奇怪的小gc发生在正常的小gc之后

Question

2013-11-26T10:19:30.011+0800: [GC [ParNew: 2432484K->19997K(2696640K), 0.0378270 secs] 5560240K->3155822K(7691712K), 0.0398670 secs] [Times: user=0.18 sys=0.00, real=0.04 secs] 
2013-11-26T10:19:36.277+0800: [GC [ParNew: 2417093K->15777K(2696640K), 0.0372550 secs] 5552917K->3151795K(7691712K), 0.0388490 secs] [Times: user=0.28 sys=0.00, real=0.04 secs] 
**2013-11-26T10:19:36.325+0800: [GC [ParNew: 20441K->16452K(2696640K), 0.0186420 secs] 3156459K->3153092K(7691712K), 0.0200280 secs] [Times: user=0.17 sys=0.00, real=0.02 secs]** 
2013-11-26T10:19:41.464+0800: [GC [ParNew: 2413508K->22811K(2696640K), 0.0426440 secs] 5550148K->3160128K(7691712K), 0.0444710 secs] [Times: user=0.25 sys=0.00, real=0.04 secs] 

很明显，小的gc每隔5或6秒就会发生一次。

在2013-11-26T10:19:36.277之后，在2013-11-26T10:19:36.325有一个次要的gc只使用了20441K！！！

为什么会发生小的gc（行上方的blod）？谁知道啊？

更多详细信息：

内存总数16322984 KB

jvm（热点1.6.0_26)的参数是：

-xms7804m-xmx7804m-xmn2926m-xx:permsize=256m-xx:maxpermsize=256m-xx:survivorratio=8-xx:targetsurvivorratio=70-xx:maxtenuringthreshold=10-server-xss256k-xx:+heapdumponoutofmemoryerror-xx:+printgcdetails-xx:+printgcdatestamps-xx:+useconcmarksweepgc-xx:+disableexplicitgc

@Alexey Ragozin更多日志：

2013-11-27T23:34:47.352+0800: [GC [ParNew: 2496458K->81521K(2696640K), 0.0381510 secs]     5104803K->2690529K(7691712K), 0.0406260 secs] [Times: user=0.28 sys=0.00, real=0.04 secs] 
2013-11-27T23:34:51.745+0800: [GC [ParNew: 2478577K->57599K(2696640K), 0.0535680 secs] 5087585K->2716762K(7691712K), 0.0554400 secs] [Times: user=0.28 sys=0.01, real=0.06 secs] 
2013-11-27T23:34:55.728+0800: [GC [ParNew: 2454747K->19841K(2696640K), 0.0300150 secs] 5113910K->2679701K(7691712K), 0.0320030 secs] [Times: user=0.18 sys=0.00, real=0.03 secs] 
**2013-11-27T23:34:55.769+0800: [GC [ParNew: 21438K->16389K(2696640K), 0.0171200 secs] 2681299K->2676788K(7691712K), 0.0187850 secs] [Times: user=0.13 sys=0.00, real=0.02 secs]** 
2013-11-27T23:34:59.888+0800: [GC [ParNew: 2413445K->16017K(2696640K), 0.0251400 secs] 5073844K->2676744K(7691712K), 0.0271570 secs] [Times: user=0.15 sys=0.00, real=0.02 secs] 
2013-11-27T23:35:04.374+0800: [GC [ParNew: 2413073K->16059K(2696640K), 0.0240360 secs] 5073800K->2677460K(7691712K), 0.0259960 secs] [Times: user=0.15 sys=0.00, real=0.03 secs] 
... ...
2013-11-28T02:56:57.719+0800: [GC [ParNew: 2449927K->45500K(2696640K), 0.0360740 secs] 6195838K->3791742K(7691712K), 0.0379370 secs] [Times: user=0.23 sys=0.00, real=0.03 secs] 
2013-11-28T02:57:37.987+0800: [GC [ParNew: 2442556K->54097K(2696640K), 0.0383490 secs] 6188798K->3800678K(7691712K), 0.0402170 secs] [Times: user=0.23 sys=0.00, real=0.04 secs] 
2013-11-28T02:57:38.036+0800: [GC [1 CMS-initial-mark: 3746580K(4995072K)] 3801777K(7691712K), 0.0694940 secs] [Times: user=0.06 sys=0.00, real=0.07 secs] 
2013-11-28T02:57:38.770+0800: [CMS-concurrent-mark: 0.661/0.662 secs] [Times: user=2.15 sys=0.00, real=0.66 secs] 
2013-11-28T02:57:38.806+0800: [CMS-concurrent-preclean: 0.035/0.035 secs] [Times: user=0.06 sys=0.01, real=0.04 secs] 
 CMS: abort preclean due to time 2013-11-28T02:57:43.862+0800: [CMS-concurrent-abortable-preclean: 5.016/5.056 secs] [Times: user=6.82 sys=0.19, real=5.05 secs] 
**2013-11-28T02:57:43.872+0800: [GC[YG occupancy: 407766 K (2696640 K)]2013-11-28T02:57:43.872+0800: [GC [ParNew: 407766K->35780K(2696640K), 0.0236470 secs] 4154346K->3782623K(7691712K), 0.0256460 secs] [Times: user=0.20 sys=0.00, real=0.03 secs]** 
[Rescan (parallel) , 0.0119390 secs][weak refs processing, 0.8478360 secs][class unloading, 0.0661530 secs][scrub symbol & string tables, 0.0146780 secs] [1 CMS-remark: 3746843K(4995072K)] 3782623K(7691712K), 1.1138910 secs] [Times: user=1.41 sys=0.00, real=1.12 secs] 
2013-11-28T02:57:48.965+0800: [CMS-concurrent-sweep: 3.893/3.977 secs] [Times: user=5.65 sys=0.15, real=3.97 secs] 
2013-11-28T02:57:48.977+0800: [CMS-concurrent-reset: 0.012/0.012 secs] [Times: user=0.02 sys=0.00, real=0.02 secs]

Answer 1

-xx:+CMSScavengeBeForereMark是您奇怪的次要集合的一个原因。

CMS必须执行2次“停止世界”暂停以完成旧的空间收集周期

• 初始标记
• 备注

有关CMS机制的更多详细信息-了解JVM中的GC暂停，HotSpot的CMS收集器
有关JVM GC选项的更多信息-HotSpot JVM垃圾收集选项备忘单

更新
从技术上讲，还有另一个原因可能导致早产的年轻GC。如果应用程序试图分配放不下Eden的大数组，JVM可能会启动收集以释放空间。这是有可能的，但更有可能的是JVM会选择在旧空间中直接分配这个数组。