Let’s get familiar with it today, about JVMCommon tuning some parameters.

Anything starting with

格式：-XX:[+-]<name> 表示啟用或者禁用name屬性。
例子：-XX:+UseG1GC（表示啟用G1垃圾收集器）

View the relevant parameters currently set by JVM:

JVM parameter classification

The parameter type can be distinguished according to the beginning of the JVM parameter, there are three types in total Classes: "-", "-X", "-XX",

Standard parameters (-): all JVM implementations The functions of these parameters must be implemented and backward compatible;

Examples: -verbose:class, -verbose:gc, -verbose:jni... …

Non-standard parameters (-X): The default jvm implements the functions of these parameters, but it does not guarantee that all jvm implementations are satisfied, and backward compatibility is not guaranteed;

Example : Xms20m, -Xmx20m, -Xmn20m, -Xss128k...

Non-Stable parameters (-XX) : Each jvm implementation of such parameters will be different and may be canceled at any time in the future, so you need to use it with caution;

Example: -XX: PrintGCDetails, -XX:-UseParallelGC, -XX: PrintGCTimeStamps......

Heap Parameter settings

-Xms Initial heap size, ms is the abbreviation of memory start, equivalent to -XX:InitialHeapSize-Xmx Maximum heap size, mx is the abbreviation of memory max, equivalent to the parameter -XX:MaxHeapSize

Note: Under normal circumstances, server projects During operation, the heap space will continue to shrink and expand, which will inevitably cause unnecessary system pressure.

So in a production environment, Xms and Xmx of JVM should be set to the same size to avoid GCUnnecessary pressure caused by adjusting the heap size.

-XX:NewSize=n Set the young generation size -XX:NewRatio=n Set the ratio of the young generation to the old generation.

For example: -XX:NewRatio=3, which means that the ratio between the young generation and the old generation is 1:3, and the young generation accounts for the total sum of the young generation and the old generation. 1/4, The default ratio between the new generation and the old generation is 1:2. -XX:SurvivorRatio=n The ratio of the Eden area to the two Survivor areas in the young generation.

Note that there are two Survivor areas, the default is 8, which means: Eden:S0:S1=8:1:1

For example: -XX: SurvivorRatio=3, means Eden: Survivor=3:2, one Survivor area accounts for 1/5 of the entire young generation.

Metaspace parameters

-XX:MetaspaceSize：Metaspace The initial size of the space, if If not set, the default is 20.79M. This initial size is the threshold that triggers the first Metaspace Full GC.

For example: -XX:MetaspaceSize=256M

-XX:MaxMetaspaceSize：Metaspace The maximum value, the default is not to limit the size, but Recommended settings for online environments.

For example: -XX:MaxMetaspaceSize=256M

##-XX:MinMetaspaceFreeRatio: Minimum idle ratio when Metaspace occurs After GC, the free ratio of Metaspace will be calculated. If the free ratio (free space/current Metaspace size) is less than this value, Metaspace expansion will be triggered. The default value is 40, which is 40%, for example -XX:MinMetaspaceFreeRatio=40

: Maximum idle ratio, when GC occurs in Metaspace, The free ratio of Metaspace will be calculated. If the free ratio (free space/current Metaspace size) is greater than this value, Metaspace will be triggered to release the space. The default value is 70, which is 70%, for example -XX:MaxMetaspaceFreeRatio=70<blockquote data-tool="mdnice編輯器" style="border-top: none;border-right: none;border-bottom: none;font-size: 0.9em;overflow: auto;color: rgb(106, 115, 125);padding: 10px 10px 10px 20px;margin-bottom: 20px;margin-top: 20px;border-left-color: rgb(239, 112, 96);background: rgb(255, 249, 249);"><p style="font-size: 16px;padding-top: 8px;padding-bottom: 8px;color: black;line-height: 26px;">It is recommended to set <code style="font-size: 14px;padding: 2px 4px;border-radius: 4px;margin-right: 2px;margin-left: 2px;background-color: rgba(27, 31, 35, 0.05);font-family: "Operator Mono", Consolas, Monaco, Menlo, monospace;word-break: break-all;color: rgb(239, 112, 96);">MetaspaceSize and MaxMetaspaceSize to the same size to avoid frequent expansion.

Stack parameter setting

-Xss: stack space size, the stack is exclusive to the thread, so Is the size of the stack space used by a thread.

For example: -Xss256K, if this parameter is not set, the default value is 1M. Generally speaking, setting it to 256K is enough.

Collector parameter settings

Serial garbage collector (new generation)

Enable: -XX: UseSerialGC Turn off: -XX:-UseSerialGC //The new generation uses Serial and the old generation uses SerialOld

ParNew garbage collector (new generation)

Enable -XX: UseParNewGC Turn off -XX:-UseParNewGC //The new generation uses the function ParNew and the old generation uses the function CMS

Parallel Scavenge collector (new generation)

Turn on -XX: UseParallelOldGC Turn off -XX:-UseParallelOldGC //The new generation uses the function Parallel Scavenge. The old generation will use the Parallel Old collector

ParallelOl garbage collector (old generation)

Enable -XX: UseParallelGC Turn off -XX:-UseParallelGC //The new generation uses the function Parallel Scavenge. The old generation will use the Parallel Old collector

CMS garbage collector (old generation)

Enable -XX: UseConcMarkSweepGC Turn off -XX:-UseConcMarkSweepGC

G1 Garbage Collector

Turn on -XX: UseG1GC Close -XX:-UseG1GC

##GC strategy parameter configuration

-XX:MaxGCPauseMillis

-XX:InitiatingHeapOccupancyPercent=n

-XX:PretenureSizeThreshold=1000000 //

Enter the minimum GC age of the old generation, and the young generation object is converted to the minimum age value of the old generation object. The default value is 7

-XX:InitialTenuringThreshol=7

Upgrade the age of the old generation, the maximum value is 15

-XX:MaxTenuringThreshold

Number of GC parallel execution threads

-XX: ParallelGCThreads=16

Disable System.gc(). Since this method triggers FGC by default and ignores UseG1GC and UseConcMarkSweepGC in the parameters, this method can be disabled if necessary.

-XX:- DisableExplicitGC

Set the throughput size, default is 99

XX:GCTimeRatio

Turn on the adaptive strategy, the ratio of each area, the age of promotion to the old generation and other parameters will be automatically adjusted. To achieve a balance between throughput and pause time.

XX:UseAdaptiveSizePolicy

Set the percentage of GC time occupied by program running time

GCTimeRatio

Dump異?？煺?/span>

-XX:+HeapDumpOnOutOfMemoryError

-XX:HeapDumpPath

堆內(nèi)存出現(xiàn)OOM的概率是所有內(nèi)存耗盡異常中最高的，出錯時的堆內(nèi)信息對解決問題非常有幫助。

所以給JVM設(shè)置這個參數(shù)(-XX:+HeapDumpOnOutOfMemoryError)，讓JVM遇到OOM異常時能輸出堆內(nèi)信息，并通過（-XX:+HeapDumpPath）參數(shù)設(shè)置堆內(nèi)存溢出快照輸出的文件地址。

這對于特別是對相隔數(shù)月才出現(xiàn)的OOM異常尤為重要。

-Xms10M -Xmx10M -Xmn2M -XX:SurvivorRatio=8 -XX:+HeapDumpOnOutOfMemoryError 
-XX:HeapDumpPath=D:\study\log_hprof\gc.hprof

-XX:OnOutOfMemoryError

表示發(fā)生OOM后，運(yùn)行jconsole.exe程序。

這里可以不用加“”，因?yàn)?code style="font-size: 14px;padding: 2px 4px;border-radius: 4px;margin-right: 2px;margin-left: 2px;background-color: rgba(27, 31, 35, 0.05);font-family: "Operator Mono", Consolas, Monaco, Menlo, monospace;word-break: break-all;color: rgb(239, 112, 96);">jconsole.exe路徑Program Files含有空格。利用這個參數(shù)，我們可以在系統(tǒng)OOM后，自定義一個腳本，可以用來發(fā)送郵件告警信息，可以用來重啟系統(tǒng)等等。

-XX:OnOutOfMemoryError="C:\Program Files\Java\jdk1.8.0_151\bin\jconsole.exe"

8G內(nèi)存的服務(wù)器該如何設(shè)置

java -Xmx3550m -Xms3550m -Xss128k -XX:NewRatio=4 -XX:SurvivorRatio=4 -XX:MaxPermSize=16m -XX:MaxTenuringThreshold=0

-Xmx3500m 設(shè)置JVM最大可用內(nèi)存為3550M。

-Xms3500m 設(shè)置JVM初始內(nèi)存為3550m。此值可以設(shè)置與-Xmx相同，以避免每次垃圾回收完成后JVM重新分配內(nèi)存。-Xmn2g 設(shè)置年輕代大小為2G。

整個堆大小=年輕代大小 + 年老代大小 + 方法區(qū)大小

-Xss128k 設(shè)置每個線程的堆棧大小。

JDK1.5以后每個線程堆棧大小為1M，以前每個線程堆棧大小為256K。更具應(yīng)用的線程所需內(nèi)存大小進(jìn)行調(diào)整。在相同物理內(nèi)存下，減小這個值能生成更多的線程。但是操作系統(tǒng)對一個進(jìn)程內(nèi)的線程數(shù)還是有限制的，不能無限生成，經(jīng)驗(yàn)值在3000~5000左右。

-XX:NewRatio=4 設(shè)置年輕代（包括Eden和兩個Survivor區(qū)）與年老代的比值（除去持久代）。設(shè)置為4，則年輕代與年老代所占比值為1：4，年輕代占整個堆棧的1/5 。

-XX:SurvivorRatio=4 設(shè)置年輕代中Eden區(qū)與Survivor區(qū)的大小比值。

設(shè)置為4，則兩個Survivor區(qū)與一個Eden區(qū)的比值為2:4，一個Survivor區(qū)占整個年輕代的1/6 -XX:MaxPermSize=16m 設(shè)置持久代大小為16m。

-XX:MaxTenuringThreshold=0 設(shè)置垃圾最大年齡。

如果設(shè)置為0的話，則年輕代對象不經(jīng)過Survivor區(qū)，直接進(jìn)入年老代。對于年老代比較多的應(yīng)用，可以提高效率。如果將此值設(shè)置為一個較大值，則年輕代對象會在Survivor區(qū)進(jìn)行多次復(fù)制，這樣可以增加對象在年輕代的存活時間，增加在年輕代即被回收的概論。

項(xiàng)目中，GC日志配置

比如，我們啟動一個user-service項(xiàng)目：

 java  
 -XX:+PrintGCDetails -XX:+PrintGCDateStamps 
 -XX:+UseGCLogFileRotation 
 -XX:+PrintHeapAtGC -XX:NumberOfGCLogFiles=5  
 -XX:GCLogFileSize=20M    
 -Xloggc:/opt/user-service-gc-%t.log  
 -jar user-service-1.0-SNAPSHOT.jar

參數(shù)解釋：

 -Xloggc:/opt/app/ard-user/user-service-gc-%t.log   設(shè)置日志目錄和日志名稱
 -XX:+UseGCLogFileRotation           開啟滾動生成日志
 -XX:NumberOfGCLogFiles=5            滾動GC日志文件數(shù)，默認(rèn)0，不滾動
 -XX:GCLogFileSize=20M               GC文件滾動大小，需開啟UseGCLogFileRotation
 -XX:+PrintGCDetails                 開啟記錄GC日志詳細(xì)信息（包括GC類型、各個操作使用的時間）,并且在程序運(yùn)行結(jié)束打印出JVM的內(nèi)存占用情況
 -XX:+ PrintGCDateStamps             記錄系統(tǒng)的GC時間           
 -XX:+PrintGCCause                   產(chǎn)生GC的原因(默認(rèn)開啟)

項(xiàng)目中沒用過怎么辦？

對于很多沒用過的人來說，面試官問項(xiàng)目中這些參數(shù)是怎么用？此時，很容易選擇妥協(xié)，傻傻的回答沒用過。

偷偷的告訴你，很多面試官也沒有用過。

另外，你可以自己搞個小項(xiàng)目，把JVM參數(shù)設(shè)置小點(diǎn)，使用測試工具JMeter，多線程測試一下。