JVM tuning notes

java -Djava.security.egd=file:/dev/./urandom  -Xms128m -Xmx128m -XX:MetaspaceSize=128M -XX:MaxMetaspaceSize=128M -XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=/var/log/javadump -jar user.jar &

java
-Djava.security.egd=file:/dev/./urandom
-Xms128m
-Xmx128m
-XX:MetaspaceSize=128M
-XX:MaxMetaspaceSize=128M
-XX:+HeapDumpOnOutOfMemoryError
-XX:HeapDumpPath=/var/log/javadump
-jar user.jar &

1）-Djava.security.egd=file:/dev/./urandom : Non blocking entropy source is adopted (entropy source)
It is related to random number generation ,tomcat In the server sessionId.
/dev/random The device will return random bytes less than the total entropy pool noise .
/dev/random It can generate high randomness public key or one-time codebook . If entropy pool is empty , Yes /dev/random The read operation of will be blocked , Until enough ambient noise is collected
/dev/urandom It's a non blocking generator :
（”unlocked”, Nonblocking random number generator ）, It reuses the data in the entropy pool to produce pseudo-random data . This is right. /dev/urandom The read operation of will not block , But the entropy of its output may be less than /dev/random Of . It can be used as a pseudo-random number generator to generate lower strength passwords , Not recommended for generating high-strength long-term passwords .
But why is the value here in dev and random Add a point between them ？ It's because of one jdk Of bug, In this bug There's feedback from someone in your connection securerandom.source Set to /dev/urandom It's still used /dev/random, There are alternative solutions , One of the workarounds is to securerandom.source Set to /dev/./urandom Talent .
I have seen Jdk Related source code , When java.security.egd The value is file:/dev/urandom perhaps file:/dev/random when , All use blocked entropy sources , So that's why file:/dev/urandom Don't take effect , And a little more in the middle will take effect .

2）java Setting of heap memory in , Generally set to the same size
-Xms128m : The initial heap size is 128M
-Xmx128m : Maximum heap size is 128M

3） Meta space , The memory used is local memory
-XX:MetaspaceSize=128M
-XX:MaxMetaspaceSize=128M : This parameter is used to limit Metaspace The ceiling on growth , To prevent... From being caused by certain circumstances Metaspace Unlimited use of local memory , Affecting other procedures . The default value of this parameter on this machine is 4294967295B（ about 4096MB）.

4） out of memory
-XX:+HeapDumpOnOutOfMemoryError : Generated when memory overflows heapdump file
Parameters ：
-XX:HeapDumpPath=/var/log/javadump ： After memory overflow ,heapdump Where documents are stored

/dev/urandom：
Started TestApplication in 7.388 seconds (JVM running for 8.37)
Started TestApplication in 7.064 seconds (JVM running for 8.06)
Started TestApplication in 7.376 seconds (JVM running for 8.361)
/dev/random
Started TestApplication in 7.144 seconds (JVM running for 8.196)
Started TestApplication in 7.38 seconds (JVM running for 8.372)

5） Other parameters ：

-Xmn:64M: The memory of the new generation is 64M
-XX:SurvivorRatio=2: The proportion ：Eden/From
-XX:NewRatio=2: The proportion ： Old age / The new generation 
-XX:PermSize=10M: Method area （ Forever ） Memory size 
-XX:MaxPermSize=10M: Method area （ Forever ） The maximum memory is 10M
-XX:MaxDirectMemorySize: Direct memory settings , If you don't specify , Default and Java Heap maximum （-Xmx Appoint ） equally 
-XX:+PrintGCDetails: see GC journal 
-XX:+UseSerialGC: Configure the serial recycler 
-XX:+PrintCommandLineFlags: You can output implicit or explicit parameters to the virtual machine