0. Preface

GC (Garbage Collection) is a very important part of the Java virtual machine. In many cases, we need to view its logs. The following is an introduction to how to view the GC logs.

1. GET GC log

1.1 Output log parameters

To view the GC log, you need to set the parameters of the jvm. There are the following parameters for outputting GC logs

-XX:+PrintGC output GC log

-XX:+PrintGCDetails output detailed GC log

-XX:+PrintGCTimeStamps Print GC timestamps (in the form of base time)

-XX:+PrintGCDateStamps Output GC timestamps (in the form of dates, such as 2013-05-04T21:53:59.234+0800)

-XX:+PrintHeapAtGC Print out heap information before and after GC

-Xloggc:../logs/gc.log Output path to log file

Here I use the parameter PrintGCDetails .

1.2 Write a demo class

Next, I first write a demo class, the code is as follows

package edu.jyu.jvm;
/**
 * GC日志演示
 * @author Jason
 *
 */
public class GCLogDemo {

    public static void main(String[] args) {
        int _1m = 1024 * 1024;
        byte[] data = new byte[_1m];
        // 将data置为null即让它成为垃圾
        data = null;
        // 通知垃圾回收器回收垃圾
        System.gc();
    }
}

1.3 Running

Well, now to run this demo class, but add the virtual machine parameter PrintGCDetails when running, I will run this program in Eclipse and the command line respectively.

1.3.1 Running in Eclipse

First set the Run Configurations
parameter settings
settings as follows

and then click Run to run. After running, the following results are output. These are the GC logs.

[GC (System.gc()) [PSYoungGen: 3686K->664K(38400K)] 3686K->672K(125952K), 0.0016607 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[Full GC (System.gc()) [PSYoungGen: 664K->0K(38400K)] [ParOldGen: 8K->537K(87552K)] 672K->537K(125952K), [Metaspace: 2754K->2754K(1056768K)], 0.0059024 secs] [Times: user=0.00 sys=0.00, real=0.01 secs] 
Heap
 PSYoungGen      total 38400K, used 333K [0x00000000d5c00000, 0x00000000d8680000, 0x0000000100000000)
  eden space 33280K, 1% used [0x00000000d5c00000,0x00000000d5c534a8,0x00000000d7c80000)
  from space 5120K, 0% used [0x00000000d7c80000,0x00000000d7c80000,0x00000000d8180000)
  to   space 5120K, 0% used [0x00000000d8180000,0x00000000d8180000,0x00000000d8680000)
 ParOldGen       total 87552K, used 537K [0x0000000081400000, 0x0000000086980000, 0x00000000d5c00000)
  object space 87552K, 0% used [0x0000000081400000,0x00000000814864a0,0x0000000086980000)
 Metaspace       used 2761K, capacity 4486K, committed 4864K, reserved 1056768K
  class space    used 299K, capacity 386K, committed 512K, reserved 1048576K

1.3.2 Running from the command line

Because Eclipse has compiled the class, so I directly enter the project directory to run.
command line run
output result

[GC (System.gc()) [PSYoungGen: 3702K->784K(38400K)] 3702K->792K(125952K), 0.0014644 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
[Full GC (System.gc()) [PSYoungGen: 784K->0K(38400K)] [ParOldGen: 8K->648K(87552K)] 792K->648K(125952K), [Metaspace: 2763K->2763K(1056768K)], 0.0057136 secs] [Times: user=0.00 sys=0.00, real=0.01 secs]
Heap
 PSYoungGen      total 38400K, used 333K [0x00000000d5c00000, 0x00000000d8680000, 0x0000000100000000)
  eden space 33280K, 1% used [0x00000000d5c00000,0x00000000d5c534a8,0x00000000d7c80000)
  from space 5120K, 0% used [0x00000000d7c80000,0x00000000d7c80000,0x00000000d8180000)
  to   space 5120K, 0% used [0x00000000d8180000,0x00000000d8180000,0x00000000d8680000)
 ParOldGen       total 87552K, used 648K [0x0000000081400000, 0x0000000086980000, 0x00000000d5c00000)
  object space 87552K, 0% used [0x0000000081400000,0x00000000814a2318,0x0000000086980000)
 Metaspace       used 2770K, capacity 4486K, committed 4864K, reserved 1056768K
  class space    used 300K, capacity 386K, committed 512K, reserved 1048576K

2. Analyze logs

Ok, the GC log has been obtained, and now it is analysis. I put the GC log output after Eclipse runs here for analysis.

[GC (System.gc()) [PSYoungGen: 3686K->664K(38400K)] 3686K->672K(125952K), 0.0016607 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 
[Full GC (System.gc()) [PSYoungGen: 664K->0K(38400K)] [ParOldGen: 8K->537K(87552K)] 672K->537K(125952K), [Metaspace: 2754K->2754K(1056768K)], 0.0059024 secs] [Times: user=0.00 sys=0.00, real=0.01 secs] 
Heap
 PSYoungGen      total 38400K, used 333K [0x00000000d5c00000, 0x00000000d8680000, 0x0000000100000000)
  eden space 33280K, 1% used [0x00000000d5c00000,0x00000000d5c534a8,0x00000000d7c80000)
  from space 5120K, 0% used [0x00000000d7c80000,0x00000000d7c80000,0x00000000d8180000)
  to   space 5120K, 0% used [0x00000000d8180000,0x00000000d8180000,0x00000000d8680000)
 ParOldGen       total 87552K, used 537K [0x0000000081400000, 0x0000000086980000, 0x00000000d5c00000)
  object space 87552K, 0% used [0x0000000081400000,0x00000000814864a0,0x0000000086980000)
 Metaspace       used 2761K, capacity 4486K, committed 4864K, reserved 1056768K
  class space    used 299K, capacity 386K, committed 512K, reserved 1048576K

GC日志开头的”[GC”和”[Full GC”说明了这次垃圾收集的停顿类型，如果有”Full”，说明这次GC发生了”Stop-The-World”。因为是调用了System.gc()方法触发的收集，所以会显示”[Full GC (System.gc())”，不然是没有后面的(System.gc())的。
“[PSYoungGen”和”[ParOldGen”是指GC发生的区域，分别代表使用Parallel Scavenge垃圾收集器的新生代和使用Parallel old垃圾收集器的老生代。为什么是这两个垃圾收集器组合呢？因为我的jvm开启的模式是Server，而Server模式的默认垃圾收集器组合便是这个，在命令行输入java -version就可以看到自己的jvm默认开启模式。还有一种是client模式，默认组合是Serial收集器和Serial Old收集器组合。
在方括号中”PSYoungGen:”后面的”3686K->664K(38400K)”代表的是”GC前该内存区域已使用的容量->GC后该内存区域已使用的容量(该内存区域总容量)”
在方括号之外的”3686K->672K(125952K)”代表的是”GC前Java堆已使用容量->GC后Java堆已使用容量(Java堆总容量)”
再往后的”0.0016607 sec”代表该内存区域GC所占用的时间，单位是秒。
再后面的”[Times: user=0.00 sys=0.00, real=0.00 secs]”，user代表进程在用户态消耗的CPU时间，sys代表代表进程在内核态消耗的CPU时间、real代表程序从开始到结束所用的时钟时间。这个时间包括其他进程使用的时间片和进程阻塞的时间（比如等待 I/O 完成）。
至于后面的”eden”代表的是Eden空间，还有”from”和”to”代表的是Survivor空间。

If there is any wrong place in the above content or a bad place, please give me some pointers, so that I can correct it in time.

Copyright statement: This article is an original article by the blogger and may not be reproduced without the blogger's permission. https://blog.csdn.net/TimHeath/article/details/53053106

Article label: jvm gc gc log

Personal classification: jvm

GC log view analysis