我有一个每秒约100-1000个请求的实时Web应用程序。我使用CMS GC，每当CMS开始工作时，FGC计数器增加2.为什么FullGC每次都发生 ？我的意思是CMS必须在没有FullGC的情况下工作。或者这是正常的行为？ 我的JMV选项：
-Xms6g
-Xmx6g
-XX:PermSize=512m
-XX:MaxPermSize=512m
-XX:+UseParNewGC
-XX:+UseConcMarkSweepGC
-XX:+UseCMSInitiatingOccupancyOnly
-XX:CMSInitiatingOccupancyFraction=70
-XX:+CMSClassUnloadingEnabled
-XX:+UseLargePages
-XX:LargePageSizeInBytes=2m
-verbose:gc
-Xloggc:gc.log
-XX:+PrintGCDetails
-XX:+PrintGCDateStamps
-Dsun.rmi.dgc.client.gcInterval=3600000
-Dsun.rmi.dgc.server.gcInterval=3600000
这是gc.log：
2013-08-19T10:21:33.072+0600: 123570.236: [GC 123570.237: [ParNew: 427534K->35446K(440896K), 0.1872440 secs] 4482285K->4091194K(6242496K), 0.1877660 secs] [Times: user=0.51 sys=0.01, real=0.19 secs]
2013-08-19T10:21:34.026+0600: 123571.191: [GC 123571.191: [ParNew: 427382K->34355K(440896K), 0.1993550 secs] 4483130K->4090615K(6242496K), 0.1998720 secs] [Times: user=0.53 sys=0.00, real=0.20 secs]
2013-08-19T10:21:34.724+0600: 123571.889: [GC 123571.889: [ParNew: 426291K->38786K(440896K), 0.2229880 secs] 4482551K->4095384K(6242496K), 0.2235130 secs] [Times: user=0.53 sys=0.00, real=0.23 secs]
2013-08-19T10:21:35.579+0600: 123572.744: [GC 123572.744: [ParNew: 430722K->34694K(440896K), 0.1812580 secs] 4487320K->4091801K(6242496K), 0.1817630 secs] [Times: user=0.49 sys=0.00, real=0.18 secs]
2013-08-19T10:21:36.429+0600: 123573.594: [GC 123573.594: [ParNew: 426630K->34091K(440896K), 0.1752240 secs] 4483737K->4092028K(6242496K), 0.1757390 secs] [Times: user=0.48 sys=0.00, real=0.17 secs]
2013-08-19T10:21:37.221+0600: 123574.386: [GC 123574.386: [ParNew: 426027K->36411K(440896K), 0.2047550 secs] 4483964K->4095856K(6242496K), 0.2052280 secs] [Times: user=0.50 sys=0.00, real=0.21 secs]
2013-08-19T10:21:38.059+0600: 123575.223: [GC 123575.223: [ParNew: 428347K->40113K(440896K), 0.1892000 secs] 4487792K->4100213K(6242496K), 0.1896180 secs] [Times: user=0.51 sys=0.01, real=0.19 secs]
2013-08-19T10:21:38.593+0600: 123575.757: [GC 123575.757: [ParNew: 432049K->39917K(440896K), 0.2193730 secs] 4492149K->4101831K(6242496K), 0.2198480 secs] [Times: user=0.50 sys=0.00, real=0.22 secs]
2013-08-19T10:21:38.817+0600: 123575.981: [GC [1 CMS-initial-mark: 4061914K(5801600K)] 4102165K(6242496K), 0.0901990 secs] [Times: user=0.09 sys=0.00, real=0.09 secs]
2013-08-19T10:21:38.908+0600: 123576.072: [CMS-concurrent-mark-start]
2013-08-19T10:21:39.305+0600: 123576.469: [CMS-concurrent-mark: 0.397/0.397 secs] [Times: user=4.16 sys=0.44, real=0.39 secs]
2013-08-19T10:21:39.305+0600: 123576.469: [CMS-concurrent-preclean-start]
2013-08-19T10:21:39.335+0600: 123576.499: [GC 123576.499: [ParNew: 431853K->39752K(440896K), 0.2246840 secs] 4493767K->4102434K(6242496K), 0.2251970 secs] [Times: user=0.54 sys=0.00, real=0.23 secs]
2013-08-19T10:21:39.576+0600: 123576.741: [CMS-concurrent-preclean: 0.039/0.271 secs] [Times: user=0.75 sys=0.05, real=0.27 secs]
2013-08-19T10:21:39.576+0600: 123576.741: [CMS-concurrent-abortable-preclean-start]
2013-08-19T10:21:40.193+0600: 123577.357: [GC 123577.358: [ParNew: 431688K->39319K(440896K), 0.2158020 secs] 4494370K->4102833K(6242496K), 0.2162640 secs] [Times: user=0.53 sys=0.00, real=0.22 secs]
2013-08-19T10:21:40.474+0600: 123577.638: [CMS-concurrent-abortable-preclean: 0.673/0.898 secs] [Times: user=3.19 sys=0.77, real=0.90 secs]
2013-08-19T10:21:40.479+0600: 123577.643: [GC[YG occupancy: 202771 K (440896 K)]123577.643: [Rescan (parallel) , 0.0455840 secs]123577.689: [weak refs processing, 2.2041640 secs]123579.893: [class unloading, 0.0445080 secs]123579.938: [scrub symbol & string tables, 0.0187100 secs] [1 CMS-remark: 4063513K(5801600K)] 4266284K(6242496K), 2.3260510 secs] [Times: user=3.24 sys=0.00, real=2.32 secs]
2013-08-19T10:21:42.806+0600: 123579.970: [CMS-concurrent-sweep-start]
2013-08-19T10:21:42.914+0600: 123580.079: [GC 123580.079: [ParNew: 431328K->45397K(440896K), 0.0902990 secs] 4494842K->4109425K(6242496K), 0.0908150 secs] [Times: user=0.50 sys=0.00, real=0.09 secs]
2013-08-19T10:21:43.012+0600: 123580.176: [GC 123580.176: [ParNew: 45767K->4894K(440896K), 0.1271260 secs] 4109795K->4114786K(6242496K), 0.1275320 secs] [Times: user=0.75 sys=0.04, real=0.13 secs]
2013-08-19T10:21:43.286+0600: 123580.451: [GC 123580.451: [ParNew: 396847K->48960K(440896K), 0.2152650 secs] 4506739K->4161825K(6242496K), 0.2157500 secs] [Times: user=0.59 sys=0.00, real=0.22 secs]
2013-08-19T10:21:43.509+0600: 123580.673: [GC 123580.674: [ParNew: 49831K->8172K(440896K), 0.0639450 secs] 4162696K->4159717K(6242496K), 0.0643790 secs] [Times: user=0.48 sys=0.00, real=0.06 secs]
2013-08-19T10:21:43.726+0600: 123580.890: [GC 123580.890: [ParNew: 400108K->48960K(440896K), 0.1256470 secs] 4551616K->4203961K(6242496K), 0.1261110 secs] [Times: user=0.50 sys=0.00, real=0.13 secs]
2013-08-19T10:21:43.999+0600: 123581.164: [GC 123581.164: [ParNew: 440896K->48960K(440896K), 0.1631010 secs] 4565558K->4184975K(6242496K), 0.1635960 secs] [Times: user=0.58 sys=0.01, real=0.16 secs]
2013-08-19T10:21:44.715+0600: 123581.880: [GC 123581.880: [ParNew: 440896K->39265K(440896K), 0.2020150 secs] 4374009K->3975902K(6242496K), 0.2025700 secs] [Times: user=0.53 sys=0.00, real=0.21 secs]
2013-08-19T10:21:44.924+0600: 123582.088: [GC 123582.088: [ParNew: 39857K->1876K(440896K), 0.0546600 secs] 3976335K->3973247K(6242496K), 0.0550670 secs] [Times: user=0.46 sys=0.02, real=0.06 secs]
2013-08-19T10:21:45.207+0600: 123582.371: [GC 123582.371: [ParNew: 393812K->42887K(440896K), 0.2367970 secs] 4280052K->3929127K(6242496K), 0.2372620 secs] [Times: user=0.57 sys=0.00, real=0.24 secs]
2013-08-19T10:21:46.048+0600: 123583.212: [GC 123583.213: [ParNew: 434823K->33908K(440896K), 0.1922750 secs] 4081263K->3683497K(6242496K), 0.1927880 secs] [Times: user=0.54 sys=0.00, real=0.19 secs]
这是jstat日志：
70.86 0.00 63.36 69.93 29.29 7110 1077.048 42 112.603 1189.651 unknown GCCause No GC
0.00 69.63 84.75 69.95 29.29 7111 1077.223 42 112.603 1189.826 unknown GCCause No GC
74.37 20.57 100.00 69.98 29.29 7113 1077.428 42 112.603 1190.031 unknown GCCause Allocation Failure
S0 S1 E O P YGC YGCT FGC FGCT GCT LGCC GCC
81.53 0.00 80.03 70.01 29.29 7114 1077.837 43 112.693 1190.530 CMS Initial Mark No GC
0.00 81.19 92.05 70.03 29.29 7115 1078.061 43 112.693 1190.754 unknown GCCause No GC
80.31 0.00 41.70 70.04 29.29 7116 1078.277 44 112.693 1190.970 No GC CMS Final Remark
80.31 0.00 41.70 70.04 29.29 7116 1078.277 44 112.693 1190.970 No GC CMS Final Remark
10.00 92.72 0.09 70.85 29.30 7118 1078.368 44 115.019 1193.386 No GC GCLocker Initiated GC
S0 S1 E O P YGC YGCT FGC FGCT GCT LGCC GCC
86.43 100.00 100.00 71.26 29.30 7122 1078.900 44 115.019 1193.918 unknown GCCause Allocation Failure
3.83 0.00 96.82 67.37 29.30 7124 1079.319 44 115.019 1194.338 GCLocker Initiated GC No GC
10.70 87.60 100.00 62.91 29.30 7126 1079.556 44 115.019 1194.575 unknown GCCause Allocation Failure
69.26 46.82 100.00 58.81 29.30 7127 1079.748 44 115.019 1194.767 unknown GCCause Allocation Failure
0.00 2.97 0.00 54.59 29.30 7129 1080.196 44 115.019 1195.215 GCLocker Initiated GC No GC
S0 S1 E O P YGC YGCT FGC FGCT GCT LGCC GCC
77.12 0.00 72.64 46.93 29.30 7130 1080.391 44 115.019 1195.410 unknown GCCause No GC
1.21 0.00 92.31 40.78 29.30 7132 1080.656 44 115.019 1195.675 GCLocker Initiated GC No GC
70.43 0.00 0.14 33.51 29.30 7134 1080.874 44 115.019 1195.893 unknown GCCause No GC
0.00 62.56 33.67 25.89 29.30 7135 1080.974 44 115.019 1195.992 unknown GCCause No GC
71.90 0.00 85.91 23.61 29.30 7136 1081.153 44 115.019 1196.172 unknown GCCause No GC
S0 S1 E O P YGC YGCT FGC FGCT GCT LGCC GCC
35.00 87.41 100.00 23.66 29.30 7138 1081.349 44 115.019 1196.368 unknown GCCause Allocation Failure
0.00 79.22 68.07 23.69 29.30 7139 1081.671 44 115.019 1196.690 unknown GCCause No GC
87.27 0.00 93.23 23.70 29.30 7140 1081.878 44 115.019 1196.897 unknown GCCause No GC
0.00 65.85 68.58 23.73 29.30 7141 1082.028 44 115.019 1197.047 unknown GCCause No GC
37.82 0.00 22.79 23.75 29.30 7142 1082.097 44 115.019 1197.116 unknown GCCause No GC
S0 S1 E O P YGC YGCT FGC FGCT GCT LGCC GCC
0.00 65.38 91.55 23.75 29.30 7143 1082.223 44 115.019 1197.242 unknown GCCause No GC
5.66 97.48 100.00 23.80 29.30 7146 1082.490 44 115.019 1197.509 unknown GCCause Allocation Failure
91.30 0.00 93.29 24.44 29.30 7148 1082.807 44 115.019 1197.826 unknown GCCause No GC
64.91 76.26 100.00 24.48 29.30 7150 1082.925 44 115.019 1197.944 unknown GCCause Allocation Failure
0.00 75.64 43.14 24.51 29.30 7151 1083.267 44 115.019 1198.286 unknown GCCause No GC
答案 0 :(得分：1)
主要问题是：你提到的“FullGC”计数器算什么。
1。）CMS是一个完整的系列，因为它收集了年轻一代和老一代。
2。）CMS的每次运行将包括两个世界各地的阶段（初始标记和备注），而所有其他工作同时完成。 CMS不会试图阻止世界各地的停滞，它只是为了尽可能地缩短它们。
3。）当VM的并发阶段处于活动状态时，当VM耗尽内存时，它将回退到正常的Mark-Sweep-Compact算法，这将导致长时间停止世界暂停。
检查您的GC日志我没有看到任何暗示3.）正在发生。检查jstat-log时，您可以看到FGC计数器在“CMS初始标记”和“CMS最终备注”上增加，所以它似乎只计算了两个正常的停止阶段CMS-集合。
答案 1 :(得分：1)
CMS将使用FULL GC。
所有垃圾收集器都使用次要GC或完整GC。 Minor GCs将收集年轻一代，而FullGCs将收集年轻人，老年人和烫发者。
在CMS上，你得到的只是老一代的吞吐量收集器，这意味着这个收集器将试图避免大停止世界暂停。它有不同的阶段，就像你可以在你的日志上看到的，我不会进入CMS阶段的详细信息，但您需要知道，当您的旧一代无法分配终身对象时，保证会发生完整集合。因此CMS确实执行FULLGC。
从您的jstat日志疑问开始，CMS上的FullGC将始终以2增量增加FULLGC指标，但只发生了一个FULLGC。 （我已经在官方的jstat文档中读过这个，但是现在还没有找到它，但是一旦我遇到了同样的疑问，我已经亲自在多个JVM和网站上验证了这一点。）
答案 2 :(得分：0)
尝试按照JVM参数获取详细的GC日志，我们可以进一步调查
-XX:+PrintPromotionFailure To get more details about promotion failures
-XX:PrintFLSStatistics=1 To get more details about young and old generation statistics