1、测试代码实例：使用线程池不断的创建对象
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importjava.util.Scanner;
importjava.util.concurrent.ArrayBlockingQueue;
importjava.util.concurrent.ThreadPoolExecutor;
importjava.util.concurrent.TimeUnit;
/**
* @author
* @Description:XXXXXX
* @Date: Created in 20:07 2021/1/25
*/
publicclassMem {
privatestatic final ThreadPoolExecutor executor = newThreadPoolExecutor(32, 32, 60,
TimeUnit.SECONDS, newArrayBlockingQueue<>(3), newThreadPoolExecutor.DiscardOldestPolicy());
publicstatic void main(String[] args) throwsException {
System.out.println(“Enter executor num:”);
Scanner scanner = newScanner(System.in);
int worker = Integer.valueOf(scanner.next());
for(int i = 0; i < worker; i++) {
System.out.println(“worker ” + i + ” start…”);
executor.execute(() -> {
try{
alloc(Integer.MAX_VALUE);
Thread.sleep(1);
} catch(Exception e) {
}
});
System.out.println(“worker ” + i + ” end…”);
}
System.out.println(“End”);
}
privatestatic void alloc(int n) {
byte[] block = null;
for(int i = 0; i < n; i++) {
//申请512M空间
block = newbyte[512 * 1024 * 1024];
System.out.println(“apply for memory 512M”);
}
}
}
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2、使用的命令说明
a.测试代码执行命令，堆内存18g，使用不同的GC算法
重要参数说明:
-Xms18g -Xmx18g:
-XX:+PrintGC 打印GC日志
-XX:+PrintGCTimeStamps 打印GC发送的时间
-XX:+PrintGCApplicationStoppedTime 打印应用由于GC而产生的停顿时间
-XX:+UseG1GC 指定G1垃圾回收
-XX:+UseConcMarkSweepGC 指定CMS垃圾回收
java -Xms18g -Xmx18g -XX:+UseConcMarkSweepGC -XX:+PrintGC -XX:+PrintGCTimeStamps -XX:+PrintGCApplicationStoppedTime -Xloggc:cmsgc.log Mem
java -Xms18g -Xmx18g -XX:+UseG1GC -XX:+PrintGC -XX:+PrintGCTimeStamps -XX:+PrintGCApplicationStoppedTime -Xloggc:g1gc.log Mem
b.GC情况查看命令,间隔10秒，观察30次
jstat -gcutil pid 10000 30
3、服务器配置和实验数据
CPU:Intel(R) Xeon(R) CPU E5-2609 v4 @ 1.70GHz
内存：24G
硬盘：500G
操作系统:centos
jdk:1.8.0_121-b13
堆内存:限制18G
变量:垃圾回收算法(CMS和G1)
使用UseConcMarkSweepGC的GC情况
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S0 S1 E O M CCS YGC YGCT FGC FGCT GCT
0.00 0.00 4.00 0.00 17.28 19.76 0 0.000 0 0.000 0.000
0.01 0.02 96.19 49.00 91.41 82.79 18 5.350 0 0.000 5.350
0.00 0.02 96.14 97.99 91.53 82.79 36 12.885 1 0.000 12.885
0.00 0.00 96.14 97.99 91.55 82.79 60 15.517 4 4.423 19.939
0.00 0.00 96.14 97.99 91.55 82.79 78 19.744 8 9.090 28.834
0.00 0.00 96.14 97.99 91.62 82.79 95 21.575 13 15.444 37.019
0.00 0.00 96.14 97.99 91.62 82.79 108 22.636 17 22.066 44.702
0.00 0.00 96.14 97.99 91.62 82.79 116 23.245 21 31.244 54.488
0.00 0.00 96.14 97.99 91.62 82.79 126 23.833 27 39.567 63.399
0.00 0.00 96.14 97.99 91.62 82.79 136 24.415 33 49.362 73.777
0.00 0.00 96.14 97.99 91.62 82.79 144 24.891 38 56.471 81.362
0.00 0.00 96.14 97.99 91.63 82.79 156 25.652 44 66.230 91.882
0.00 0.00 97.14 97.99 91.84 82.79 164 26.175 48 73.664 99.839
0.00 0.00 96.14 97.99 91.87 82.79 171 28.897 52 84.253 113.150
0.00 0.00 96.14 97.99 91.87 82.79 178 29.142 57 93.978 123.120
0.00 0.00 96.14 97.99 91.87 82.79 186 29.609 62 103.077 132.686
0.00 0.00 98.14 97.99 91.87 82.79 192 29.990 65 111.313 141.303
0.00 0.00 100.00 97.99 91.87 82.79 198 30.391 69 120.084 150.474
0.00 0.00 100.00 97.99 91.87 82.79 209 31.945 75 129.803 161.748
0.00 0.00 96.14 97.99 91.87 82.79 218 32.546 79 137.913 170.459
0.00 0.00 96.79 97.99 91.87 82.79 225 33.012 83 145.764 178.776
0.00 0.00 96.14 95.10 91.87 82.79 233 33.483 88 153.464 186.947
0.00 0.00 96.14 97.99 91.94 82.79 243 34.094 94 164.024 198.118
0.00 0.00 96.14 97.99 92.05 82.79 250 34.562 98 171.400 205.962
0.00 0.00 0.00 95.10 92.05 82.79 256 34.963 101 183.027 217.990
0.00 0.00 96.14 97.99 92.05 82.79 264 35.467 106 191.465 226.932
0.00 0.00 96.14 97.99 92.05 82.79 268 37.928 108 197.131 235.059
0.00 0.00 96.14 97.99 92.05 82.79 276 38.398 113 207.743 246.141
0.00 0.00 96.56 97.99 92.05 82.79 282 38.766 117 215.147 253.913
0.00 0.00 96.32 97.99 92.08 82.79 290 39.229 121 223.913 263.142
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使用G1的GC情况
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S0 S1 E O M CCS YGC YGCT FGC FGCT GCT
0.00 0.00 0.00 0.00 17.28 19.76 0 0.000 0 0.000 0.000
0.00 100.00 0.00 99.96 91.64 82.79 27 0.136 0 0.000 0.136
0.00 0.00 0.00 99.96 92.09 82.79 134 0.440 0 0.000 0.440
0.00 0.00 0.00 99.96 92.09 82.79 240 0.775 0 0.000 0.775
0.00 0.00 0.00 99.96 92.09 82.79 347 1.085 0 0.000 1.085
0.00 0.00 0.00 99.96 92.09 82.79 422 1.413 0 0.000 1.413
0.00 0.00 0.00 93.82 92.46 82.84 495 1.765 22 2.339 4.104
0.00 0.00 0.00 96.75 92.47 82.84 570 1.982 66 6.656 8.639
0.00 0.00 0.00 96.75 92.47 82.84 624 2.213 103 10.610 12.822
0.00 0.00 0.83 96.71 92.47 82.84 668 2.505 135 16.130 18.635
0.00 0.00 0.00 96.75 92.47 82.84 718 3.932 173 20.356 24.288
0.00 0.00 0.00 97.06 92.59 82.99 785 4.169 214 24.755 28.924
0.00 0.00 0.83 96.71 92.59 82.99 849 4.406 232 26.653 31.059
0.00 0.00 0.83 99.64 92.59 82.99 925 4.696 233 26.753 31.449
0.00 100.00 0.78 99.96 92.59 82.99 992 4.915 238 27.267 32.182
0.00 0.00 1.65 99.64 92.59 82.99 1049 5.109 247 28.205 33.315
0.00 0.00 1.56 99.96 92.59 82.99 1115 5.358 249 28.411 33.769
0.00 0.00 1.59 99.96 92.59 82.99 1174 5.576 250 28.513 34.089
0.00 0.00 1.56 99.96 92.59 82.99 1227 5.774 256 29.145 34.918
0.00 0.00 1.56 99.96 92.59 82.99 1283 5.964 261 29.665 35.629
0.00 0.00 1.56 99.96 92.59 82.99 1340 6.162 262 29.768 35.931
0.00 0.00 0.00 99.96 92.59 82.99 1399 6.377 264 29.979 36.356
0.00 0.00 0.00 99.96 92.59 82.99 1456 6.598 267 30.187 36.785
0.00 0.00 1.56 99.96 92.59 82.99 1515 6.796 268 30.393 37.190
0.00 0.00 1.56 99.96 92.59 82.99 1577 7.033 270 30.598 37.631
0.00 0.00 0.00 99.96 92.59 82.99 1639 7.253 272 30.806 38.059
0.00 0.00 1.56 99.96 92.59 82.99 1699 7.483 274 31.015 38.497
0.00 0.00 1.56 99.96 92.59 82.99 1752 7.668 278 31.425 39.093
0.00 0.00 1.56 99.96 92.59 82.99 1803 7.864 281 31.731 39.595
0.00 0.00 1.59 99.96 92.59 82.99 1850 8.033 286 32.271 40.304
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使用CMS的应用暂停时间(选取部分)
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491.172: [Full GC (Allocation Failure) 18350819K->17302210K(18806272K), 2.7447486 secs]
493.961: Total time for which application threads were stopped: 2.8691817 seconds, Stopping threads took: 0.0795971 seconds
494.041: [GC (Allocation Failure) 17826909K->17826500K(18806272K), 0.1132958 secs]
494.154: Total time for which application threads were stopped: 0.1934281 seconds, Stopping threads took: 0.0798435 seconds
494.239: [Full GC (Allocation Failure) 18351199K->17302210K(18806272K), 2.6556058 secs]
496.938: Total time for which application threads were stopped: 2.7832497 seconds, Stopping threads took: 0.0830667 seconds
497.018: [GC (Allocation Failure) 17826765K->17826500K(18806272K), 0.1161283 secs]
497.134: Total time for which application threads were stopped: 0.1962357 seconds, Stopping threads took: 0.0798200 seconds
497.215: [Full GC (Allocation Failure) 18350962K->17302210K(18806272K), 1.3247983 secs]
498.583: Total time for which application threads were stopped: 1.4485793 seconds, Stopping threads took: 0.0789361 seconds
498.666: [GC (Allocation Failure) 17826498K->17826500K(18806272K), 0.1134047 secs]
498.779: Total time for which application threads were stopped: 0.1967264 seconds, Stopping threads took: 0.0830367 seconds
498.861: [Full GC (Allocation Failure) 18350882K->17302210K(18806272K), 1.4982433 secs]
500.403: Total time for which application threads were stopped: 1.6238582 seconds, Stopping threads took: 0.0797511 seconds
500.485: [GC (Allocation Failure) 17826498K->17826500K(18806272K), 0.1272701 secs]
500.612: Total time for which application threads were stopped: 0.2085994 s*conds, Stopping threads took: 0.0809783 seconds
500.697: [Full GC (Allocation Failure) 18350801K->17826498K(18806272K), 0.5578431 secs]
501.304: Total time for which application threads were stopped: 0.6919552 seconds, Stopping threads took: 0.0837865 seconds
501.395: [Full GC (Allocation Failure) 18371842K->17302211K(18806272K), 2.6491250 secs]
504.090: Total time for which application threads were stopped: 2.7847448 seconds, Stopping threads took: 0.0892144 seconds
504.170: [GC (Allocation Failure) 17837454K->17826507K(18806272K), 1.2969435 secs]
505.499: Total time for which application threads were stopped: 1.4089028 seconds, Stopping threads took: 0.0793611 seconds
505.592: [Full GC (Allocation Failure) 18355521K->17302212K(18806272K), 2.8537842 secs]
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使用G1的应用暂停时间(选取部分)
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9.938: [GC pause (G1 Humongous Allocation) (young) (initial-mark) 16G->16G(18G), 0.0074400 secs]
89.946: [GC concurrent-root-region-scan-start]
89.946: [GC concurrent-root-region-scan-end, 0.0000230 secs]
89.946: [GC concurrent-mark-start]
89.946: Total time for which application threads were stopped: 0.0913843 seconds, Stopping threads took: 0.0836821 seconds
89.949: [GC pause (G1 Humongous Allocation) (young) 16G->16G(18G), 0.0024211 secs]
89.951: Total time for which application threads were stopped: 0.0029560 seconds, Stopping threads took: 0.0000470 seconds
89.954: [GC concurrent-mark-end, 0.0077998 secs]
90.032: [GC pause (G1 Humongous Allocation) (young) 16G->16G(18G), 0.0030254 secs]
90.035: Total time for which application threads were stopped: 0.0808401 seconds, Stopping threads took: 0.0769236 seconds
90.036: [Full GC (Allocation Failure) 16G->16G(18G), 0.1000621 secs]
90.136: Total time for which application threads were stopped: 0.1007767 seconds, Stopping threads took: 0.0000808 seconds
90.224: [GC pause (G1 Humongous Allocation) (young) 16G->16G(18G), 0.0028310 secs]
90.227: Total time for which application threads were stopped: 0.0905506 seconds, Stopping threads took: 0.0869216 seconds
90.227: [Full GC (Allocation Failure) 16G->16G(18G), 0.1026309 secs]
90.330: Total time for which application threads were stopped: 0.1032992 seconds, Stopping threads took: 0.0000372 seconds
90.414: [GC pause (G1 Humongous Allocation) (young) 16G->16G(18G), 0.0047862 secs]
90.419: Total time for which application threads were stopped: 0.0862279 seconds, Stopping threads took: 0.0806315 seconds
90.419: [Full GC (Allocation Failure) 16G->16G(18G), 0.1011580 secs]
90.521: Total time for which application threads were stopped: 0.1018183 seconds, Stopping threads took: 0.0000429 seconds
90.603: [GC pause (G1 Humongous Allocation) (young) 16G->16G(18G), 0.0043945 secs]
90.607: Total time for which application threads were stopped: 0.0854959 seconds, Stopping threads took: 0.0803917 seconds
90.608: [Full GC (Allocation Failure) 16G->16G(18G), 0.1055658 secs]
90.714: Total time for which application threads were stopped: 0.1061946 seconds, Stopping threads took: 0.0000364 seconds
90.794: [GC pause (G1 Humongous Allocation) (young) 16G->16G(18G), 0.0030520 secs]
90.798: Total time for which application threads were stopped: 0.0836833 seconds, Stopping threads took: 0.0799108 seconds
90.798: [Full GC (Allocation Failure) 16G->16G(18G), 0.0975948 secs]
90.896: [Full GC (Allocation Failure) 16G->16G(18G), 0.0983627 secs]
90.994: Total time for which application threads were stopped: 0.1967194 s*conds, Stopping threads took: 0.0000412 seconds
90.997: [GC pause (G1 Humongous Allocation) (young) 16G->15G(18G), 0.0028386 secs]
91.000: Total time for which application threads were stopped: 0.0034994 s*conds, Stopping threads took: 0.0000583 seconds
91.099: [GC pause (G1 Humongous Allocation) (young) 16G->16G(18G), 0.0023358 secs]
91.101: Total time for which application threads were stopped: 0.0991074 seconds, Stopping threads took: 0.0961578 seconds
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4、结论
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CMS
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G1
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YGC（年轻代GC次数）
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290
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1850
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YGCT（年轻代GC的时间）
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39.229
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8.033s
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FGC（FullGC的次数）
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121
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286
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FGCT（FullGC的总时间）
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223.913s
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32.271s
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GCT（GC的总时间）
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263.142s
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40.304s
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最大应用暂停时间
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2.869s
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0.196s
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CMS垃圾回收产生的FullGC次数是121，FullGC总的时间是223.913s,平均每次FullGC的时间是1.85s,总的GC时间是263.142s
G1垃圾回收产生的FullGC次数是286，FullGC总的时间是32.271s,平均每次FullGC的时间是0.11s,总的GC时间是40.304s
显然在实验中，G1总的GC时间短,FullGC的次数是CMS的2.36倍,CMS应用暂停时间达到最大2.869s，G1最大应用暂停时间0.196s,也跟官方给FGC的最大暂停时间（200ms）接近G1算法已经称为Java8之后的默认算法，本身的需要调整的参数很少，性能&易用性方面都比CMS好。大内存的情况下，同等条件，G1算法比CMS更适合
5、参考文档
https://www.oracle.com/cn/technical-resources/articles/java/g1gc.html
https://www.oracle.com/technetwork/tutorials/tutorials-1876574.html
Recommended Use Cases for G1
The first focus of G1 is to provide a solution for users running applications that require large heaps with limited GC latency. This means heap sizes of around 6GB or larger, and stable and predictable pause time below 0.5 seconds.
Applications running today with either the CMS or the ParallelOldGC garbage collector would benefit switching to G1 if the application has one or more of the following traits.
– Full GC durations are too long or too frequent.
– The rate of object allocation rate or promotion varies significantly.
– Undesired long garbage collection or compaction pauses (longer than 0.5 to 1 second)
Note:If you are using CMS or ParallelOldGC and your application is not experiencing long garbage collection pauses, it is fine to stay with your current collector. Changing to the G1 collector is not a requirement for using the latest JDK.
以下是对oracle官方描述的翻译
G1的推荐用例
G1的首要重点是为运行需要大量GC延迟的应用程序的用户提供一个解决方案。这意味着堆大小约为6GB或更大，并且稳定且可预测的暂停时间低于0.5秒。
如果应用程序具有以下一个或多个特性，那么使用CMS或ParallelOldGC垃圾收集器运行的应用程序将有利于切换到G1。
• Full GC 持续时间太长或太频繁
• 对象分配频率高或者提升变化明显
• 不期望GC久、压缩停顿 (超过 0.5 to 1 秒)
注意：如果您使用的是CMS或parallelloldgc，并且您的应用程序没有遇到长时间的垃圾收集暂停，那么可以使用当前收集器。使用最新的JDK不需要更改为G1收集器。
来源：freebuf.com 2021-05-12 18:09:07 by: 安全狗safedog
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