GC log analysis

One 、 Log printing

1.JVM Add a parameter in

‐Xloggc:./gc‐%t.log ‐XX:+PrintGCDetails ‐XX:+PrintGCDateStamps ‐XX:+PrintGCTimeStamps ‐XX:+PrintGCCause ‐XX:+UseGCLogFileRotation ‐XX:NumberOfGCLogFiles=10 ‐XX:GCLogFileSize=100M

2.Tomcat Add to JAVA_OPTS variable

Two 、GC Log analysis Demo

A local service , The parameters are as follows ：

java -jar -Xloggc:./gc-%t.log -XX:+PrintGCDetails -XX:+PrintGCDateStamps -XX:+PrintGCCause -XX:+UseGCLogFileRotation -XX:NumberOfGCLogFiles=10 -XX:GCLogFileSize=100M .\microservice-eureka-server.jar

Project start , open the log file , The contents are as follows ：
Configuration parameters of the project
Not only is printing configured here GC journal , And related VM Memory parameters .

GC Situation analysis

2022-06-04T14:42:00.589+0800: 2.829: [Full GC (Metadata GC Threshold) [PSYoungGen: 4997K->0K(129024K)] [ParOldGen: 2890K->7520K(94720K)] 7887K->7520K(223744K), [Metaspace: 20864K->20864K(1069056K)], 0.0381591 secs][Times: user=0.16 sys=0.00, real=0.04 secs] 

1、2.829 ： from jvm Start the calculation to this time GC Time passed
2、Full GC(Metadata GC Threshold)： This is a time full gc, In brackets is gc Why , PSYoungGen It's the younger generation GC, ParOldGen It's from the old days GC,Metaspace It's the of meta space GC.
3、[PSYoungGen: 4997K->0K(129024K)] ： The three numbers correspond to GC It used to occupy the size of the younger generation ,GC Later, the younger generation occupied , And the whole young generation Small .
4、[ParOldGen: 2890K->7520K(94720K)] ： The three numbers correspond to GC It used to occupy the size of the old age ,GC Later, the old age occupied , And the size of the whole old age .
5、7887K->7520K(223744K)： The three numbers correspond to GC The size of the heap memory previously occupied ,GC After that, the heap memory is occupied , And the entire heap memory Size .
6、[Metaspace: 20864K->20864K(1069056K)]： The three numbers correspond to GC The size of the memory that used to occupy meta space ,GC After that, the meta space memory is occupied , With And the size of the entire Metaspace memory .
7、0.0381591 secs： This point in time GC It takes time

Optimize
From the log, you can find fullgc It is caused by insufficient meta space , So we can turn up the meta space .

java -jar -Xloggc:./gc‐adjust-%t.log ‐XX:MetaspaceSize=256M ‐XX:MaxMetaspaceSize=256M -XX:+PrintGCDetails -XX:+PrintGCDateStamps ‐XX:+PrintGCTimeStamps -XX:+PrintGCCause -XX:+UseGCLogFileRotation -XX:NumberOfGCLogFiles=10 -XX:GCLogFileSize=100M .\microservice-eureka-server.jar

Let's take a look after the adjustment gc The log finds that there are no problems caused by insufficient meta space fullgc 了

3、 ... and 、gceasy visualization GC journal

Upload gc The file uses a visual interface to show GC situation
Official website address ：https://gceasy.io/
Put the GC Log file import analysis
1、 You can see the younger generation 、 Old age 、 And the memory allocation and maximum usage of the permanent generation .

2、 You can see that there is GC Before and after changes and other information

Four 、JVM Parameter summary view command

Print out the default values of all parameter options

java -XX:+PrintFlagsInitial

Print out the values of all parameter options that take effect when the program is run

java -XX:+PrintFlagsFinal

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