Java8 jvm參數

jmap輸出

[tomcat@n01 ~]$ /opt/java/jdk1.8.0_101/bin/jmap -heap 11368   
Attaching to process ID 11368, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.101-b13

using thread-local object allocation.
Parallel GC with 2 thread(s)

Heap Configuration:
   MinHeapFreeRatio         = 0
   MaxHeapFreeRatio         = 100
   MaxHeapSize              = 2684354560 (2560.0MB)
   NewSize                  = 1073741824 (1024.0MB)
   MaxNewSize               = 1073741824 (1024.0MB)
   OldSize                  = 1610612736 (1536.0MB)
   NewRatio                 = 2
   SurvivorRatio            = 8
   MetaspaceSize            = 21807104 (20.796875MB)
   CompressedClassSpaceSize = 1073741824 (1024.0MB)
   MaxMetaspaceSize         = 17592186044415 MB
   G1HeapRegionSize         = 0 (0.0MB)

Heap Usage:
PS Young Generation
Eden Space:
   capacity = 852492288 (813.0MB)
   used     = 420427144 (400.95056915283203MB)
   free     = 432065144 (412.04943084716797MB)
   49.31741317993014% used
From Space:
   capacity = 113770496 (108.5MB)
   used     = 2299712 (2.19317626953125MB)
   free     = 111470784 (106.30682373046875MB)
   2.021360617079493% used
To Space:
   capacity = 107479040 (102.5MB)
   used     = 0 (0.0MB)
   free     = 107479040 (102.5MB)
   0.0% used
PS Old Generation
   capacity = 1610612736 (1536.0MB)
   used     = 50883368 (48.526161193847656MB)
   free     = 1559729368 (1487.4738388061523MB)
   3.1592552860577903% used

27595 interned Strings occupying 3138384 bytes.

MaxHeapFreeRatio: GC後若是發現空閒堆內存佔到整個預估堆內存的N%(百分比)，則收縮堆內存的預估最大值, 預估堆內存是堆大小動態調控的重要選項之一. 堆內存預估最大值必定小於或等於固定最大值(-Xmx指定的數值). 前者會根據使用狀況動態調大或縮小, 以提升GC回收的效率
MinHeapFreeRatio: GC後若是發現空閒堆內存佔到整個預估堆內存的N%(百分比), 則放大堆內存的預估最大值

MaxHeapSize: 即-Xmx, 堆內存大小的上限
InitialHeapSize: 即-Xms, 堆內存大小的初始值

NewSize: 新生代預估堆內存佔用的默認值
MaxNewSize: 新生代佔整個堆內存的最大值

OldSize: 老年代的默認大小, default size of the tenured generation
NewRatio: 老年代對比新生代的空間大小, 好比2表明老年代空間是新生代的兩倍大小. The ratio of old generation to young generation.

SurvivorRatio: Eden/Survivor的值. 這個值的說明, 不少網上轉載的都是錯的. 8表示Survivor:Eden=1:8, 由於survivor區有2個, 因此Eden的佔比爲8/10. Ratio of eden/survivor space size. -XX:SurvivorRatio=6 sets the ratio between each survivor space and eden to be 1:6, each survivor space will be one eighth of the young generation.

MetaspaceSize: 分配給類元數據空間的初始大小(Oracle邏輯存儲上的初始高水位，the initial high-water-mark ). 此值爲估計值. MetaspaceSize設置得過大會延長垃圾回收時間. 垃圾回收事後, 引發下一次垃圾回收的類元數據空間的大小可能會變大
MaxMetaspaceSize: 是分配給類元數據空間的最大值, 超過此值就會觸發Full GC. 此值僅受限於系統內存的大小, JVM會動態地改變此值

CompressedClassSpaceSize: 類指針壓縮空間大小, 默認爲1G

G1HeapRegionSize: G1區塊的大小, 取值爲1M至32M. 其取值是要根據最小Heap大小劃分出2048個區塊. With G1 the Java heap is subdivided into uniformly sized regions. This sets the size of the individual sub-divisions. The default value of this parameter is determined ergonomically based upon heap size. The minimum value is 1Mb and the maximum value is 32Mb. Sets the size of a G1 region. The value will be a power of two and can range from 1MB to 32MB. The goal is to have around 2048 regions based on the minimum Java heap size.

指針壓縮1. 64位平臺上默認打開
  1)使用-XX:+UseCompressedOops壓縮對象指針
    "oops"指的是普通對象指針("ordinary" object pointers)。
    Java堆中對象指針會被壓縮成32位。
    使用堆基地址（若是堆在低26G內存中的話，基地址爲0）
  2)使用-XX:+UseCompressedClassPointers選項來壓縮類指針
    對象中指向類元數據的指針會被壓縮成32位
    類指針壓縮空間會有一個基地址

2. 元空間和類指針壓縮空間的區別
  1)類指針壓縮空間只包含類的元數據，好比InstanceKlass, ArrayKlass
    僅當打開了UseCompressedClassPointers選項才生效
    爲了提升性能，Java中的虛方法表也存放到這裏
    這裏到底存放哪些元數據的類型，目前仍在減小
  2)元空間包含類的其它比較大的元數據，好比方法，字節碼，常量池等。

[tomcat@n01 ~]$ /opt/java/jdk1.8.0_101/bin/jstat -gcutil 11368   
  S0     S1     E      O      M     CCS       YGC   YGCT     FGC    FGCT     GCT   
  2.02   0.00  49.32   3.16  92.13  88.17     24    1.220     3    0.637    1.857

S0: Survivor 0區的空間使用率 Survivor space 0 utilization as a percentage of the space's current capacity.
S1: Survivor 1區的空間使用率 Survivor space 1 utilization as a percentage of the space's current capacity.
E: Eden區的空間使用率 Eden space utilization as a percentage of the space's current capacity.
O: 老年代的空間使用率 Old space utilization as a percentage of the space's current capacity.
M: 元數據的空間使用率 Metaspace utilization as a percentage of the space's current capacity.
CCS: 類指針壓縮空間使用率 Compressed class space utilization as a percentage.
YGC: 新生代GC次數 Number of young generation GC events.
YGCT: 新生代GC總時長 Young generation garbage collection time.
FGC: Full GC次數 Number of full GC events.
FGCT: Full GC總時長 Full garbage collection time.
GCT: 總共的GC時長 Total garbage collection time.

記錄GC日誌

加入參數

-verbose:gc -Xloggc:$CATALINA_BASE/logs/gc.log XX:+PrintGCTimeStamps -XX:+PrintGCDetails

例如

export JAVA_HOME=/opt/java/jdk1.8.0_101
export CATALINA_HOME=/opt/tomcat/apache-tomcat-8.0.36-redis
export CATALINA_BASE=/home/tomcat/tomcat8_jdk8
export JAVA_OPTS="-server -Xms1280m -Xmx1280m -XX:MaxNewSize=896m -Djava.awt.hea
dless=true -verbose:gc -Xloggc:$CATALINA_BASE/logs/gc.log -XX:+PrintGCTimeStamps
 -XX:+PrintGCDetails"
cd $CATALINA_HOME/bin
./catalina.sh start

若是jvm進程不明緣由退出, 能夠查看syslog, 位置是 /var/log/messages

-XX:MaxTenuringThreshold     垃圾最大年齡, 若是設置爲0的話,則年輕代對象不通過Survivor區,直接進入年老代. 對於年老代比較多的應用,能夠提升效率.若是將此值設置爲一個較大值,則年輕代對象會在Survivor區進行屢次複製,這樣能夠增長對象再年輕代的存活 時間,增長在年輕代即被回收的機率. 該參數只有在串行GC時纔有效.