes/tools/windows/java.html

1.1. Type 1 ： Standard parameter options

> java -help
 usage : java [-options] class [args...]
           ( Executive class )
    or   java [-options] -jar jarfile [args...]
           ( perform  jar  file )
 The options include :
    -d32           Use  32  Bit data model  ( If available )
    -d64           Use  64  Bit data model  ( If available )
    -server        choice  "server" VM
                   Default  VM  yes  server.

    -cp < Contents and  zip/jar  Class search path for file >
    -classpath < Contents and  zip/jar  Class search path for file >
                   use  ;  Separate directories , JAR  archives 
                   and  ZIP  File list ,  Used to search for class files .
    -D< name >=< value >
                   Set system properties 
    -verbose:[class|gc|jni]
                   Enable verbose output 
    -version       Output product version and exit 
    -version:< value >
                   Warning :  This feature is out of date ,  Will be in 
                   Delete... In future releases .
                   The specified version is required to run 
    -showversion   Output the product version and continue 
    -jre-restrict-search | -no-jre-restrict-search
                   Warning :  This feature is out of date ,  Will be in 
                   Delete... In future releases .
                   Include... In version search / Exclusive to users  JRE
    -? -help       Output this help message 
    -X             Output help for non-standard options 
    -ea[:<packagename>...|:<classname>]
    -enableassertions[:<packagename>...|:<classname>]
                   Enable assertions at the specified granularity 
    -da[:<packagename>...|:<classname>]
    -disableassertions[:<packagename>...|:<classname>]
                   Disable assertions with specified granularity 
    -esa | -enablesystemassertions
                   Enable system assertion 
    -dsa | -disablesystemassertions
                   Disable system assertion 
    -agentlib:<libname>[=< Options >]
                   Load native agent Library  <libname>,  for example  -agentlib:hprof
                   See also  -agentlib:jdwp=help  and  -agentlib:hprof=help
    -agentpath:<pathname>[=< Options >]
                   Load the native agent library by the full pathname 
    -javaagent:<jarpath>[=< Options >]
                   load  Java  Programming language agent ,  see also  java.lang.instrument
    -splash:<imagepath>
                   Use the specified image to display the start screen 
 For more information ,  see also  http://www.oracle.com/technetwork/java/javase/documentation/index.html.

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Server Patterns and Client Pattern

Hotspot JVM There are two patterns , Namely server and client, Pass respectively -server and -client Mode setting

• 32 Bit system , By default Client Type of JVM. Want to use Server Pattern , The machine configuration has at least 2 More than one CPU and 2G The physical memory above .client Mode is suitable for desktop applications that require less memory , By default Serial Serial garbage collector
• 64 Bit system , Only support server Mode JVM, For applications that require large memory , Parallel garbage collectors are used by default

Official website address ：https://docs.oracle.com/javase/8/docs/technotes/guides/vm/server-class.html

How to know which mode the system uses by default ？

adopt java -version command ： You can see Server VM word , Representing the current system usage is Server Pattern

> java -version
java version "1.8.0_101"
Java(TM) SE Runtime Environment (build 1.8.0_101-b13)
Java HotSpot(TM) 64-Bit Server VM (build 25.101-b13, mixed mode)

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1.2. Type 2 ：-X Parameter options

> java -X
    -Xmixed            Mixed mode execution  ( Default )
    -Xint              Explain only mode execution 
    -Xbootclasspath:< use  ;  Separate directories and  zip/jar  file >
                       Set search paths to guide classes and resources 
    -Xbootclasspath/a:< use  ;  Separate directories and  zip/jar  file >
                       Attached at the end of the boot class path 
    -Xbootclasspath/p:< use  ;  Separate directories and  zip/jar  file >
                       Before the boot class path 
    -Xdiag             Display additional diagnostic messages 
    -Xnoclassgc        Disable garbage collection 
    -Xincgc            Enable incremental garbage collection 
    -Xloggc:<file>     take  GC  The status is recorded in the file  ( With a time stamp )
    -Xbatch            Disable background compilation 
    -Xms<size>         Set the initial  Java  Heap size 
    -Xmx<size>         Set the maximum  Java  Heap size 
    -Xss<size>         Set up  Java  Thread stack size 
    -Xprof             Output  cpu  Profile data 
    -Xfuture           Enable the most stringent checks ,  Expected future defaults 
    -Xrs               Reduce  Java/VM  Use of operating system signals  ( See documentation )
    -Xcheck:jni        Yes  JNI  Function to perform other checks 
    -Xshare:off        Don't try to use shared class data 
    -Xshare:auto       Use shared class data when possible  ( Default )
    -Xshare:on         Require shared data ,  Otherwise it will fail .
    -XshowSettings     Show all settings and continue 
    -XshowSettings:all
                       Show all settings and continue 
    -XshowSettings:vm  Show all and  vm  Related settings and continue 
    -XshowSettings:properties
                       Show all property settings and continue 
    -XshowSettings:locale
                       Show all locale related settings and continue 

-X  Options are nonstandard options ,  If there is any change ,  Without notice .

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How do you know JVM Mixed mode is used by default ？

similarly , adopt java -version command ： You can see mixed mode word , Representing the current system using mixed mode

1.3. Type 3 ：-XX Parameter options

Boolean Type format

-XX:+<option>   Enable option attribute 
-XX:-<option>   Ban option attribute 

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Not Boolean Type format

-XX:<option>=<number>   Set up option The number , You can bring units like k/K/m/M/g/G
-XX:<option>=<string>   Set up option Character value 

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2. add to JVM Parameter options

eclipse and idea I don't need to say much about the configuration in , stay Run Configurations in VM Options Medium configuration is ok , Be the same in essentials while differing in minor points

function jar package

java -Xms100m -Xmx100m -XX:+PrintGCDetails -XX:+PrintGCDateStamps -XX:+PrintGCTimeStamps -jar demo.jar

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Tomcat function war package

# linux Next catalina.sh add to 
JAVA_OPTS="-Xms512M -Xmx1024M"
# windows Next catalina.bat add to 
set "JAVA_OPTS=-Xms512M -Xmx1024M"

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The program is running

#  Set up Boolean Type parameter 
jinfo -flag [+|-]<name> <pid>
#  Set not to Boolean Type parameter 
jinfo -flag <name>=<value> <pid>

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3. frequently-used JVM Parameter options

3.1. Print settings for XX Options and values

-XX:+PrintCommandLineFlags  Program runtime JVM Default settings or manually set by the user XX Options 
-XX:+PrintFlagsInitial  Print all XX The default value of the option 
-XX:+PrintFlagsFinal  Print all XX The actual value of the option 
-XX:+PrintVMOptions  Print JVM Parameters of 

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3.2. Pile up 、 Stack 、 Method area and other memory size settings

#  Stack 
-Xss128k <==> -XX:ThreadStackSize=128k  Set the thread stack size to 128K

#  Pile up 
-Xms2048m <==> -XX:InitialHeapSize=2048m  Set up JVM The initial heap memory is 2048M
-Xmx2048m <==> -XX:MaxHeapSize=2048m  Set up JVM The maximum heap memory is 2048M
-Xmn2g <==> -XX:NewSize=2g -XX:MaxNewSize=2g  Set the size of the younger generation to 2G
-XX:SurvivorRatio=8  Set up Eden District and Survivor Area ratio , The default is 8
-XX:NewRatio=2  Set the ratio between the old and the young , The default is 2
-XX:+UseAdaptiveSizePolicy  Set size scale adaptive , Default on 
-XX:PretenureSizeThreadshold=1024  The setting allows objects larger than this threshold to be allocated directly to the older generation , Only right Serial、ParNew Collector valid 
-XX:MaxTenuringThreshold=15  Set the age limit for the new generation to promote the old generation , The default is 15
-XX:TargetSurvivorRatio  Set up MinorGC After the end Survivor The expected proportion of space occupied by the zone 

#  Method area 
-XX:MetaspaceSize / -XX:PermSize=256m  Set up the meta space / The initial value of permanent generation is 256M
-XX:MaxMetaspaceSize / -XX:MaxPermSize=256m  Set up the meta space / The maximum value of permanent generation is 256M
-XX:+UseCompressedOops  Use compressed objects 
-XX:+UseCompressedClassPointers  Use compressed class pointers 
-XX:CompressedClassSpaceSize  Set up Klass Metaspace Size , Default 1G

#  Direct memory 
-XX:MaxDirectMemorySize  Appoint DirectMemory Capacity , Default equal to Java Heap maximum 

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3.3. OutOfMemory Related options

-XX:+HeapDumpOnOutMemoryError  Memory appears OOM Time generation Heap Dump files , The two are mutually exclusive 
-XX:+HeapDumpBeforeFullGC  appear FullGC Time generation Heap Dump files , The two are mutually exclusive 
-XX:HeapDumpPath=<path>  Appoint heap The storage path of the dump file , Default current directory 
-XX:OnOutOfMemoryError=<path>  Specify the path to the feasibility program or script , Happen when OOM When the script is executed 

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3.4. Garbage collector related options

First of all, we need to understand the collocation relationship between garbage collectors

• The red dotted line indicates that jdk8 When I was Deprecate,jdk9 Is deleted
• The dotted green line indicates that jdk14 When I was Deprecate
• The empty green box means jdk9 When I was Deprecate,jdk14 Is deleted

# Serial Recyclers 
-XX:+UseSerialGC   The younger generation uses Serial GC,  Used in the old days Serial Old GC
# ParNew Recyclers 
-XX:+UseParNewGC   The younger generation uses ParNew GC
-XX:ParallelGCThreads   Set the number of threads for the younger generation of parallel collectors .
     In a general way , Best and CPU Equal quantity , To avoid the impact of too many threads on garbage collection performance .

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$$ ParallelGCThreads = \begin{cases} CPU_Count & \text (CPU_Count <= 8) \ 3 + (5 * CPU＿Count / 8) & \text (CPU_Count > 8) \end{cases} $$

# Parallel Recyclers 
-XX:+UseParallelGC   The younger generation uses  Parallel Scavenge GC, Activate each other 
-XX:+UseParallelOldGC   Used in the old days  Parallel Old GC, Activate each other 
-XX:ParallelGCThreads
-XX:MaxGCPauseMillis   Set the maximum pause time for the garbage collector （ namely STW Time for ）, In milliseconds .
     In order to control the pause time as much as possible MaxGCPauseMills within , The collector adjusts as it works Java Heap size or some other parameters .
     For users , The shorter the pause, the better the experience ; But the server side focuses on high concurrency , Overall throughput .
     So the server side is suitable for Parallel, Control . This parameter should be used with caution .
-XX:GCTimeRatio   The proportion of garbage collection time to total time （1 / (N＋1)）, Used to measure throughput 
     Value range （0,100）, The default value is 99, That is, the garbage collection time should not exceed 1％.
     With the previous -XX：MaxGCPauseMillis There are some contradictions in the parameters . The longer the pause is ,Radio The parameter is easy to exceed the set ratio .
-XX:+UseAdaptiveSizePolicy   Set up Parallel Scavenge The collector has an adaptive tuning strategy .
     In this mode , The size of the younger generation 、Eden and Survivor The proportion of 、 Parameters such as the age of the person who is promoted to the old age will be automatically adjusted , To reach the heap size 、 The balance between throughput and pause time .
     Where manual tuning is difficult , You can use this adaptive approach directly , Only specify the maximum heap for the virtual machine 、 Target throughput （GCTimeRatio） And pause time （MaxGCPauseMills）, Let the virtual machine do its own tuning .

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# CMS Recyclers 
-XX:+UseConcMarkSweepGC   The younger generation uses CMS GC.
     When this parameter is turned on, the -XX：＋UseParNewGC open . namely ：ParNew（Young District ）+ CMS（Old District ）+ Serial Old The combination of 
-XX:CMSInitiatingOccupanyFraction   Set the threshold for heap memory usage , Once that threshold is reached , And we started recycling .JDK5 The default values for and before are 68,DK6 And above, the default value is 92％.
     If memory growth is slow , You can set a slightly larger value , A large threshold can effectively reduce CMS The trigger frequency of , Reducing the number of recycling in the old age can significantly improve application performance .
     conversely , If application memory usage is growing fast , The threshold should be lowered , To avoid frequent triggering of old age serial collectors .
     Therefore, this option can effectively reduce Fu1l GC Number of executions .
-XX:+UseCMSInitiatingOccupancyOnly   Whether it is dynamically adjustable , send CMS Keep pressing CMSInitiatingOccupancyFraction Set the value to start 
-XX:+UseCMSCompactAtFullCollection   Used to specify at the end of execution Full GC After the memory space compression finishing 
     To avoid memory fragmentation . However, due to memory compression, the grooming process cannot be executed concurrently , The problem is that the pause is getting longer .
-XX:CMSFullGCsBeforeCompaction   Set how many times to execute Full GC After the memory space compression finishing .
-XX:ParallelCMSThreads   Set up CMS Number of threads .
    CMS  The default number of threads to start is (ParallelGCThreads＋3)/4,ParallelGCThreads  Is the number of threads in the younger generation of parallel collectors .
     When CPU  When resources are tight , suffer CMS The impact of collector threads , The performance of an application can be very poor during the garbage collection phase .
-XX:ConcGCThreads   Set the number of threads for concurrent garbage collection , The default value is based on ParallelGCThreads Calculated 
-XX:+CMSScavengeBeforeRemark   mandatory hotspot stay cms remark Do it before the stage minor gc, For improvement remark The speed of the stage 
-XX:+CMSClassUnloadingEnable   If any , Enable recycle Perm  District （JDK8 Before ）
-XX:+CMSParallelInitialEnabled   For opening CMS initial-mark The stage is marked by multi thread 
     Used to increase marking speed , stay Java8 Start has been turned on by default 
-XX:+CMSParallelRemarkEnabled   User on CMS remark The stage is marked again by multi thread , Default on 
-XX:+ExplicitGCInvokesConcurrent
-XX:+ExplicitGCInvokesConcurrentAndUnloadsClasses
     These two parameters are user specified hotspot Virtual reality is performing System.gc() When using CMS cycle 
-XX:+CMSPrecleaningEnabled   Appoint CMS Is it necessary to Pre cleaning Stage 

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# G1 Recyclers 
-XX:+UseG1GC  Manually specify the use of G1 The collector performs the memory reclaim task .
-XX:G1HeapRegionSize  Set each Region Size .
     The value is 2 The power of , The scope is 1MB To 32MB Between , The goal is based on the smallest Java The heap size is divided into about 2048 Regions . The default is heap memory 1/2000.
-XX:MaxGCPauseMillis   Set the maximum expected GC Pause time indicator （JVM I will try my best to achieve , But there is no guarantee that ）. The default value is 200ms
-XX:ParallelGCThread   Set up STW when GC The value of the number of threads . The maximum setting is 8
-XX:ConcGCThreads   Set the number of threads for concurrent tagging . take n Set to the number of parallel garbage collection threads （ParallelGCThreads） Of 1/4 about .
-XX:InitiatingHeapOccupancyPercent  Set trigger concurrency GC Periodic Java Heap occupancy threshold . Beyond this value , It triggers GC. The default value is 45.
-XX:G1NewSizePercent   The minimum percentage of the whole heap memory occupied by the new generation （ Default 5％）
-XX:G1MaxNewSizePercent   The largest percentage of the whole heap memory occupied by the new generation （ Default 60％）
-XX:G1ReservePercent=10   Reserve memory area , prevent  to space（Survivor Medium to District ） overflow 

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How to choose a garbage collector ？

• Give priority to JVM The adaptive , Size the heap
• Serial collector ： Memory less than 100M; Single core 、 Stand alone program , And there is no pause time requirement
• Parallel collector ： many CPU、 High throughput 、 Allow a pause longer than 1 second
• concurrent collector ： many CPU、 Pursue low pause time 、 Respond quickly （ For example, the delay cannot exceed 1 second , Such as Internet applications ）
• The official recommendation G1, High performance . Now the Internet Project , It's basically using G1

In particular ：

• There is no best collector , There is no universal collector
• Tuning is always for specific scenarios 、 Specific needs , There is no collector for once and for all

3.5. GC Log related options

-XX:+PrintGC <==> -verbose:gc   Print brief log information 
-XX:+PrintGCDetails             Print detailed log information 
-XX:+PrintGCTimeStamps   Print program starts to GC When it happened , collocation -XX:+PrintGCDetails Use 
-XX:+PrintGCDateStamps   Print GC Time stamp when it happened , collocation -XX:+PrintGCDetails Use 
-XX:+PrintHeapAtGC   Print GC Front and back heap information , Here's the picture 
-Xloggc:<file>  Output GC In the file under the specified path 

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-XX:+TraceClassLoading   Monitor the loading of classes 
-XX:+PrintGCApplicationStoppedTime   Print GC The pause time of the thread 
-XX:+PrintGCApplicationConcurrentTime   Apply uninterrupted execution time before printing garbage collection 
-XX:+PrintReferenceGC  Print how many different reference types are recycled 
-XX:+PrintTenuringDistribution   Print JVM In every time MinorGC After the current use of Survivor The age distribution of the subjects in the study 
-XX:+UseGCLogFileRotation  Enable GC Automatic dump of log files 
-XX:NumberOfGCLogFiles=1   Set up GC Number of cycles for the log file 
-XX:GCLogFileSize=1M   Set up GC The size of the log file 

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3.6. The other parameters

-XX:+DisableExplicitGC   Ban hotspot perform System.gc(), Default disabled 
-XX:ReservedCodeCacheSize=<n>[g|m|k]、-XX:InitialCodeCacheSize=<n>[g|m|k]   Specify the size of the code cache 
-XX:+UseCodeCacheFlushing   Give up some compiled code , Avoid when the code cache is full JVM Switch to interpreted-only The situation of 
-XX:+DoEscapeAnalysis   Open escape analysis 
-XX:+UseBiasedLocking   Open the bias lock 
-XX:+UseLargePages   Open and use the big page 
-XX:+PrintTLAB   Print TLAB Usage situation 
-XX:TLABSize   Set up TLAB size 

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4. adopt Java Code acquisition JVM Parameters

Java Provides java.lang.management Packages are used to monitor and manage Java Virtual machine and Java Other components in the runtime , It allows local or remote monitoring and management of running Java virtual machine . among ManagementFactory Class is more commonly used , in addition Runtime Class to get memory 、CPU Kernel number and other related data . By using these api, You can monitor the heap memory usage of the application server , Set some thresholds for alarm and other processing .

public class MemoryMonitor {
    public static void main(String[] args) {
        MemoryMXBean memorymbean = ManagementFactory.getMemoryMXBean();
        MemoryUsage usage = memorymbean.getHeapMemoryUsage();
        System.out.println("INIT HEAP: " + usage.getInit() / 1024 / 1024 + "m");
        System.out.println("MAX HEAP: " + usage.getMax() / 1024 / 1024 + "m");
        System.out.println("USE HEAP: " + usage.getUsed() / 1024 / 1024 + "m");
        System.out.println("\nFull Information:");
        System.out.println("Heap Memory Usage: " + memorymbean.getHeapMemoryUsage());
        System.out.println("Non-Heap Memory Usage: " + memorymbean.getNonHeapMemoryUsage());

        System.out.println("======================= adopt java To get the relevant system status ============================ ");
        System.out.println(" Current heap memory size totalMemory " + (int) Runtime.getRuntime().totalMemory() / 1024 / 1024 + "m");//  Current heap memory size 
        System.out.println(" Free heap memory size freeMemory " + (int) Runtime.getRuntime().freeMemory() / 1024 / 1024 + "m");//  Free heap memory size 
        System.out.println(" Maximum available total heap memory maxMemory " + Runtime.getRuntime().maxMemory() / 1024 / 1024 + "m");//  Maximum available total heap memory size 

    }
}

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