频繁的主要gc而不是空闲堆？

Question

运行几个小时后，我的http服务器开始频繁地使用主gc，但没有释放堆。

几次主要gc之后，出现了promotion failed和concurrent mode failure，然后堆被释放。我的gc日志如下：

{Heap before GC invocations=7172 (full 720):
 par new generation   total 737280K, used 667492K [0x000000076b800000, 0x000000079d800000, 0x000000079d800000)
  eden space 655360K, 100% used [0x000000076b800000, 0x0000000793800000, 0x0000000793800000)
  from space 81920K,  14% used [0x0000000793800000, 0x00000007943d91d0, 0x0000000798800000)
  to   space 81920K,   0% used [0x0000000798800000, 0x0000000798800000, 0x000000079d800000)
 concurrent mark-sweep generation total 1482752K, used 1479471K [0x000000079d800000, 0x00000007f8000000, 0x00000007f8000000)
 concurrent-mark-sweep perm gen total 131072K, used 58091K [0x00000007f8000000, 0x0000000800000000, 0x0000000800000000)
2015-11-19T21:50:02.692+0800: 113963.532: [GC2015-11-19T21:50:02.692+0800: 113963.532: [ParNew (promotion failed)
Desired survivor size 41943040 bytes, new threshold 15 (max 15)
- age   1:    3826144 bytes,    3826144 total
- age   2:     305696 bytes,    4131840 total
- age   3:     181416 bytes,    4313256 total
- age   4:     940632 bytes,    5253888 total
- age   5:      88368 bytes,    5342256 total
- age   6:     159840 bytes,    5502096 total
- age   7:     733856 bytes,    6235952 total
- age   8:      64712 bytes,    6300664 total
- age   9:     314304 bytes,    6614968 total
- age  10:     587160 bytes,    7202128 total
- age  11:      38728 bytes,    7240856 total
- age  12:     221160 bytes,    7462016 total
- age  13:     648376 bytes,    8110392 total
- age  14:      33296 bytes,    8143688 total
- age  15:     380768 bytes,    8524456 total
: 667492K->665908K(737280K), 0.7665810 secs]2015-11-19T21:50:03.459+0800: 113964.299: [CMS2015-11-19T21:50:05.161+0800: 113966.001: [CMS-concurrent-mark: 3.579/4.747 secs] [Times: user=13.41 sys=0.35, rea
l=4.75 secs] 
 (concurrent mode failure): 1479910K->44010K(1482752K), 4.7267420 secs] 2146964K->44010K(2220032K), [CMS Perm : 58091K->57795K(131072K)], 5.4939440 secs] [Times: user=9.07 sys=0.13, real=5.49 secs] 
Heap after GC invocations=7173 (full 721):
 par new generation   total 737280K, used 0K [0x000000076b800000, 0x000000079d800000, 0x000000079d800000)
  eden space 655360K,   0% used [0x000000076b800000, 0x000000076b800000, 0x0000000793800000)
  from space 81920K,   0% used [0x0000000798800000, 0x0000000798800000, 0x000000079d800000)
  to   space 81920K,   0% used [0x0000000793800000, 0x0000000793800000, 0x0000000798800000)
 concurrent mark-sweep generation total 1482752K, used 44010K [0x000000079d800000, 0x00000007f8000000, 0x00000007f8000000)
 concurrent-mark-sweep perm gen total 131072K, used 57795K [0x00000007f8000000, 0x0000000800000000, 0x0000000800000000)
}  

CMS GC似乎没有任何意义。你能给我解释一下吗？

这是我的gc配置：

/usr/local/jdk1.7.0_79/bin/java 
-server 
-Xms2248m 
-Xmx2248m 
-Xmn800m 
-XX:PermSize=128m 
-XX:MaxPermSize=128m 
-XX:MaxTenuringThreshold=15 
-XX:+UseCMSCompactAtFullCollection 
-XX:CMSFullGCsBeforeCompaction=0 
-XX:+UseConcMarkSweepGC 
-XX:+PrintGCDetails 
-XX:+PrintGCTimeStamps 
-XX:+PrintGCDateStamps 
-Xloggc:gc.log 
-XX:+PrintHeapAtGC 
-XX:+PrintTenuringDistribution 
-XX:+UseFastAccessorMethods

更新

自服务器启动以来，有一个定期任务。它的工作是从mysql加载数据并保存在jvm堆中。当客户端请求出现时，服务器应该使用数据进行计算。这样的任务代码：

private volatile List<ActivityInfo> activityInfos;

public void run () {
    activityInfos = db.loadActivity();
}

public ActivityInfo getActivityByClient() {
    //
    List<ActivityInfo> local = activityInfos; 
    // biz code
    ActivityInfo response = // biz code
    return response;
}

// executor 
executor.scheduleWithFixedDelay(task, 0, 1, TimeUnit.MINUTES);

最让我困惑的是，为什么堆在满gc之后才被释放，而不是在主gc之后释放？

更新

full gc log is here

Answer 1

这意味着您的运行非常接近您的最大堆大小，因此有频繁的GC，但很少被释放。试着大幅增加，比如增加1.5倍或2倍。

Answer 2

为了解决这个问题，您可以使用Eclipse 。它将向您详细展示所有这些与内存和线程相关的问题。您还可以使用jConsole来实现它。