我有一个Java应用程序在G1 Gc上花费了大量时间，特别是在年轻人身上。
[Ext Root Scanning（ms）：Min：10.4，Avg：21.4，Max：137.2，Diff：126.8，Sum：385.2]
花费大量时间进行Ext Root Scanning。我无法弄清楚为什么这样做。什么是调试这个的好方法？任何帮助表示赞赏。
我之前已经读过，当代码缓存变大时会发生这种情况。代码高速缓存大小约为90 MB。我有几个节点运行相同的应用程序，它们具有更高的代码缓存大小，并且在年轻的gc中花费的时间明显更少。
我的Java版本：
java version "1.8.0_40"
Java(TM) SE Runtime Environment (build 1.8.0_40-b26)
Java HotSpot(TM) 64-Bit Server VM (build 25.40-b25, mixed mode)
我们使用的标志：
-XX:+AlwaysPreTouch -XX:G1HeapRegionSize=8388608 -XX:GCLogFileSize=26214400
-XX:+HeapDumpOnOutOfMemoryError -XX:InitialHeapSize=17179869184 -XX:+ManagementServer
-XX:MaxGCPauseMillis=40 -XX:MaxHeapSize=17179869184 -XX:NumberOfGCLogFiles=40
-XX:+ParallelRefProcEnabled -XX:+PrintAdaptiveSizePolicy -XX:+PrintCommandLineFlags
-XX:+PrintGC -XX:+PrintGCApplicationStoppedTime
-XX:+PrintGCDateStamps -XX:+PrintGCDetails -XX:+PrintGCTimeStamps
-XX:+PrintTenuringDistribution -XX:+UnlockExperimentalVMOptions
-XX:+UseCompressedClassPointers -XX:+UseCompressedOops
-XX:+UseG1GC -XX:+UseGCLogFileRotation
以下是GC Log的示例。
2018-02-25T10:26:27.325+0000: 252536.066: [GC pause (GCLocker Initiated GC) (young)
Desired survivor size 54525952 bytes, new threshold 15 (max 15)
- age 1: 4584256 bytes, 4584256 total
- age 2: 507616 bytes, 5091872 total
- age 3: 522784 bytes, 5614656 total
- age 4: 1195744 bytes, 6810400 total
- age 5: 884192 bytes, 7694592 total
- age 6: 2016552 bytes, 9711144 total
- age 7: 778576 bytes, 10489720 total
- age 8: 3066312 bytes, 13556032 total
- age 9: 584736 bytes, 14140768 total
- age 10: 297152 bytes, 14437920 total
- age 11: 709296 bytes, 15147216 total
- age 12: 608328 bytes, 15755544 total
- age 13: 609584 bytes, 16365128 total
- age 14: 637952 bytes, 17003080 total
- age 15: 2049208 bytes, 19052288 total
252536.068: [G1Ergonomics (CSet Construction) start choosing CSet, _pending_cards: 6415, predicted base time: 64.92 ms, remaining time: 0.00 ms, target pause time: 40.00 ms]
252536.068: [G1Ergonomics (CSet Construction) add young regions to CSet, eden: 95 regions, survivors: 8 regions, predicted young region time: 68.43 ms]
252536.068: [G1Ergonomics (CSet Construction) finish choosing CSet, eden: 95 regions, survivors: 8 regions, old: 0 regions, predicted pause time: 133.35 ms, target pause time: 40.00 ms]
, 0.1635855 secs]
[Parallel Time: 139.4 ms, GC Workers: 18]
[GC Worker Start (ms): Min: 252536073.2, Avg: 252536073.8, Max: 252536074.4, Diff: 1.3]
[Ext Root Scanning (ms): Min: 10.4, Avg: 21.4, Max: 137.2, Diff: 126.8, Sum: 385.2]
[Update RS (ms): Min: 0.0, Avg: 18.6, Max: 28.5, Diff: 28.5, Sum: 335.3]
[Processed Buffers: Min: 0, Avg: 63.7, Max: 91, Diff: 91, Sum: 1147]
[Scan RS (ms): Min: 0.0, Avg: 18.3, Max: 23.7, Diff: 23.7, Sum: 329.4]
[Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.1]
[Object Copy (ms): Min: 0.4, Avg: 60.4, Max: 64.8, Diff: 64.4, Sum: 1088.1]
[Termination (ms): Min: 0.0, Avg: 19.5, Max: 20.9, Diff: 20.9, Sum: 350.3]
[GC Worker Other (ms): Min: 0.1, Avg: 0.3, Max: 0.5, Diff: 0.4, Sum: 4.8]
[GC Worker Total (ms): Min: 137.8, Avg: 138.5, Max: 139.1, Diff: 1.3, Sum: 2493.0]
[GC Worker End (ms): Min: 252536212.2, Avg: 252536212.3, Max: 252536212.4, Diff: 0.3]
[Code Root Fixup: 0.8 ms]
[Code Root Purge: 0.0 ms]
[Clear CT: 7.9 ms]
[Other: 15.5 ms]
[Choose CSet: 0.0 ms]
[Ref Proc: 2.7 ms]
[Ref Enq: 0.1 ms]
[Redirty Cards: 0.3 ms]
[Humongous Reclaim: 0.0 ms]
[Free CSet: 1.8 ms]
[Eden: 760.0M(752.0M)->0.0B(776.0M) Survivors: 64.0M->40.0M Heap: 7861.5M(16.0G)->7084.8M(16.0G)]
[Times: user=2.55 sys=0.02, real=0.17 secs]