JVM Options

Apr 20, 2017 (updated: May 24, 2017)

1. 1. Options
   1. 1.1. Sizing
   2. 1.2. Performance Options
   3. 1.3. Misc
      1. 1.3.1. Print All Options
      2. 1.3.2. Enable GC Logs
      3. 1.3.3. Alter DNS Cache
      4. 1.3.4. Do Not Wait Antropy For SecurityRandom
      5. 1.3.5. Dump on OOM
      6. 1.3.6. Script on OOM
      7. 1.3.7. Enable JMX
2. 2. Carbage Collector
   1. 2.1. Young Generation Collector
      1. 2.1.1. Copy
      2. 2.1.2. PS Scavenge
      3. 2.1.3. ParNew
      4. 2.1.4. G1
   2. 2.2. Old Generation Collector
      1. 2.2.1. MarkSweepCompact
      2. 2.2.2. PS MarkSweep
      3. 2.2.3. ConcurrentMarkSweep
      4. 2.2.4. G1 Mixed
   3. 2.3. Combinations
   4. 2.4. Example
      1. 2.4.1. Parallel Collector
      2. 2.4.2. CMS Collector
      3. 2.4.3. Phases
      4. 2.4.4. G1 Collector
3. 3. Tools
   1. 3.1. jmap
      1. 3.1.1. Dump The Heap
      2. 3.1.2. Dump The Heap, Live Only
      3. 3.1.3. Print Distribution
   2. 3.2. jstat
      1. 3.2.1. Loaded/Unloaded Classes Info
      2. 3.2.2. Compiler Info
      3. 3.2.3. Capacities (KB) and GC Info
      4. 3.2.4. Capacities (Ratio) and GC Info
      5. 3.2.5. Capacities (KB) Info
   3. 3.3. jstack
   4. 3.4. jinfo

Options

Sizing

Option	Description	Example
-Xms	min heap size	-Xms8G
-Xmx	max heap size	-Xmx8G
-Xmn	min and max young size	-Xmn2G
-Xss	thread stack size, equivalent to -XX:ThreadStackSize	-Xss1M
–XX:NewRatio	young size = heap/(NewRatio + 1), or NewRatio = old / young	default: –XX:NewRatio=2
-XX:NewSize	min young size	-XX:NewSize=3G
-XX:MaxNewSize	max young size	-XX:MaxNewSize=3G
-XX:SurvivorRatio	each survivor size = young/(SurvivorRatio + 2), or SurvivorRatio = eden / each survivor	Default: -XX:SurvivorRatio=8
-XX:PermSize	min PermGen size, Java < 8	-XX:PermSize=512M
-XX:MaxPermSize	max PermGen size, Java < 8	-XX:PermSize=512M
-XX:MaxMetaspaceSize	max metaspace size, Java >= 8	-XX:MaxMetaspaceSize=1G
-XX:ThreadStackSize	thread stack size, equivalent to -Xss	-XX:ThreadStackSize=1M

Notice

• Set -Xms and -Xmx the same value to avoid heap resizing in production

Performance Options

Option	Description
-XX:+UseLargePages	enable use large pages
-XX:LargePageSizeInBytes	large page size

Misc

Print All Options

java -server -XX:+AggressiveOpts -XX:+UnlockDiagnosticVMOptions -XX:+UnlockExperimentalVMOptions -XX:+PrintFlagsFinal -version

Enable GC Logs

-verbose:gc 
-XX:+PrintGCApplicationStoppedTime 
-XX:+PrintGCDateStamps 
-XX:+PrintGCDetails 
-XX:+PrintGCTimeStamps 
-XX:+PrintHeapAtGC 
-XX:+PrintTenuringDistribution 
-Xloggc:../logs/gc-`date +%F_%H-%M-%S`.log 
-XX:+UseGCLogFileRotation 
-XX:NumberOfGCLogFiles=20 
-XX:GCLogFileSize=200M

Alter DNS Cache

-Dsun.net.inetaddr.ttl=<TTL in seconds>

Do Not Wait Antropy For SecurityRandom

-Djava.security.egd=file:/dev/./urandom

Dump on OOM

-XX:+HeapDumpOnOutOfMemoryError 
-XX:HeapDumpPath=../logs/dump-`date +%F_%H-%M-%S`.hprof

Script on OOM

-XX:OnOutOfMemoryError="/path/to/script.sh

Enable JMX

-Djava.rmi.server.hostname=<external IP>
-Dcom.sun.management.jmxremote.port=<port> 
-Dcom.sun.management.jmxremote.authenticate=false 
-Dcom.sun.management.jmxremote.ssl=false

Carbage Collector

Young Generation Collector

Copy

Uses one thread to copy surviving objects from Eden to Survivor spaces and between Survivor spaces.

-XX:+UseSerialGC

PS Scavenge

Like the Copy collector, but uses multiple threads. (With PS Old Collector)

-XX:+UseParallelGC

ParNew

Like the Copy collector, but uses multiple threads. (With Concurrent Old Collector)

-XX:+UseParNewGC

G1

Old is also G1.

-XX:+UseG1GC

Old Generation Collector

MarkSweepCompact

The serial mark-sweep collector, uses one thread full mark-sweep garbage collection algorithm, with optional compaction.

-XX:+UseSerialGC

PS MarkSweep

The parallel scavenge mark-sweep collector, multiple threads version of MarkSweepCompact.

-XX:+UseParallelOldGC

ConcurrentMarkSweep

The CMS collector.

-XX:+UseConcMarkSweepGC

G1 Mixed

The G1 collector.

-XX:+UseG1GC

Combinations

Option	Young	Old
-XX:+UseSerialGC	Copy	MarkSweepCompact
-XX:+UseG1GC	G1 Young	G1 Mixed
-XX:+UseParallelGC	PS Scavenge	PS MarkSweep
-XX:+UseConcMarkSweepGC	ParNew	MarkSweepCompact

Example

Parallel Collector

For backend application, for its high throughput.

-server -Xms4G -Xmx4G

CMS Collector

For frontend application, for its low latency.

-server -Xms6G -Xmx6G -Xmn2G -XX:SurvivorRatio=2 
-XX:+UseConcMarkSweepGC -XX:+CMSParallelRemarkEnabled -XX:+UseCMSInitiatingOccupancyOnly 
-XX:+ScavengeBeforeFullGC -XX:+CMSScavengeBeforeRemark -XX:CMSInitiatingOccupancyFraction=65 
-XX:+ParallelRefProcEnabled -XX:+AlwaysPreTouch

Phases

• Initial Mark: stop-the-world, identify direct reachable objects from GC roots
• Concurrent Mark: mark the objects reachable from initially marked
• Precleaning: mark new objects as live/dead in the OOP table
• Remark: stop-the-world, retrace from GC roots and remark objects as live/dead in the OOP table
• Concurent Sweep: mark dead objects as free space
• Reset: reset the state to get ready for next collection

OOP = Ordinary Object Pointer

G1 Collector

For applications that use big memorys (at least 8G), or you want to play it safe.

-server -Xms12G -Xmx12G -XX:+UseG1GC -XX:MaxGCPauseMillis=100

Tools

jmap

Dump The Heap

jmap -dump:format=b,file=dump.hprof <pid>

Dump The Heap, Live Only

jmap -dump:live,format=b,file=dump.hprof <pid>

Print Distribution

jmap -heap <pid>

jstat

Loaded/Unloaded Classes Info

jstat –class [-h15] <pid> [interval] [count]

Compiler Info

jstat -compiler [-h15] <pid> [interval] [count]

Capacities (KB) and GC Info

jstat -gc [-h15] <pid> [interval] [count]

Capacities (Ratio) and GC Info

jstat -gcutil [-h15] <pid> [interval] [count]

Capacities (KB) Info

jstat -gccapacity [-h15] <pid> [interval] [count]

Column	Description
S0C	Survivor0 space capacity (KB)
S1C	Survivor1 space capacity (KB)
S0U	Survivor0 space utilization (KB)
S1U	Survivor1 space utilization (KB)
EC	Eden space capacity (KB)
EU	Eden space utilization (KB)
OC	Old space capacity (KB)
OU	Old space utilization (KB)
PC	Permanent space capacity (KB)
PU	Permanent space utilization (KB)
YGC	number of Yong GC events
YGCT	used time for Yong GC
FGC	number of Full GC events
FGCT	used time for Full GC
GCT	total used time for GC
MC	Metaspace capacity (KB)
MU	Metacspace utilization (KB)
CCSC	Compressed class space capacity (KB)
CCSU	Compressed class space used (KB)
NGCMN	Young space min capacity (KB)
NGCMX	Young space max capacity (KB)
NGC	Young space current capacity (KB)
OGCMN	Old space min capacity (KB)
OGCMX	Old space max capacity (KB)
OGC	Old space current capacity (KB)

jstack

Print threads information

jstack <pid>

jinfo

Check the option value of a running JVM

jinfo -flag <option> <pid>