4種GC日志解讀和分析

目標

并行GC演示和分析
串行GC演示和分析
CMSGC演示和分析
G1GC演示和分析
GC 如何選擇

如下是模擬記憶體溢位的Java代碼

案例代碼存在生產垃圾物件，當垃圾物件超過最大堆記憶體時，就造成記憶體溢位

import java.util.Random;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.LongAdder;
/*
演示GC日志生成與解讀
*/
public class GCLogAnalysis {
    private static Random random = new Random();
    public static void main(String[] args) {
        // 當前毫秒時間戳
        long startMillis = System.currentTimeMillis();
        // 持續運行毫秒數; 可根據需要進行修改
        long timeoutMillis = TimeUnit.SECONDS.toMillis(1);
        // 結束時間戳
        long endMillis = startMillis + timeoutMillis;
        LongAdder counter = new LongAdder();
        System.out.println("正在執行...");
        // 快取一部分物件; 進入老年代
        int cacheSize = 2000;
        Object[] cachedGarbage = new Object[cacheSize];
        // 在此時間范圍內,持續回圈
        while (System.currentTimeMillis() < endMillis) {
            // 生成垃圾物件
            Object garbage = generateGarbage(100*1024);
            counter.increment();
            int randomIndex = random.nextInt(2 * cacheSize);
            if (randomIndex < cacheSize) {
                cachedGarbage[randomIndex] = garbage;
            }
        }
        System.out.println("執行結束!共生成物件次數:" + counter.longValue());
    }

    // 生成物件
    private static Object generateGarbage(int max) {
        int randomSize = random.nextInt(max);
        int type = randomSize % 4;
        Object result = null;
        switch (type) {
            case 0:
                result = new int[randomSize];
                break;
            case 1:
                result = new byte[randomSize];
                break;
            case 2:
                result = new double[randomSize];
                break;
            default:
                StringBuilder builder = new StringBuilder();
                String randomString = "randomString-Anything";
                while (builder.length() < randomSize) {
                    builder.append(randomString);
                    builder.append(max);
                    builder.append(randomSize);
                }
                result = builder.toString();
                break;
        }
        return result;
    }
}

演示GC日志生成與解讀

當時在win下編譯的，需要加引數-encoding UTF-8，linux不需要（win默認使用GBDK字符，如果檔案是UTF-8不能編譯）

D:\F盤\JavaCourseCodes\01jvm>javac -encoding UTF-8 GCLogAnalysis.java

執行GCLogAnalysis檔案，列印的GC日志生產到當前目錄下demo.log

D:\F盤\JavaCourseCodes\01jvm>java -Xloggc:gc.demo.log -XX:+PrintGCDetails -XX:+PrintGCDateStamps
-XX:MaxHeapSize=4244185088,如果沒有設定最大堆記憶體，默認是服務器的最大記憶體的1/4大小，默認采用UseParallelGC 并行GC

Java HotSpot(TM) 64-Bit Server VM (25.201-b09) for windows-amd64 JRE (1.8.0_201-b09), built on Dec 15 2018 18:36:39 by "java_re" with MS VC++ 10.0 (VS2010)
Memory: 4k page, physical 16578848k(2260340k free), swap 30350536k(4206764k free)
CommandLine flags: -XX:InitialHeapSize=265261568 -XX:MaxHeapSize=4244185088 -XX:+PrintGC -XX:+PrintGCDateStamps -XX:+PrintGCDetails -XX:+PrintGCTimeStamps -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:-UseLargePagesIndividualAllocation -XX:+UseParallelGC
//-XX:MaxHeapSize=4244185088,如果沒有設定最大堆記憶體，默認是服務器的最大記憶體的1/4大小 ，默認采用UseParallelGC 并行GC

2021-02-23T07:34:15.501+0800: 0.274: [GC (Allocation Failure) [PSYoungGen: 65024K->10747K(75776K)] 65024K->23007K(249344K), 0.0072693 secs] [Times: user=0.03 sys=0.08, real=0.01 secs] 
//65024K->10747K(75776K)] 65024K->23007K(249344K), 0.0072693 secs] :young區從65M >10M，Old區從65M > 23M  ，old區減到23M，說明此時部分物件從young提升到old區
//Times: user=0.03   用戶執行緒耗時  sys=0.08   系統耗時, real=0.01 secs STW暫停時間
2021-02-23T07:34:15.531+0800: 0.304: [GC (Allocation Failure) [PSYoungGen: 75204K->10744K(140800K)] 87463K->42977K(314368K), 0.0097131 secs] [Times: user=0.03 sys=0.05, real=0.01 secs] 
2021-02-23T07:34:15.610+0800: 0.382: [GC (Allocation Failure) [PSYoungGen: 140564K->10747K(140800K)] 172797K->88342K(314368K), 0.0153475 secs] [Times: user=0.03 sys=0.08, real=0.02 secs] 
2021-02-23T07:34:15.664+0800: 0.436: [GC (Allocation Failure) [PSYoungGen: 140795K->10749K(270848K)] 218390K->127729K(444416K), 0.0157620 secs] [Times: user=0.02 sys=0.08, real=0.02 secs] 
2021-02-23T07:34:15.810+0800: 0.583: [GC (Allocation Failure) [PSYoungGen: 270845K->10747K(270848K)] 387825K->204045K(465408K), 0.0291165 secs] [Times: user=0.03 sys=0.19, real=0.03 secs] 
2021-02-23T07:34:15.839+0800: 0.612: [Full GC (Ergonomics) [PSYoungGen: 10747K->0K(270848K)] [ParOldGen: 193297K->170244K(330240K)] 204045K->170244K(601088K), [Metaspace: 2628K->2628K(1056768K)], 0.0301415 secs] [Times: user=0.16 sys=0.00, real=0.03 secs] 
//經過五次youngGC（每次GC耗時遞增），old區越來越大，此時進行fullGC一次，， 0.0301415 secs對比youngGC，fullGC耗時非常多
//fungGC時，young區直接減到0，old區從193M > 170M
2021-02-23T07:34:15.941+0800: 0.713: [GC (Allocation Failure) [PSYoungGen: 260096K->78120K(546816K)] 430340K->248364K(877056K), 0.0249608 secs] [Times: user=0.02 sys=0.06, real=0.03 secs] 
2021-02-23T07:34:16.189+0800: 0.961: [GC (Allocation Failure) [PSYoungGen: 546600K->101880K(588800K)] 716844K->353984K(919040K), 0.0510303 secs] [Times: user=0.11 sys=0.22, real=0.05 secs] 
2021-02-23T07:34:16.240+0800: 1.013: [Full GC (Ergonomics) [PSYoungGen: 101880K->0K(588800K)] [ParOldGen: 252103K->266559K(460800K)] 353984K->266559K(1049600K), [Metaspace: 2628K->2628K(1056768K)], 0.0374054 secs] [Times: user=0.20 sys=0.00, real=0.04 secs] 
2021-02-23T07:34:16.398+0800: 1.171: [GC (Allocation Failure) [PSYoungGen: 486345K->142934K(982016K)] 752905K->409493K(1442816K), 0.0431602 secs] [Times: user=0.05 sys=0.22, real=0.04 secs] 
Heap
 PSYoungGen      total 982016K, used 176414K [0x000000076bb00000, 0x00000007b5680000, 0x00000007c0000000)
  eden space 821760K, 4% used [0x000000076bb00000,0x000000076dbb2160,0x000000079dd80000)
  from space 160256K, 89% used [0x00000007a8e00000,0x00000007b1995908,0x00000007b2a80000)
  to   space 180736K, 0% used [0x000000079dd80000,0x000000079dd80000,0x00000007a8e00000)
 ParOldGen       total 460800K, used 266559K [0x00000006c3000000, 0x00000006df200000, 0x000000076bb00000)
  object space 460800K, 57% used [0x00000006c3000000,0x00000006d344fda0,0x00000006df200000)
 Metaspace       used 2634K, capacity 4486K, committed 4864K, reserved 1056768K
  class space    used 291K, capacity 386K, committed 512K, reserved 1048576K

執行結束!共生成物件次數:8885

設定初始最小和最大對大小一樣時，

共生成物件次數:12035,性能對沒有設定初始化最小堆時提升39%，解讀：Xms設定與-Xmx相同，以避免每次垃圾回收完成后JVM重新分配記憶體

D:\F盤\JavaCourseCodes\01jvm>java  -Xms4g -Xmx4g  -XX:+PrintGCDetails -XX:+PrintGCDateStamps GCLogAnalysis
正在執行...
2021-02-24T06:48:52.516+0800: [GC (Allocation Failure) [PSYoungGen: 1048576K->174589K(1223168K)] 1048576K->235252K(4019712K), 0.0441413 secs] [Times: user=0.02 sys=0.20, real=0.04 secs]
2021-02-24T06:48:52.718+0800: [GC (Allocation Failure) [PSYoungGen: 1223165K->174591K(1223168K)] 1283828K->357504K(4019712K), 0.1024186 secs] [Times: user=0.23 sys=0.52, real=0.10 secs]
2021-02-24T06:48:52.978+0800: [GC (Allocation Failure) [PSYoungGen: 1223167K->174577K(1223168K)] 1406080K->473105K(4019712K), 0.4661695 secs] [Times: user=0.27 sys=2.50, real=0.47 secs]
執行結束!共生成物件次數:12035
Heap
 PSYoungGen      total 1223168K, used 216650K [0x000000076ab00000, 0x00000007c0000000, 0x00000007c0000000)
  eden space 1048576K, 4% used [0x000000076ab00000,0x000000076d416238,0x00000007aab00000)
  from space 174592K, 99% used [0x00000007aab00000,0x00000007b557c750,0x00000007b5580000)
  to   space 174592K, 0% used [0x00000007b5580000,0x00000007b5580000,0x00000007c0000000)
 ParOldGen       total 2796544K, used 298527K [0x00000006c0000000, 0x000000076ab00000, 0x000000076ab00000)
  object space 2796544K, 10% used [0x00000006c0000000,0x00000006d2387f40,0x000000076ab00000)
 Metaspace       used 2634K, capacity 4486K, committed 4864K, reserved 1056768K
  class space    used 291K, capacity 386K, committed 512K, reserved 1048576K

D:\F盤\JavaCourseCodes\01jvm>

圖解說明并行GC的記憶體變化

每次youngGC，young區空間減少很多，Eden和S0全部清空，部分物件復制到S1區，old區越來越大，然后進行fullGC，下一次youngGC會把Eden和S1清空，

延伸問題：年輕代和老年代的垃圾回收都會觸發STW 事件，在年輕代使用標記-復制（mark-copy）演算法，在老年代使用標記-清除-整理（mark-sweepcompact）演算法，-XX：ParallelGCThreads=N 來指定GC 執行緒數，其默認值為CPU 核心數，

fullGC，會把Eden ，S0，S1區全部清空
默認yungGC次數15次，超過15次還存活的物件采用移動方式晉升到老年代區，老年代能減少些但不多

串行GC演示（ -XX:+UseSerialGC）

當前最大堆記憶體512M

D:\F盤\JavaCourseCodes\01jvm>java -XX:+UseSerialGC -Xms512m -Xmx512m  -XX:+PrintGCDetails -XX:+PrintGCDateStamps GCLogAnalysis
正在執行...
2021-02-23T22:35:13.453+0800: [GC (Allocation Failure) 2021-02-23T22:35:13.453+0800: [DefNew: 139776K->17472K(157248K), 0.0324775 secs] 139776K->48832K(506816K), 0.0330806 secs] [Times: user=0.03 sys=0.00, real=0.03 secs]
2021-02-23T22:35:13.521+0800: [GC (Allocation Failure) 2021-02-23T22:35:13.521+0800: [DefNew: 157127K->17471K(157248K), 0.0427665 secs] 188488K->98605K(506816K), 0.0434445 secs] [Times: user=0.03 sys=0.00, real=0.04 secs]
2021-02-23T22:35:13.607+0800: [GC (Allocation Failure) 2021-02-23T22:35:13.608+0800: [DefNew: 157247K->17471K(157248K), 0.0323721 secs] 238381K->140525K(506816K), 0.0331904 secs] [Times: user=0.03 sys=0.00, real=0.03 secs]
2021-02-23T22:35:13.682+0800: [GC (Allocation Failure) 2021-02-23T22:35:13.682+0800: [DefNew: 157247K->17471K(157248K), 0.0337171 secs] 280301K->184841K(506816K), 0.0344639 secs] [Times: user=0.02 sys=0.01, real=0.03 secs]
2021-02-23T22:35:13.757+0800: [GC (Allocation Failure) 2021-02-23T22:35:13.758+0800: [DefNew: 156573K->17472K(157248K), 0.0318421 secs] 323942K->226784K(506816K), 0.0327063 secs] [Times: user=0.01 sys=0.02, real=0.03 secs]
2021-02-23T22:35:13.831+0800: [GC (Allocation Failure) 2021-02-23T22:35:13.832+0800: [DefNew: 157248K->17471K(157248K), 0.0357931 secs] 366560K->273563K(506816K), 0.0364656 secs] [Times: user=0.02 sys=0.02, real=0.04 secs]
2021-02-23T22:35:13.909+0800: [GC (Allocation Failure) 2021-02-23T22:35:13.910+0800: [DefNew: 157098K->17471K(157248K), 0.0321501 secs] 413190K->312630K(506816K), 0.0330849 secs] [Times: user=0.02 sys=0.02, real=0.03 secs]
2021-02-23T22:35:13.981+0800: [GC (Allocation Failure) 2021-02-23T22:35:13.981+0800: [DefNew: 157247K->17470K(157248K), 0.0342253 secs] 452406K->354246K(506816K), 0.0349803 secs] [Times: user=0.02 sys=0.02, real=0.04 secs]
2021-02-23T22:35:14.053+0800: [GC (Allocation Failure) 2021-02-23T22:35:14.054+0800: [DefNew: 157246K->157246K(157248K), 0.0002210 secs]2021-02-23T22:35:14.054+0800: [Tenured: 336776K->273961K(349568K), 0.0635833 secs] 494022K->273961K(506816K), [Metaspace: 2628K->2628K(1056768K)], 0.0645362 secs] [Times: user=0.06 sys=0.00, real=0.07 secs]
2021-02-23T22:35:14.155+0800: [GC (Allocation Failure) 2021-02-23T22:35:14.155+0800: [DefNew: 139776K->17471K(157248K), 0.0137345 secs] 413737K->324486K(506816K), 0.0144490 secs] [Times: user=0.01 sys=0.00, real=0.02 secs]
2021-02-23T22:35:14.207+0800: [GC (Allocation Failure) 2021-02-23T22:35:14.207+0800: [DefNew: 157247K->157247K(157248K), 0.0002225 secs]2021-02-23T22:35:14.207+0800: [Tenured: 307014K->301297K(349568K), 0.0715632 secs] 464262K->301297K(506816K), [Metaspace: 2628K->2628K(1056768K)], 0.0724522 secs] [Times: user=0.06 sys=0.00, real=0.07 secs]
執行結束!共生成物件次數:6288
Heap
 def new generation   total 157248K, used 126441K [0x00000000e0000000, 0x00000000eaaa0000, 0x00000000eaaa0000)
  eden space 139776K,  90% used [0x00000000e0000000, 0x00000000e7b7a608, 0x00000000e8880000)
  from space 17472K,   0% used [0x00000000e9990000, 0x00000000e9990000, 0x00000000eaaa0000)
  to   space 17472K,   0% used [0x00000000e8880000, 0x00000000e8880000, 0x00000000e9990000)
 tenured generation   total 349568K, used 301297K [0x00000000eaaa0000, 0x0000000100000000, 0x0000000100000000)
   the space 349568K,  86% used [0x00000000eaaa0000, 0x00000000fd0dc620, 0x00000000fd0dc800, 0x0000000100000000)
 Metaspace       used 2634K, capacity 4486K, committed 4864K, reserved 1056768K
  class space    used 291K, capacity 386K, committed 512K, reserved 1048576K

D:\F盤\JavaCourseCodes\01jvm>

當前最大堆記憶體128M時，出現記憶體溢位例外了

fullGC對垃圾回收幾乎沒有作用，垃圾一直占的記憶體空間

D:\F盤\JavaCourseCodes\01jvm>java -XX:+UseSerialGC -Xms128m -Xmx128m  -XX:+PrintGCDetails -XX:+PrintGCDateStamps GCLogAnalysis
正在執行...
2021-02-23T22:42:07.153+0800: [GC (Allocation Failure) 2021-02-23T22:42:07.154+0800: [DefNew: 34915K->4351K(39296K), 0.0105513 secs] 34915K->12378K(126720K), 0.0115875 secs] [Times: user=0.02 sys=0.00, real=0.01 secs]
2021-02-23T22:42:07.182+0800: [GC (Allocation Failure) 2021-02-23T22:42:07.182+0800: [DefNew: 39197K->4349K(39296K), 0.0123236 secs] 47223K->22965K(126720K), 0.0138073 secs] [Times: user=0.00 sys=0.02, real=0.01 secs]
2021-02-23T22:42:07.213+0800: [GC (Allocation Failure) 2021-02-23T22:42:07.214+0800: [DefNew: 38618K->4343K(39296K), 0.0091259 secs] 57234K->33897K(126720K), 0.0100570 secs] [Times: user=0.01 sys=0.02, real=0.01 secs]
2021-02-23T22:42:07.237+0800: [GC (Allocation Failure) 2021-02-23T22:42:07.238+0800: [DefNew: 39146K->4351K(39296K), 0.0104840 secs] 68701K->45751K(126720K), 0.0111278 secs] [Times: user=0.00 sys=0.00, real=0.01 secs]
2021-02-23T22:42:07.265+0800: [GC (Allocation Failure) 2021-02-23T22:42:07.266+0800: [DefNew: 38929K->4338K(39296K), 0.0086908 secs] 80329K->55306K(126720K), 0.0096184 secs] [Times: user=0.02 sys=0.00, real=0.01 secs]
2021-02-23T22:42:07.288+0800: [GC (Allocation Failure) 2021-02-23T22:42:07.288+0800: [DefNew: 39269K->4348K(39296K), 0.0137101 secs] 90238K->68836K(126720K), 0.0144757 secs] [Times: user=0.00 sys=0.02, real=0.01 secs]
2021-02-23T22:42:07.313+0800: [GC (Allocation Failure) 2021-02-23T22:42:07.313+0800: [DefNew: 39292K->4349K(39296K), 0.0142091 secs] 103780K->83540K(126720K), 0.0150465 secs] [Times: user=0.02 sys=0.00, real=0.02 secs]
2021-02-23T22:42:07.344+0800: [GC (Allocation Failure) 2021-02-23T22:42:07.350+0800: [DefNew: 39008K->39008K(39296K), 0.0011520 secs]2021-02-23T22:42:07.352+0800: [Tenured: 79190K->87419K(87424K), 0.0237885 secs] 118199K->90743K(126720K), [Metaspace: 2628K->2628K(1056768K)], 0.0361671 secs] [Times: user=0.02 sys=0.00, real=0.04 secs]
2021-02-23T22:42:07.394+0800: [Full GC (Allocation Failure) 2021-02-23T22:42:07.396+0800: [Tenured: 87419K->87397K(87424K), 0.0191946 secs] 126695K->99685K(126720K), [Metaspace: 2628K->2628K(1056768K)], 0.0212197 secs] [Times: user=0.01 sys=0.00, real=0.02 secs]
2021-02-23T22:42:07.429+0800: [Full GC (Allocation Failure) 2021-02-23T22:42:07.429+0800: [Tenured: 87397K->87232K(87424K), 0.0221254 secs] 126683K->106532K(126720K), [Metaspace: 2628K->2628K(1056768K)], 0.0232227 secs] [Times: user=0.01 sys=0.00, real=0.02 secs]
2021-02-23T22:42:07.462+0800: [Full GC (Allocation Failure) 2021-02-23T22:42:07.463+0800: [Tenured: 87304K->87268K(87424K), 0.0306981 secs] 126540K->108960K(126720K), [Metaspace: 2628K->2628K(1056768K)], 0.0328040 secs] [Times: user=0.03 sys=0.00, real=0.03 secs]
2021-02-23T22:42:07.504+0800: [Full GC (Allocation Failure) 2021-02-23T22:42:07.506+0800: [Tenured: 87268K->87268K(87424K), 0.0079315 secs] 126362K->113405K(126720K), [Metaspace: 2628K->2628K(1056768K)], 0.0106992 secs] [Times: user=0.00 sys=0.02, real=0.01 secs]
2021-02-23T22:42:07.520+0800: [Full GC (Allocation Failure) 2021-02-23T22:42:07.529+0800: [Tenured: 87268K->87268K(87424K), 0.0050639 secs] 126526K->116875K(126720K), [Metaspace: 2628K->2628K(1056768K)], 0.0144243 secs] [Times: user=0.02 sys=0.00, real=0.02 secs]
2021-02-23T22:42:07.541+0800: [Full GC (Allocation Failure) 2021-02-23T22:42:07.550+0800: [Tenured: 87268K->87398K(87424K), 0.0116193 secs] 126410K->118705K(126720K), [Metaspace: 2628K->2628K(1056768K)], 0.0215769 secs] [Times: user=0.00 sys=0.00, real=0.02 secs]
2021-02-23T22:42:07.569+0800: [Full GC (Allocation Failure) 2021-02-23T22:42:07.574+0800: [Tenured: 87398K->87249K(87424K), 0.0315818 secs] 126427K->116175K(126720K), [Metaspace: 2628K->2628K(1056768K)], 0.0369570 secs] [Times: user=0.03 sys=0.00, real=0.04 secs]
2021-02-23T22:42:07.613+0800: [Full GC (Allocation Failure) 2021-02-23T22:42:07.616+0800: [Tenured: 87249K->87249K(87424K), 0.0091864 secs] 126101K->119359K(126720K), [Metaspace: 2628K->2628K(1056768K)], 0.0126861 secs] [Times: user=0.00 sys=0.00, real=0.01 secs]
2021-02-23T22:42:07.634+0800: [Full GC (Allocation Failure) 2021-02-23T22:42:07.636+0800: [Tenured: 87249K->87249K(87424K), 0.0047839 secs] 126005K->121203K(126720K), [Metaspace: 2628K->2628K(1056768K)], 0.0077264 secs] [Times: user=0.00 sys=0.00, real=0.01 secs]
2021-02-23T22:42:07.651+0800: [Full GC (Allocation Failure) 2021-02-23T22:42:07.651+0800: [Tenured: 87249K->87249K(87424K), 0.0049760 secs] 126470K->123275K(126720K), [Metaspace: 2628K->2628K(1056768K)], 0.0061458 secs] [Times: user=0.00 sys=0.00, real=0.01 secs]
2021-02-23T22:42:07.669+0800: [Full GC (Allocation Failure) 2021-02-23T22:42:07.680+0800: [Tenured: 87357K->87423K(87424K), 0.0342295 secs] 126618K->121943K(126720K), [Metaspace: 2628K->2628K(1056768K)], 0.0459635 secs] [Times: user=0.03 sys=0.00, real=0.05 secs]
2021-02-23T22:42:07.717+0800: [Full GC (Allocation Failure) 2021-02-23T22:42:07.717+0800: [Tenured: 87423K->87423K(87424K), 0.0109540 secs] 126119K->123382K(126720K), [Metaspace: 2628K->2628K(1056768K)], 0.0122313 secs] [Times: user=0.02 sys=0.00, real=0.01 secs]
2021-02-23T22:42:07.737+0800: [Full GC (Allocation Failure) 2021-02-23T22:42:07.742+0800: [Tenured: 87423K->87423K(87424K), 0.0046415 secs] 126694K->124383K(126720K), [Metaspace: 2628K->2628K(1056768K)], 0.0101329 secs] [Times: user=0.00 sys=0.00, real=0.01 secs]
2021-02-23T22:42:07.753+0800: [Full GC (Allocation Failure) 2021-02-23T22:42:07.768+0800: [Tenured: 87423K->87423K(87424K), 0.0114733 secs] 126437K->124440K(126720K), [Metaspace: 2628K->2628K(1056768K)], 0.0268546 secs] [Times: user=0.00 sys=0.00, real=0.03 secs]
2021-02-23T22:42:07.786+0800: [Full GC (Allocation Failure) 2021-02-23T22:42:07.794+0800: [Tenured: 87423K->87362K(87424K), 0.0319215 secs] 126709K->124372K(126720K), [Metaspace: 2628K->2628K(1056768K)], 0.0403032 secs] [Times: user=0.03 sys=0.00, real=0.04 secs]
2021-02-23T22:42:07.831+0800: [Full GC (Allocation Failure) 2021-02-23T22:42:07.832+0800: [Tenured: 87362K->87362K(87424K), 0.0037068 secs] 126024K->125099K(126720K), [Metaspace: 2628K->2628K(1056768K)], 0.0073521 secs] [Times: user=0.00 sys=0.00, real=0.01 secs]
2021-02-23T22:42:07.840+0800: [Full GC (Allocation Failure) 2021-02-23T22:42:07.842+0800: [Tenured: 87362K->87362K(87424K), 0.0046366 secs] 125758K->125758K(126720K), [Metaspace: 2628K->2628K(1056768K)], 0.0095181 secs] [Times: user=0.00 sys=0.00, real=0.01 secs]
2021-02-23T22:42:07.851+0800: [Full GC (Allocation Failure) 2021-02-23T22:42:07.866+0800: [Tenured: 87362K->87279K(87424K), 0.0246226 secs] 125758K->125675K(126720K), [Metaspace: 2628K->2628K(1056768K)], 0.0399895 secs] [Times: user=0.01 sys=0.00, real=0.04 secs]
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
        at GCLogAnalysis.generateGarbage(GCLogAnalysis.java:48)
        at GCLogAnalysis.main(GCLogAnalysis.java:25)
Heap
 def new generation   total 39296K, used 38845K [0x00000000f8000000, 0x00000000faaa0000, 0x00000000faaa0000)
  eden space 34944K, 100% used [0x00000000f8000000, 0x00000000fa220000, 0x00000000fa220000)
  from space 4352K,  89% used [0x00000000fa660000, 0x00000000faa2f408, 0x00000000faaa0000)
  to   space 4352K,   0% used [0x00000000fa220000, 0x00000000fa220000, 0x00000000fa660000)
 tenured generation   total 87424K, used 87279K [0x00000000faaa0000, 0x0000000100000000, 0x0000000100000000)
   the space 87424K,  99% used [0x00000000faaa0000, 0x00000000fffdbe90, 0x00000000fffdc000, 0x0000000100000000)
 Metaspace       used 2658K, capacity 4486K, committed 4864K, reserved 1056768K
  class space    used 294K, capacity 386K, committed 512K, reserved 1048576K

D:\F盤\JavaCourseCodes\01jvm>

當前最大堆記憶體2048M時，

此時GC次數只有兩次了，但是GC耗時很久，對比設定最大堆記憶體512M時產生的物件減少了20%，也就是說性能反而較少，Xmx不是越大越好

D:\F盤\JavaCourseCodes\01jvm>java -XX:+UseSerialGC -Xms2048m -Xmx2048m  -XX:+PrintGCDetails -XX:+PrintGCDateStamps GCLogAnalysis
正在執行...
2021-02-23T22:47:56.035+0800: [GC (Allocation Failure) 2021-02-23T22:47:56.035+0800: [DefNew: 559232K->69888K(629120K), 0.1193025 secs] 559232K->157347K(2027264K), 0.1198591 secs] [Times: user=0.06 sys=0.05, real=0.12 secs]
2021-02-23T22:47:56.304+0800: [GC (Allocation Failure) 2021-02-23T22:47:56.304+0800: [DefNew: 629120K->69888K(629120K), 0.1559310 secs] 716579K->289236K(2027264K), 0.1568711 secs] [Times: user=0.09 sys=0.06, real=0.16 secs]
執行結束!共生成物件次數:4917
Heap
 def new generation   total 629120K, used 291713K [0x0000000080000000, 0x00000000aaaa0000, 0x00000000aaaa0000)
  eden space 559232K,  39% used [0x0000000080000000, 0x000000008d8a07a8, 0x00000000a2220000)
  from space 69888K, 100% used [0x00000000a2220000, 0x00000000a6660000, 0x00000000a6660000)
  to   space 69888K,   0% used [0x00000000a6660000, 0x00000000a6660000, 0x00000000aaaa0000)
 tenured generation   total 1398144K, used 219348K [0x00000000aaaa0000, 0x0000000100000000, 0x0000000100000000)
   the space 1398144K,  15% used [0x00000000aaaa0000, 0x00000000b80d53e8, 0x00000000b80d5400, 0x0000000100000000)
 Metaspace       used 2634K, capacity 4486K, committed 4864K, reserved 1056768K
  class space    used 291K, capacity 386K, committed 512K, reserved 1048576K

D:\F盤\JavaCourseCodes\01jvm>

CMSGC演示（ -XX:+UseConcMarkSweepGC）

當前最大堆記憶體512M

當發生5五次youngGC后，就進行CMSGC，

[GC (CMS Initial Mark) [1 CMS-initial-mark: 207528K(349568K)] 225479K(506816K), 0.0009352 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] Initial Mark(初始標記)：當前步驟需要JVM暫停STW，標記的根物件GC root
[CMS-concurrent-mark-start] Concurrent Mark(并發標記)
[CMS-concurrent-mark: 0.002/0.002 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] Concurrent Preclean(并發預清理)
[CMS-concurrent-preclean-start] Final Remark(最終標記)
[CMS-concurrent-preclean: 0.001/0.001 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] Concurrent Sweep(并發清除)
[CMS-concurrent-abortable-preclean-start] Concurrent Reset(并發重置)
一共6個步驟，5個步驟是并發執行,CMS 使用的并發執行緒數等于CPU 核心數的1/4

CMS GC 的設計目標是避免在老年代垃圾收集時出現長時間的卡頓

D:\F盤\JavaCourseCodes\01jvm>java -XX:+UseConcMarkSweepGC  -Xms512m -Xmx512m  -XX:+PrintGCDetails -XX:+PrintGCDateStamps GCLogAnalysis
正在執行...
2021-02-24T07:02:07.839+0800: [GC (Allocation Failure) 2021-02-24T07:02:07.840+0800: [ParNew: 139776K->17468K(157248K), 0.0079185 secs] 139776K->44649K(506816K), 0.0085489 secs] [Times: user=0.00 sys=0.00, real=0.01 secs]
2021-02-24T07:02:07.879+0800: [GC (Allocation Failure) 2021-02-24T07:02:07.879+0800: [ParNew: 157199K->17459K(157248K), 0.0130003 secs] 184380K->97930K(506816K), 0.0133819 secs] [Times: user=0.03 sys=0.09, real=0.01 secs]
2021-02-24T07:02:07.923+0800: [GC (Allocation Failure) 2021-02-24T07:02:07.923+0800: [ParNew: 157235K->17472K(157248K), 0.0219433 secs] 237706K->138548K(506816K), 0.0223873 secs] [Times: user=0.08 sys=0.02, real=0.02 secs]
2021-02-24T07:02:07.972+0800: [GC (Allocation Failure) 2021-02-24T07:02:07.972+0800: [ParNew: 157248K->17472K(157248K), 0.0225044 secs] 278324K->181476K(506816K), 0.0231603 secs] [Times: user=0.09 sys=0.02, real=0.02 secs]
2021-02-24T07:02:08.023+0800: [GC (Allocation Failure) 2021-02-24T07:02:08.024+0800: [ParNew: 156802K->17471K(157248K), 0.0235213 secs] 320806K->225000K(506816K), 0.0239944 secs] [Times: user=0.17 sys=0.03, real=0.02 secs]
2021-02-24T07:02:08.049+0800: [GC (CMS Initial Mark) [1 CMS-initial-mark: 207528K(349568K)] 225479K(506816K), 0.0009352 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
2021-02-24T07:02:08.050+0800: [CMS-concurrent-mark-start]
2021-02-24T07:02:08.052+0800: [CMS-concurrent-mark: 0.002/0.002 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
2021-02-24T07:02:08.053+0800: [CMS-concurrent-preclean-start]
2021-02-24T07:02:08.055+0800: [CMS-concurrent-preclean: 0.001/0.001 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
2021-02-24T07:02:08.055+0800: [CMS-concurrent-abortable-preclean-start]
2021-02-24T07:02:08.082+0800: [GC (Allocation Failure) 2021-02-24T07:02:08.083+0800: [ParNew: 157247K->17471K(157248K), 0.0207909 secs] 364776K->262029K(506816K), 0.0213737 secs] [Times: user=0.09 sys=0.00, real=0.02 secs]
2021-02-24T07:02:08.130+0800: [GC (Allocation Failure) 2021-02-24T07:02:08.131+0800: [ParNew: 157247K->17470K(157248K), 0.0221870 secs] 401805K->300663K(506816K), 0.0227283 secs] [Times: user=0.17 sys=0.02, real=0.02 secs]
2021-02-24T07:02:08.176+0800: [GC (Allocation Failure) 2021-02-24T07:02:08.176+0800: [ParNew: 157246K->17471K(157248K), 0.0268265 secs] 440439K->348831K(506816K), 0.0273241 secs] [Times: user=0.17 sys=0.01, real=0.03 secs]
2021-02-24T07:02:08.205+0800: [CMS-concurrent-abortable-preclean: 0.006/0.146 secs] [Times: user=0.51 sys=0.03, real=0.15 secs]
2021-02-24T07:02:08.206+0800: [GC (CMS Final Remark) [YG occupancy: 25957 K (157248 K)]2021-02-24T07:02:08.207+0800: [Rescan (parallel) , 0.0004543 secs]2021-02-24T07:02:08.207+0800: [weak refs processing, 0.0000867 secs]2021-02-24T07:02:08.207+0800: [class unloading, 0.0008495 secs]2021-02-24T07:02:08.208+0800: [scrub symbol table, 0.0004637 secs]2021-02-24T07:02:08.209+0800: [scrub string table, 0.0002037 secs][1 CMS-remark: 331359K(349568K)] 357316K(506816K), 0.0035973 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
2021-02-24T07:02:08.210+0800: [CMS-concurrent-sweep-start]
2021-02-24T07:02:08.211+0800: [CMS-concurrent-sweep: 0.001/0.001 secs] [Times: user=0.02 sys=0.00, real=0.00 secs]
2021-02-24T07:02:08.214+0800: [CMS-concurrent-reset-start]
2021-02-24T07:02:08.222+0800: [CMS-concurrent-reset: 0.001/0.001 secs] [Times: user=0.00 sys=0.00, real=0.01 secs]
2021-02-24T07:02:08.234+0800: [GC (Allocation Failure) 2021-02-24T07:02:08.234+0800: [ParNew: 157247K->17471K(157248K), 0.0133071 secs] 444418K->351340K(506816K), 0.0137356 secs] [Times: user=0.09 sys=0.00, real=0.01 secs]
2021-02-24T07:02:08.249+0800: [GC (CMS Initial Mark) [1 CMS-initial-mark: 333869K(349568K)] 351485K(506816K), 0.0008960 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
2021-02-24T07:02:08.250+0800: [CMS-concurrent-mark-start]
2021-02-24T07:02:08.253+0800: [CMS-concurrent-mark: 0.002/0.002 secs] [Times: user=0.00 sys=0.00, real=0.01 secs]
2021-02-24T07:02:08.260+0800: [CMS-concurrent-preclean-start]
2021-02-24T07:02:08.262+0800: [CMS-concurrent-preclean: 0.001/0.001 secs] [Times: user=0.02 sys=0.00, real=0.00 secs]
2021-02-24T07:02:08.263+0800: [CMS-concurrent-abortable-preclean-start]
2021-02-24T07:02:08.263+0800: [CMS-concurrent-abortable-preclean: 0.000/0.000 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
2021-02-24T07:02:08.265+0800: [GC (CMS Final Remark) [YG occupancy: 79605 K (157248 K)]2021-02-24T07:02:08.265+0800: [Rescan (parallel) , 0.0004118 secs]2021-02-24T07:02:08.266+0800: [weak refs processing, 0.0001727 secs]2021-02-24T07:02:08.266+0800: [class unloading, 0.0011286 secs]2021-02-24T07:02:08.267+0800: [scrub symbol table, 0.0004355 secs]2021-02-24T07:02:08.268+0800: [scrub string table, 0.0001893 secs][1 CMS-remark: 333869K(349568K)] 413475K(506816K), 0.0038621 secs] [Times: user=0.00 sys=0.00, real=0.01 secs]
2021-02-24T07:02:08.275+0800: [CMS-concurrent-sweep-start]
2021-02-24T07:02:08.278+0800: [CMS-concurrent-sweep: 0.001/0.001 secs] [Times: user=0.03 sys=0.00, real=0.00 secs]
2021-02-24T07:02:08.278+0800: [CMS-concurrent-reset-start]
2021-02-24T07:02:08.280+0800: [CMS-concurrent-reset: 0.001/0.001 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
2021-02-24T07:02:08.292+0800: [GC (Allocation Failure) 2021-02-24T07:02:08.292+0800: [ParNew: 157112K->17470K(157248K), 0.0115372 secs] 387069K->288883K(506816K), 0.0123590 secs] [Times: user=0.00 sys=0.00, real=0.01 secs]
2021-02-24T07:02:08.306+0800: [GC (CMS Initial Mark) [1 CMS-initial-mark: 271412K(349568K)] 289062K(506816K), 0.0008636 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
2021-02-24T07:02:08.307+0800: [CMS-concurrent-mark-start]
2021-02-24T07:02:08.309+0800: [CMS-concurrent-mark: 0.002/0.002 secs] [Times: user=0.05 sys=0.00, real=0.00 secs]
2021-02-24T07:02:08.310+0800: [CMS-concurrent-preclean-start]
2021-02-24T07:02:08.311+0800: [CMS-concurrent-preclean: 0.001/0.001 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
2021-02-24T07:02:08.312+0800: [CMS-concurrent-abortable-preclean-start]
2021-02-24T07:02:08.334+0800: [GC (Allocation Failure) 2021-02-24T07:02:08.335+0800: [ParNew: 156988K->17471K(157248K), 0.0125048 secs] 428400K->327382K(506816K), 0.0129594 secs] [Times: user=0.11 sys=0.00, real=0.01 secs]
2021-02-24T07:02:08.375+0800: [GC (Allocation Failure) 2021-02-24T07:02:08.375+0800: [ParNew: 157199K->157199K(157248K), 0.0001566 secs]2021-02-24T07:02:08.376+0800: [CMS2021-02-24T07:02:08.376+0800: [CMS-concurrent-abortable-preclean: 0.003/0.060 secs] [Times: user=0.16 sys=0.00, real=0.06 secs]
 (concurrent mode failure): 309910K->289393K(349568K), 0.0562658 secs] 467109K->289393K(506816K), [Metaspace: 2628K->2628K(1056768K)], 0.0569689 secs] [Times: user=0.05 sys=0.00, real=0.06 secs]
2021-02-24T07:02:08.459+0800: [GC (Allocation Failure) 2021-02-24T07:02:08.459+0800: [ParNew: 139776K->17472K(157248K), 0.0088130 secs] 429169K->336093K(506816K), 0.0093098 secs] [Times: user=0.09 sys=0.00, real=0.01 secs]
2021-02-24T07:02:08.470+0800: [GC (CMS Initial Mark) [1 CMS-initial-mark: 318621K(349568K)] 336237K(506816K), 0.0009292 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
2021-02-24T07:02:08.471+0800: [CMS-concurrent-mark-start]
2021-02-24T07:02:08.475+0800: [CMS-concurrent-mark: 0.002/0.002 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
2021-02-24T07:02:08.475+0800: [CMS-concurrent-preclean-start]
2021-02-24T07:02:08.477+0800: [CMS-concurrent-preclean: 0.001/0.001 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
2021-02-24T07:02:08.479+0800: [CMS-concurrent-abortable-preclean-start]
2021-02-24T07:02:08.486+0800: [CMS-concurrent-abortable-preclean: 0.000/0.000 secs] [Times: user=0.02 sys=0.00, real=0.01 secs]
2021-02-24T07:02:08.488+0800: [GC (CMS Final Remark) [YG occupancy: 88636 K (157248 K)]2021-02-24T07:02:08.491+0800: [Rescan (parallel) , 0.0004964 secs]2021-02-24T07:02:08.492+0800: [weak refs processing, 0.0001096 secs]2021-02-24T07:02:08.492+0800: [class unloading, 0.0059392 secs]2021-02-24T07:02:08.498+0800: [scrub symbol table, 0.0006367 secs]2021-02-24T07:02:08.499+0800: [scrub string table, 0.0002811 secs][1 CMS-remark: 318621K(349568K)] 407257K(506816K), 0.0135319 secs] [Times: user=0.00 sys=0.00, real=0.02 secs]
2021-02-24T07:02:08.503+0800: [CMS-concurrent-sweep-start]
2021-02-24T07:02:08.516+0800: [GC (Allocation Failure) 2021-02-24T07:02:08.516+0800: [ParNew: 157248K->17471K(157248K), 0.0143191 secs] 443039K->350400K(506816K), 0.0145845 secs] [Times: user=0.08 sys=0.00, real=0.01 secs]
2021-02-24T07:02:08.530+0800: [CMS-concurrent-sweep: 0.001/0.016 secs] [Times: user=0.09 sys=0.00, real=0.04 secs]
2021-02-24T07:02:08.540+0800: [CMS-concurrent-reset-start]
2021-02-24T07:02:08.547+0800: [CMS-concurrent-reset: 0.000/0.000 secs] [Times: user=0.02 sys=0.00, real=0.01 secs]
2021-02-24T07:02:08.560+0800: [GC (Allocation Failure) 2021-02-24T07:02:08.561+0800: [ParNew: 157247K->157247K(157248K), 0.0001587 secs]2021-02-24T07:02:08.561+0800: [CMS: 328412K->312126K(349568K), 0.0522486 secs] 485660K->312126K(506816K), [Metaspace: 2628K->2628K(1056768K)], 0.0537724 secs] [Times: user=0.05 sys=0.00, real=0.05 secs]
2021-02-24T07:02:08.614+0800: [GC (CMS Initial Mark) [1 CMS-initial-mark: 312126K(349568K)] 312630K(506816K), 0.0007227 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
2021-02-24T07:02:08.615+0800: [CMS-concurrent-mark-start]
2021-02-24T07:02:08.618+0800: [CMS-concurrent-mark: 0.002/0.002 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
2021-02-24T07:02:08.618+0800: [CMS-concurrent-preclean-start]
2021-02-24T07:02:08.620+0800: [CMS-concurrent-preclean: 0.001/0.001 secs] [Times: user=0.03 sys=0.00, real=0.00 secs]
2021-02-24T07:02:08.620+0800: [CMS-concurrent-abortable-preclean-start]
2021-02-24T07:02:08.625+0800: [CMS-concurrent-abortable-preclean: 0.000/0.000 secs] [Times: user=0.00 sys=0.00, real=0.01 secs]
2021-02-24T07:02:08.631+0800: [GC (CMS Final Remark) [YG occupancy: 88449 K (157248 K)]2021-02-24T07:02:08.632+0800: [Rescan (parallel) , 0.0004337 secs]2021-02-24T07:02:08.632+0800: [weak refs processing, 0.0014522 secs]2021-02-24T07:02:08.634+0800: [class unloading, 0.0100824 secs]2021-02-24T07:02:08.644+0800: [scrub symbol table, 0.0004037 secs]2021-02-24T07:02:08.645+0800: [scrub string table, 0.0002123 secs][1 CMS-remark: 312126K(349568K)] 400576K(506816K), 0.0142744 secs] [Times: user=0.00 sys=0.00, real=0.01 secs]
2021-02-24T07:02:08.646+0800: [CMS-concurrent-sweep-start]
2021-02-24T07:02:08.656+0800: [GC (Allocation Failure) 2021-02-24T07:02:08.663+0800: [ParNew: 139566K->139566K(157248K), 0.0001837 secs]2021-02-24T07:02:08.663+0800: [CMS2021-02-24T07:02:08.663+0800: [CMS-concurrent-sweep: 0.001/0.008 secs] [Times: user=0.02 sys=0.00, real=0.02 secs]
 (concurrent mode failure): 311478K->323317K(349568K), 0.0660418 secs] 451044K->323317K(506816K), [Metaspace: 2628K->2628K(1056768K)], 0.0729739 secs] [Times: user=0.05 sys=0.00, real=0.07 secs]
執行結束!共生成物件次數:8831
Heap
 par new generation   total 157248K, used 111872K [0x00000000e0000000, 0x00000000eaaa0000, 0x00000000eaaa0000)
  eden space 139776K,  80% used [0x00000000e0000000, 0x00000000e6d400b8, 0x00000000e8880000)
  from space 17472K,   0% used [0x00000000e9990000, 0x00000000e9990000, 0x00000000eaaa0000)
  to   space 17472K,   0% used [0x00000000e8880000, 0x00000000e8880000, 0x00000000e9990000)
 concurrent mark-sweep generation total 349568K, used 323317K [0x00000000eaaa0000, 0x0000000100000000, 0x0000000100000000)
 Metaspace       used 2634K, capacity 4486K, committed 4864K, reserved 1056768K
  class space    used 291K, capacity 386K, committed 512K, reserved 1048576K

D:\F盤\JavaCourseCodes\01jvm>

圖解GC變化

G1GC演示（-XX:+UseG1GC）

當前最大堆記憶體512M

當發生5五次youngGC后，就進行CMSGC，

階段1: Initial Mark(初始標記)
階段2: Root Region Scan(Root區掃描)
階段3:Evacuation Pause (mixed)(轉移暫停: 混合模式)
階段4: Remark(再次標記)
階段5: Cleanup(清理)


D:\F盤\JavaCourseCodes\01jvm>java -XX:+UseG1GC  -Xms512m -Xmx512m  -XX:+PrintGC -XX:+PrintGCDateStamps GCLogAnalysis
正在執行...
2021-02-24T07:28:03.140+0800: [GC pause (G1 Evacuation Pause) (young) 32M->11M(512M), 0.0028508 secs]       //Evacuation Pause: young(純年輕代模式轉移暫停)
2021-02-24T07:28:03.153+0800: [GC pause (G1 Evacuation Pause) (young) 39M->20M(512M), 0.0027907 secs]
2021-02-24T07:28:03.178+0800: [GC pause (G1 Evacuation Pause) (young) 67M->37M(512M), 0.0042861 secs]
2021-02-24T07:28:03.208+0800: [GC pause (G1 Evacuation Pause) (young) 102M->60M(512M), 0.0052239 secs]
2021-02-24T07:28:03.276+0800: [GC pause (G1 Evacuation Pause) (young) 214M->108M(512M), 0.0072020 secs]
2021-02-24T07:28:03.316+0800: [GC pause (G1 Evacuation Pause) (young) 242M->149M(512M), 0.0077214 secs]
2021-02-24T07:28:03.383+0800: [GC pause (G1 Humongous Allocation) (young) (initial-mark) 342M->202M(512M), 0.0342600 secs]   //Concurrent Marking(并發標記)
2021-02-24T07:28:03.417+0800: [GC concurrent-root-region-scan-start]
2021-02-24T07:28:03.419+0800: [GC concurrent-root-region-scan-end, 0.0012973 secs]
2021-02-24T07:28:03.419+0800: [GC concurrent-mark-start]
2021-02-24T07:28:03.422+0800: [GC concurrent-mark-end, 0.0024423 secs]
2021-02-24T07:28:03.422+0800: [GC remark, 0.0017725 secs]
2021-02-24T07:28:03.424+0800: [GC cleanup 228M->228M(512M), 0.0012586 secs]
     //階段1: Initial Mark(初始標記)
     //階段2: Root Region Scan(Root區掃描)
     //階段3:Evacuation Pause (mixed)(轉移暫停: 混合模式)
     //階段4: Remark(再次標記)
     //階段5: Cleanup(清理)
2021-02-24T07:28:03.471+0800: [GC pause (G1 Evacuation Pause) (young)-- 406M->307M(512M), 0.0083992 secs]
2021-02-24T07:28:03.480+0800: [GC pause (G1 Evacuation Pause) (mixed) 310M->299M(512M), 0.0048689 secs]             //Evacuation Pause (mixed)(轉移暫停: 混合模式)
2021-02-24T07:28:03.487+0800: [GC pause (G1 Humongous Allocation) (young) (initial-mark) 303M->300M(512M), 0.0011917 secs]
2021-02-24T07:28:03.489+0800: [GC concurrent-root-region-scan-start]
2021-02-24T07:28:03.497+0800: [GC concurrent-root-region-scan-end, 0.0085547 secs]
2021-02-24T07:28:03.504+0800: [GC concurrent-mark-start]
2021-02-24T07:28:03.507+0800: [GC concurrent-mark-end, 0.0026832 secs]
2021-02-24T07:28:03.508+0800: [GC remark, 0.0113277 secs]
2021-02-24T07:28:03.520+0800: [GC cleanup 383M->383M(512M), 0.0010538 secs]
2021-02-24T07:28:03.529+0800: [GC pause (G1 Evacuation Pause) (young) 412M->332M(512M), 0.0084896 secs]
2021-02-24T07:28:03.541+0800: [GC pause (G1 Evacuation Pause) (mixed) 349M->291M(512M), 0.0047697 secs]
2021-02-24T07:28:03.552+0800: [GC pause (G1 Evacuation Pause) (mixed) 316M->278M(512M), 0.0044825 secs]
2021-02-24T07:28:03.558+0800: [GC pause (G1 Humongous Allocation) (young) (initial-mark) 283M->279M(512M), 0.0016376 secs]
2021-02-24T07:28:03.560+0800: [GC concurrent-root-region-scan-start]
2021-02-24T07:28:03.569+0800: [GC concurrent-root-region-scan-end, 0.0091800 secs]

圖解GC變化
延伸知識

通過劃分多個記憶體區域做增量整理和回收，進一步降低延遲
G1 的全稱是Garbage-First，意為垃圾優先，哪一塊的垃圾最多就優先清理它
G1 GC 最主要的設計目標是：將STW 停頓的時間和分布，變成可預期且可配置的
堆不再分成年輕代和老年代，而是劃分為多個（通常是2048 個）可以存放物件的小塊堆區域(smaller heap regions)，每個小塊，可能一會被定義成Eden 區，一會被指定為Survivor區或者Old 區，在邏輯上，所有的Eden 區和Survivor區合起來就是年輕代，所有的Old 區拼在一起那就是老年
Java8默認并行GC，Java9， Java11 默認G1GC

GC 如何選擇

如果系統考慮吞吐優先，CPU 資源都用來最大程度處理業務，用Parallel GC；
如果系統考慮低延遲有限，每次GC 時間盡量短，用CMS GC；
如果系統記憶體堆較大，同時希望整體來看平均GC 時間可控，使用G1 GC，.對于記憶體大小的考量：
1. 一般4G 以上，算是比較大，用G1 的性價比較高，
2. 一般超過8G，比如16G-64G 記憶體，非常推薦使用G1 GC

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