Question

我们调整GC以实现最小的“停止世界”暂停。彼尔姆和终身世代表现良好。年轻人大多数时间都工作正常，停顿时间通常不超过500毫秒（注意[时间：用户= 0.35 sys = 0.02，真实= 0.06秒]）：

{Heap before GC invocations=11603 (full 60):
 par new generation   total 3640320K, used 3325226K [0x0000000600000000, 0x00000006f6e00000, 0x00000006f6e00000)
  eden space 3235840K, 100% used [0x0000000600000000, 0x00000006c5800000, 0x00000006c5800000)
  from space 404480K,  22% used [0x00000006de300000, 0x00000006e3a4a898, 0x00000006f6e00000)
  to   space 404480K,   0% used [0x00000006c5800000, 0x00000006c5800000, 0x00000006de300000)
 concurrent mark-sweep generation total 4147200K, used 1000363K [0x00000006f6e00000, 0x00000007f4000000, 0x00000007f4000000)
 concurrent-mark-sweep perm gen total 196608K, used 133030K [0x00000007f4000000, 0x0000000800000000, 0x0000000800000000)
2012-07-16T14:36:05.641+0200: 427048.412: [GC 427048.412: [ParNew
Desired survivor size 207093760 bytes, new threshold 4 (max 4)
- age   1:    8688880 bytes,    8688880 total
- age   2:   12432832 bytes,   21121712 total
- age   3:   18200488 bytes,   39322200 total
- age   4:   20473336 bytes,   59795536 total
: 3325226K->75635K(3640320K), 0.0559610 secs] 4325590K->1092271K(7787520K), 0.0562630 secs] [Times: user=0.35 sys=0.02, real=0.06 secs] 
Heap after GC invocations=11604 (full 60):
 par new generation   total 3640320K, used 75635K [0x0000000600000000, 0x00000006f6e00000, 0x00000006f6e00000)
  eden space 3235840K,   0% used [0x0000000600000000, 0x0000000600000000, 0x00000006c5800000)
  from space 404480K,  18% used [0x00000006c5800000, 0x00000006ca1dcf40, 0x00000006de300000)
  to   space 404480K,   0% used [0x00000006de300000, 0x00000006de300000, 0x00000006f6e00000)
 concurrent mark-sweep generation total 4147200K, used 1016635K [0x00000006f6e00000, 0x00000007f4000000, 0x00000007f4000000)
 concurrent-mark-sweep perm gen total 196608K, used 133030K [0x00000007f4000000, 0x0000000800000000, 0x0000000800000000)
}

然而，有时候，通常每天一次，我们得到特别长的Young垃圾收集时间（注意[时间：用户= 0.41 sys = 0.01，真实= 5.51秒]）：

{Heap before GC invocations=7884 (full 37):
 par new generation   total 3640320K, used 3304448K [0x0000000600000000, 0x00000006f6e00000, 0x00000006f6e00000)
  eden space 3235840K, 100% used [0x0000000600000000, 0x00000006c5800000, 0x00000006c5800000)
  from space 404480K,  16% used [0x00000006c5800000, 0x00000006c9b00370, 0x00000006de300000)
  to   space 404480K,   0% used [0x00000006de300000, 0x00000006de300000, 0x00000006f6e00000)
 concurrent mark-sweep generation total 4147200K, used 1967225K [0x00000006f6e00000, 0x00000007f4000000, 0x00000007f4000000)
 concurrent-mark-sweep perm gen total 189100K, used 112825K [0x00000007f4000000, 0x00000007ff8ab000, 0x0000000800000000)
2012-07-15T01:23:25.474+0200: 293088.245: [GC 293093.636: [ParNew
Desired survivor size 207093760 bytes, new threshold 4 (max 4)
- age   1:   30210472 bytes,   30210472 total
- age   2:   11614600 bytes,   41825072 total
- age   3:    8591680 bytes,   50416752 total
- age   4:    7779600 bytes,   58196352 total
: 3304448K->73854K(3640320K), 0.1158280 secs] 5271674K->2046454K(7787520K), 0.1181990 secs] [Times: user=0.41 sys=0.01, real=5.51 secs] 
Heap after GC invocations=7885 (full 37):
 par new generation   total 3640320K, used 73854K [0x0000000600000000, 0x00000006f6e00000, 0x00000006f6e00000)
  eden space 3235840K,   0% used [0x0000000600000000, 0x0000000600000000, 0x00000006c5800000)
  from space 404480K,  18% used [0x00000006de300000, 0x00000006e2b1fb40, 0x00000006f6e00000)
  to   space 404480K,   0% used [0x00000006c5800000, 0x00000006c5800000, 0x00000006de300000)
 concurrent mark-sweep generation total 4147200K, used 1972599K [0x00000006f6e00000, 0x00000007f4000000, 0x00000007f4000000)
 concurrent-mark-sweep perm gen total 189100K, used 112825K [0x00000007f4000000, 0x00000007ff8ab000, 0x0000000800000000)
}

以下是jstat -gccause：

的相关输出

Timestamp         S0     S1     E      O      P     YGC     YGCT    FGC    FGCT     GCT    LGCC                 GCC                 
   293083.2  16.96   0.00  96.26  47.44  59.66   7884  772.326    74   10.893  783.219 unknown GCCause      No GC               
   293083.2  16.96   0.00  96.26  47.44  59.66   7884  772.326    74   10.893  783.219 unknown GCCause      No GC               
   293084.2  16.96   0.00  97.69  47.44  59.66   7884  772.326    74   10.893  783.219 unknown GCCause      No GC               
   293085.3  16.96   0.00  98.32  47.44  59.66   7884  772.326    74   10.893  783.219 unknown GCCause      No GC               
   293086.3  16.96   0.00  99.32  47.44  59.66   7884  772.326    74   10.893  783.219 unknown GCCause      No GC               
Timestamp         S0     S1     E      O      P     YGC     YGCT    FGC    FGCT     GCT    LGCC                 GCC                 
   293087.3  16.96   0.00  99.55  47.44  59.66   7884  772.326    74   10.893  783.219 unknown GCCause      No GC               
   293088.3  16.96   0.00 100.00  47.44  59.66   7884  772.326    74   10.893  783.219 unknown GCCause      Allocation Failure  
   293089.3  16.96   0.00 100.00  47.44  59.66   7884  772.326    74   10.893  783.219 unknown GCCause      Allocation Failure  
   293090.3  16.96   0.00 100.00  47.44  59.66   7884  772.326    74   10.893  783.219 unknown GCCause      Allocation Failure  
   293091.3  16.96   0.00 100.00  47.44  59.66   7884  772.326    74   10.893  783.219 unknown GCCause      Allocation Failure  
   293092.3  16.96   0.00 100.00  47.44  59.66   7884  772.326    74   10.893  783.219 unknown GCCause      Allocation Failure  
   293093.3  16.96   0.00 100.00  47.44  59.66   7884  772.326    74   10.893  783.219 unknown GCCause      Allocation Failure  
   293084.2  16.96   0.00 100.00  47.44  59.66   7884  772.326    74   10.893  783.219 unknown GCCause      Allocation Failure  
   293094.3   0.00  18.26   6.23  47.56  59.66   7885  772.442    74   10.893  783.334 unknown GCCause      No GC               
   293094.6   0.00  18.26   6.71  47.56  59.66   7885  772.442    74   10.893  783.334 unknown GCCause      No GC               
   293095.3   0.00  18.26   6.85  47.56  59.66   7885  772.442    74   10.893  783.334 unknown GCCause      No GC               
   293095.6   0.00  18.26   6.92  47.56  59.66   7885  772.442    74   10.893  783.334 unknown GCCause      No GC               
   293096.2   0.00  18.26   9.84  47.56  59.66   7885  772.442    74   10.893  783.334 unknown GCCause      No GC               
   293096.6   0.00  18.26  10.11  47.56  59.66   7885  772.442    74   10.893  783.334 unknown GCCause      No GC

“分配失败”在其他地方也显示为GC原因，但始终作为单个条目。当它出现在这样的序列中时，它与长GC暂停相关联。我查看了Oracle JVM源代码，“分配失败”看起来非常自然：Young没有足够的空间来获取新数据，现在是时候清理了。

我在暂停发生前检查了系统中任何内存密集，意外的操作，但没有发现任何可疑行为。

请注意，在暂停时间内，年轻垃圾收集时间不会增加太多。我的内存和GC设置如下（省略了日志设置）：

-Xms6000m 
-Xmn2950m 
-Xmx6000m 
-XX:MaxPermSize=192m 
-XX:+UseConcMarkSweepGC 
-XX:+UseParNewGC 
-Dsun.rmi.dgc.client.gcInterval=9223372036854775807 
-Dsun.rmi.dgc.server.gcInterval=9223372036854775807
-XX:CMSInitiatingOccupancyFraction=50 
-XX:+CMSScavengeBeforeRemark 
-XX:+CMSClassUnloadingEnabled

还测试了8000米和12000米的堆。机器：

8核，16GB内存
24核，50GB内存

所以我的基本问题是：为什么ParNewGC偶然会这样做？

Answer 1

在执行GC之前，必须将每个线程都置于安全点（它不会立即停止每个线程）。如果您长时间运行JNI呼叫或系统呼叫，则可能需要很长时间才能达到安全点。你在这种情况下看到的是一个长时间的停顿，即使收集本身具有正常的时间长度。

长期附带的年轻垃圾收集暂停

1 个答案: