java - 如何确定我使用的是哪个 GC？

Question

我没有指定任何 GC，我想我的 JVM 没有默认启用任何 GC。

我当然知道OpenJDK8默认使用ParallelGC，但我觉得应该可以通过命令行打印，像这样:

java -XX:+PrintFlagsFinal|grep 使用|grep GC

我希望输出包含 bool UseParallelOldGC = true {product} 但它不是:

     bool ParGCUseLocalOverflow                     = false                               {product}
     bool UseAdaptiveGCBoundary                     = false                               {product}
     bool UseAdaptiveSizeDecayMajorGCCost           = true                                {product}
     bool UseAdaptiveSizePolicyWithSystemGC         = false                               {product}
     bool UseAutoGCSelectPolicy                     = false                               {product}
     bool UseConcMarkSweepGC                        = false                               {product}
     bool UseDynamicNumberOfGCThreads               = false                               {product}
     bool UseG1GC                                   = false                               {product}
     bool UseGCLogFileRotation                      = false                               {product}
     bool UseGCOverheadLimit                        = true                                {product}
     bool UseGCTaskAffinity                         = false                               {product}
     bool UseMaximumCompactionOnSystemGC            = true                                {product}
     bool UseParNewGC                               = false                               {product}
     bool UseParallelGC                             = false                               {product}
     bool UseParallelOldGC                          = false                               {product}
     bool UseSerialGC                               = false                               {product}

和

java -XX:+PrintCommandLineFlags -version

我希望输出包含:XX:+UseParallelGC 但它也不是:

-XX:InitialHeapSize=460493056 -XX:MaxHeapSize=7367888896 -XX:+PrintCommandLineFlags -XX:+UseCompressedClassPointers -XX:+UseCompressedOops 
.

我的 JVM 选项:

-XX:+PrintGCDetails -XX:+PrintGCTimeStamps -XX:+PrintGCDateStamps -XX:+PrintHeapAtGC -verbose:gc -XX:+PrintTenuringDistribution -XX:+PrintGCApplicationStoppedTime
.

上面的输出显示没有启用任何GC(我认为是这样)，我对这种情况感到很困惑。

GC 记录输出如下:

OpenJDK 64-Bit Server VM (25.181-b13) for linux-amd64 JRE (1.8.0_181-b13), built on Oct 23 2018 11:39:12 by "buildozer" with gcc 6.4.0
Memory: 4k page, physical 28780816k(6283132k free), swap 0k(0k free)
CommandLine flags: -XX:InitialHeapSize=460493056 -XX:MaxHeapSize=7367888896 -XX:+PrintGC -XX:+PrintGCApplicationStoppedTime -XX:+PrintGCDateStamps -XX:+PrintGCDeta
ils -XX:+PrintGCTimeStamps -XX:+PrintHeapAtGC -XX:+PrintTenuringDistribution -XX:+UseCompressedClassPointers -XX:+UseCompressedOops 
{Heap before GC invocations=0 (full 0):
 def new generation   total 135168K, used 120192K [0x0000000608c00000, 0x0000000611ea0000, 0x000000069b2a0000)
  eden space 120192K, 100% used [0x0000000608c00000, 0x0000000610160000, 0x0000000610160000)
  from space 14976K,   0% used [0x0000000610160000, 0x0000000610160000, 0x0000000611000000)
  to   space 14976K,   0% used [0x0000000611000000, 0x0000000611000000, 0x0000000611ea0000)
 tenured generation   total 300416K, used 0K [0x000000069b2a0000, 0x00000006ad800000, 0x00000007c0000000)
   the space 300416K,   0% used [0x000000069b2a0000, 0x000000069b2a0000, 0x000000069b2a0200, 0x00000006ad800000)
 Metaspace       used 20532K, capacity 20780K, committed 20992K, reserved 1067008K
  class space    used 2628K, capacity 2726K, committed 2816K, reserved 1048576K
2019-02-25T06:29:46.105+0000: 2.890: [GC (Allocation Failure) 2019-02-25T06:29:46.105+0000: 2.890: [DefNew
Desired survivor size 7667712 bytes, new threshold 1 (max 15)
- age   1:   15335424 bytes,   15335424 total
: 120192K->14976K(135168K), 0.0238110 secs] 120192K->18041K(435584K), 0.0238765 secs] [Times: user=0.01 sys=0.01, real=0.02 secs] 
Heap after GC invocations=1 (full 0):
 def new generation   total 135168K, used 14976K [0x0000000608c00000, 0x0000000611ea0000, 0x000000069b2a0000)
  eden space 120192K,   0% used [0x0000000608c00000, 0x0000000608c00000, 0x0000000610160000)
  from space 14976K, 100% used [0x0000000611000000, 0x0000000611ea0000, 0x0000000611ea0000)
  to   space 14976K,   0% used [0x0000000610160000, 0x0000000610160000, 0x0000000611000000)
 tenured generation   total 300416K, used 3065K [0x000000069b2a0000, 0x00000006ad800000, 0x00000007c0000000)
   the space 300416K,   1% used [0x000000069b2a0000, 0x000000069b59e660, 0x000000069b59e800, 0x00000006ad800000)
 Metaspace       used 20532K, capacity 20780K, committed 20992K, reserved 1067008K
  class space    used 2628K, capacity 2726K, committed 2816K, reserved 1048576K
}
2019-02-25T06:29:46.129+0000: 2.914: Total time for which application threads were stopped: 0.0241189 seconds, Stopping threads took: 0.0000181 seconds
{Heap before GC invocations=1 (full 0):
 def new generation   total 135168K, used 21860K [0x0000000608c00000, 0x0000000611ea0000, 0x000000069b2a0000)
  eden space 120192K,   5% used [0x0000000608c00000, 0x00000006092b93f8, 0x0000000610160000)
  from space 14976K, 100% used [0x0000000611000000, 0x0000000611ea0000, 0x0000000611ea0000)
  to   space 14976K,   0% used [0x0000000610160000, 0x0000000610160000, 0x0000000611000000)
 tenured generation   total 300416K, used 3065K [0x000000069b2a0000, 0x00000006ad800000, 0x00000007c0000000)
   the space 300416K,   1% used [0x000000069b2a0000, 0x000000069b59e660, 0x000000069b59e800, 0x00000006ad800000)
 Metaspace       used 20982K, capacity 21132K, committed 21248K, reserved 1067008K
  class space    used 2667K, capacity 2758K, committed 2816K, reserved 1048576K
2019-02-25T06:29:46.187+0000: 2.972: [Full GC (Metadata GC Threshold) 2019-02-25T06:29:46.187+0000: 2.972: [Tenured: 3065K->9617K(300416K), 0.0270556 secs] 24926K-
>9617K(435584K), [Metaspace: 20982K->20982K(1067008K)], 0.0271334 secs] [Times: user=0.03 sys=0.00, real=0.03 secs] 
Heap after GC invocations=2 (full 1):
 def new generation   total 135296K, used 0K [0x0000000608c00000, 0x0000000611ec0000, 0x000000069b2a0000)
  eden space 120320K,   0% used [0x0000000608c00000, 0x0000000608c00000, 0x0000000610180000)
  from space 14976K,   0% used [0x0000000610180000, 0x0000000610180000, 0x0000000611020000)
  to   space 14976K,   0% used [0x0000000611020000, 0x0000000611020000, 0x0000000611ec0000)
 tenured generation   total 300416K, used 9617K [0x000000069b2a0000, 0x00000006ad800000, 0x00000007c0000000)
   the space 300416K,   3% used [0x000000069b2a0000, 0x000000069bc04698, 0x000000069bc04800, 0x00000006ad800000)
 Metaspace       used 20982K, capacity 21132K, committed 21248K, reserved 1067008K
  class space    used 2667K, capacity 2758K, committed 2816K, reserved 1048576K
}

(从应用开始到第一次主要 GC)

日志显示JVM堆分为new和tenured，但没有打印GC类型。

可悲的是，我也不能使用 jmap -heap {pid} 来获取 GC 类型，因为 jmap 在我的环境中没有这个选项(-heap)。

所以我想知道:

1. 我真正使用的是哪个 GC？
2. 命令行选项(-XX:+PrintCommandLineFlags和-XX:+PrintFlagsFinal)输出信息是否正确？

我的环境: k8s+docker、Alpine OpenJKD8

Answer 1

jcmd <pid> VM.flags 应该有所帮助。

例如

$ /usr/java/jdk1.8.0_202/bin/java Test
...
$ jcmd 28815 VM.flags
28815:
-XX:CICompilerCount=3 -XX:InitialHeapSize=266338304 -XX:MaxHeapSize=4257218560 -XX:MaxNewSize=1418723328 -XX:MinHeapDeltaBytes=524288 -XX:NewSize=88604672 -XX:OldSize=177733632 -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseParallelGC

最后打印的标志是-XX:+UseParallelGC即使尚未明确指定。

然后如果我运行以 G1 作为默认收集器的 JDK 11，我将得到以下输出:

$ /usr/java/jdk11.0.2/bin/java Test
...
$ jcmd 28862 VM.flags
28862:
-XX:CICompilerCount=3 -XX:ConcGCThreads=1 -XX:G1ConcRefinementThreads=4 -XX:G1HeapRegionSize=1048576 -XX:GCDrainStackTargetSize=64 -XX:InitialHeapSize=266338304 -XX:MarkStackSize=4194304 -XX:MaxHeapSize=4257218560 -XX:MaxNewSize=2554331136 -XX:MinHeapDeltaBytes=1048576 -XX:NonNMethodCodeHeapSize=5830092 -XX:NonProfiledCodeHeapSize=122914074 -XX:ProfiledCodeHeapSize=122914074 -XX:ReservedCodeCacheSize=251658240 -XX:+SegmentedCodeCache -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseFastUnorderedTimeStamps -XX:+UseG1GC

现在最后打印的选项是-XX:+UseG1GC .

如果由于某种原因你没有 jcmd可用，你可以试试 jattach 这是 jcmd 的轻量级独立替代品:

$ jattach 28862 jcmd VM.flags

更新

如果没有-XX:Use*GC flags 被打印出来，这意味着使用了 Serial GC。这只发生在 JDK 8 及更早版本上。从 JDK 9 开始，Use*GC 之一标志由 JVM 自动设置。参见 JDK-8068582了解详情。