JVM系列【6】GC與調優5-日誌分析

JVM系列筆記目錄

  • 虛擬機的基礎概念
  • class文件結構
  • class文件載入過程
  • jvm記憶體模型
  • JVM常用指令
  • GC與調優

主要內容

分析PS、CMS、G1的回收日誌，目標使大概能讀懂GC日誌。

測試程式

java的版本是1.8，測試思路是用死循環中不停分配1M大小的數組 ，這樣在啟動時候指定較小的您記憶體空間，很快就會產生GC。

import java.util.LinkedList;
import java.util.List;

public class HelloGC {
    public static void main(String[] args) {
        System.out.println("HelloGC!");
        List list = new LinkedList();
        for(;;) {
            // 死循環中不停分配1M大小的數組 
            byte[] b = new byte[1024*1024];
            list.add(b);
        }
    }
}

PS日誌分析

  1. 啟動命令：java -Xmn10M -Xms40M -Xmx60M -XX:+PrintCommandLineFlags -XX:+PrintGC -XX:+Us eParallelGC -XX:+PrintGCDetails HelloGC。指定JVM醉倒記憶體為60M,初始記憶體為40M,年輕代的大小是10M,-XX:+UseParallelGC指定垃圾回收器為PS+PO，-XX:+PrintGCDetails列印出詳細的GC回收日誌。

  2. 日誌

    D:\>java -Xmn10M -Xms40M -Xmx60M -XX:+PrintCommandLineFlags -XX:+PrintGC -XX:+UseParallelGC   -XX:+PrintGCDetails  HelloGC
-XX:InitialHeapSize=41943040 -XX:MaxHeapSize=62914560 -XX:MaxNewSize=10485760 -XX:NewSize=10485760 -XX:+PrintCommandLineFlags -XX:+PrintGC -XX:+PrintGCDetails -XX:+UseCompresse
dClassPointers -XX:+UseCompressedOops -XX:-UseLargePagesIndividualAllocation -XX:+UseParallelGC
HelloGC!
[GC (Allocation Failure) [PSYoungGen: 7292K->784K(9216K)] 7292K->6936K(39936K), 0.0035951 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
[GC (Allocation Failure) [PSYoungGen: 8112K->840K(9216K)] 14264K->14160K(39936K), 0.0034203 secs] [Times: user=0.03 sys=0.06, real=0.00 secs]

[GC (Allocation Failure) [PSYoungGen: 8160K->736K(9216K)] 21481K->21224K(39936K), 0.0038512 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]

[GC (Allocation Failure) [PSYoungGen: 8059K->832K(9216K)] 28547K->28488K(39936K), 0.0031367 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]

[Full GC (Ergonomics) [PSYoungGen: 832K->0K(9216K)] [ParOldGen: 27656K->28287K(46080K)] 28488K->28287K(55296K), [Metaspace: 2710K->2710K(1056
768K)], 0.0172805 secs] [Times: user=0.02 sys=0.00, real=0.02 secs]
[GC (Allocation Failure) [PSYoungGen: 7325K->224K(8704K)] 35612K->35679K(54784K), 0.0034935 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]

[GC (Allocation Failure) [PSYoungGen: 7542K->1248K(8704K)] 42997K->42847K(54784K), 0.0032856 secs] [Times: user=0.00 sys=0.00, real=0.00 secs
]
[Full GC (Ergonomics) [PSYoungGen: 1248K->0K(8704K)] [ParOldGen: 41599K->42623K(51200K)] 42847K->42623K(59904K), [Metaspace: 2710K->2710K(105
6768K)], 0.0056374 secs] [Times: user=0.00 sys=0.00, real=0.01 secs]
[GC (Allocation Failure) [PSYoungGen: 6278K->1216K(8704K)] 48902K->48960K(59904K), 0.0034283 secs] [Times: user=0.00 sys=0.00, real=0.00 secs
]
[Full GC (Ergonomics) [PSYoungGen: 1216K->0K(8704K)] [ParOldGen: 47744K->48768K(51200K)] 48960K->48768K(59904K), [Metaspace: 2710K->2710K(105
6768K)], 0.0044365 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
[Full GC (Ergonomics) [PSYoungGen: 6272K->4096K(8704K)] [ParOldGen: 48768K->50816K(51200K)] 55040K->54912K(59904K), [Metaspace: 2710K->2710K(
1056768K)], 0.0060174 secs] [Times: user=0.09 sys=0.02, real=0.01 secs]
[Full GC (Ergonomics) [PSYoungGen: 6268K->6144K(8704K)] [ParOldGen: 50816K->50816K(51200K)] 57084K->56960K(59904K), [Metaspace: 2710K->2710K(
1056768K)], 0.0043812 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
[Full GC (Allocation Failure) [PSYoungGen: 6144K->6144K(8704K)] [ParOldGen: 50816K->50803K(51200K)] 56960K->56948K(59904K), [Metaspace: 2710K
->2710K(1056768K)], 0.0164153 secs] [Times: user=0.09 sys=0.00, real=0.02 secs]
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
        at HelloGC.main(HelloGC.java:9)
Heap
 PSYoungGen      total 8704K, used 6385K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
  eden space 7168K, 89% used 0x00000000ff600000,0x00000000ffc3c518,0x00000000ffd00000)
  from space 1536K, 0% used [0x00000000ffd00000,0x00000000ffd00000,0x00000000ffe80000)
  to   space 1536K, 0% used [0x00000000ffe80000,0x00000000ffe80000,0x0000000100000000)
 ParOldGen       total 51200K, used 50803K [0x00000000fc400000, 0x00000000ff600000, 0x00000000ff600000)
  object space 51200K, 99% used [0x00000000fc400000,0x00000000ff59cfd0,0x00000000ff600000)
 Metaspace       used 2739K, capacity 4486K, committed 4864K, reserved 1056768K
  class space    used 304K, capacity 386K, committed 512K, reserved 1048576K
    • YGC分析

file

  • FGC分析

file

  • 堆空間分析

file

CMS日誌

回顧一下CMS回收主要4個階段：初始標記、並發標記、最終標記、並發回收。

補充一個Card Table的概念

由於做YGC時，需要掃描整個OLD區，效率非常低，所以JVM設計了CardTable， 如果一個OLD區CardTable中某個Card有對象指向Y區，就將它設為Dirty，下次掃描時，只需要掃描Dirty Card。在結構上，Card Table用BitMap來實現。

  1. 啟動命令java -Xmn20M -Xmx20M -XX:+PrintCommandLineFlags -XX:+PrintGC -XX:+PrintGCDetails -XX:+UseConcMarkSweepGC HelloGC。參數 -XX:+UseConcMarkSweepGC指定使用CMS垃圾回收器。

  2. 日誌分析

    • YGC

      [GC (Allocation Failure) [ParNew: 6144K->640K(6144K), 0.0265885 secs] 6585K->2770K(19840K), 0.0268035 secs] [Times: user=0.02 sys=0.00, real=0.02 secs]

      ParNew：年輕代收集器

      6144->640：收集前後的對比

      （6144）：整個年輕代容量

      6585 -> 2770：整個堆的情況

      （19840）：整個堆大小

    • CMS的回收

      // 初始標記
//8511 (13696) : 老年代使用（最大）
//9866 (19840) : 整個堆使用（最大）
[GC (CMS Initial Mark) [1 CMS-initial-mark: 8511K(13696K)] 9866K(19840K), 0.0040321 secs] [Times: user=0.01 sys=0.00, real=0.00 secs] 

//並發標記 這裡的時間意義不大，因為是並發執行
[CMS-concurrent-mark-start]
[CMS-concurrent-mark: 0.018/0.018 secs] [Times: user=0.01 sys=0.00, real=0.02 secs] 

// 標記Card為Dirty，也稱為Card Marking
[CMS-concurrent-preclean-start]
[CMS-concurrent-preclean: 0.000/0.000 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 

//最終標記 STW階段
//YG occupancy:年輕代佔用及容量
//[Rescan (parallel)：STW下的存活對象標記
//weak refs processing: 弱引用處理
//class unloading: 卸載用不到的class
//scrub symbol(string) table: 官方的解釋cleaning up symbol and string tables which //hold class-level metadata and internalized string respectively
//CMS-remark: 8511K(13696K): 階段過後的老年代佔用及容量
//10108K(19840K): 階段過後的堆佔用及容量
[GC (CMS Final Remark) [YG occupancy: 1597 K (6144 K)][Rescan (parallel) , 0.0008396 secs][weak refs processing, 0.0000138 secs][class unloading, 0.0005404 secs][scrub symbol table, 0.0006169 secs][scrub string table, 0.0004903 secs][1 CMS-remark: 8511K(13696K)] 10108K(19840K), 0.0039567 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 

// 最終標記 標記已經完成，進行並發清理
[CMS-concurrent-sweep-start]
[CMS-concurrent-sweep: 0.005/0.005 secs] [Times: user=0.00 sys=0.00, real=0.01 secs] 

//重置內部結構，為下次GC做準備
[CMS-concurrent-reset-start]
[CMS-concurrent-reset: 0.000/0.000 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]

G1日誌

回顧一下，G1是分region回收的，有YGC、MixedGC(類似CMS)、FGC。

官方提供了 Garbage First Garbage Collector Tuning， 可以參考。

  1. 啟動命令java -Xms20M -Xmx20M -XX:+PrintGCDetails -XX:+UseConcMarkSweepGC HelloGC，其中-XX:+UseG1GC指定使用G1。

  2. 日誌分析

    //GC暫停
// (G1 Humongous Allocation) (young) 大對象分配
// initial-mark初始標記，這裡是YGC混合老年代回收MixedGC
[GC pause (G1 Humongous Allocation) (young) (initial-mark), 0.0026947 secs]

//這是一個GC執行緒 多執行緒回收的
   [Parallel Time: 2.1 ms, GC Workers: 8]
      [GC Worker Start (ms): Min: 137.2, Avg: 137.2, Max: 137.3, Diff: 0.1]
      [Ext Root Scanning (ms): Min: 0.0, Avg: 0.4, Max: 0.6, Diff: 0.6, Sum: 3.5]
      // 更新Rset
      [Update RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
         [Processed Buffers: Min: 0, Avg: 0.0, Max: 0, Diff: 0, Sum: 0]
      [Scan RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.2, Diff: 0.2, Sum: 0.2]
      //拷貝存活對象
      [Object Copy (ms): Min: 1.1, Avg: 1.2, Max: 1.3, Diff: 0.2, Sum: 9.7]
      [Termination (ms): Min: 0.0, Avg: 0.1, Max: 0.1, Diff: 0.1, Sum: 0.5]
         [Termination Attempts: Min: 1, Avg: 1.0, Max: 1, Diff: 0, Sum: 8]
      [GC Worker Other (ms): Min: 0.1, Avg: 0.3, Max: 0.7, Diff: 0.6, Sum: 2.1]
      [GC Worker Total (ms): Min: 2.0, Avg: 2.0, Max: 2.0, Diff: 0.1, Sum: 16.0]
      [GC Worker End (ms): Min: 139.2, Avg: 139.3, Max: 139.3, Diff: 0.0]
   [Code Root Fixup: 0.0 ms]
   [Code Root Purge: 0.0 ms]
   [Clear CT: 0.1 ms]
   [Other: 0.5 ms]
      [Choose CSet: 0.0 ms]
      [Ref Proc: 0.1 ms]
      [Ref Enq: 0.0 ms]
      [Redirty Cards: 0.1 ms]
      [Humongous Register: 0.0 ms]
      [Humongous Reclaim: 0.0 ms]
      [Free CSet: 0.0 ms]
   [Eden: 2048.0K(12.0M)->0.0B(4096.0K) Survivors: 0.0B->1024.0K Heap: 5122.1K(20.0M)->4880.1K(20.0M)]
 [Times: user=0.00 sys=0.00, real=0.01 secs]
 // 以下是MixedGC的其它階段 類似CMS
[GC concurrent-root-region-scan-start]
[GC concurrent-root-region-scan-end, 0.0008043 secs]
[GC concurrent-mark-start]
[GC concurrent-mark-end, 0.0001578 secs]
[GC remark [Finalize Marking, 0.0001140 secs] [GC ref-proc, 0.0001270 secs] 

// 卸載
[Unloading, 0.0011719 secs], 0.0017205 secs]
 [Times: user=0.00 sys=0.00, real=0.00 secs]
 // 清理
[GC cleanup 8034K->8034K(20M), 0.0004763 secs]
 [Times: user=0.00 sys=0.00, real=0.00 secs]

//其它的GC執行緒類似
......

//無法分配進行FGC
[Full GC (Allocation Failure)  9855K->9842K(20M), 0.0029134 secs]
   [Eden: 0.0B(1024.0K)->0.0B(1024.0K) Survivors: 0.0B->0.0B Heap: 9855.3K(20.0M)->9843.0K(20.0M)], [Metaspace: 2710K->2710K(1056768K)]
 [Times: user=0.00 sys=0.00, real=0.00 secs]
[GC concurrent-mark-abort]
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
        at HelloGC.main(HelloGC.java:9)
        
// 堆空間的情況
Heap
 garbage-first heap   total 20480K, used 9842K [0x00000000fec00000, 0x00000000fed000a0, 0x0000000100000000)
  region size 1024K, 1 young (1024K), 0 survivors (0K)
 Metaspace       used 2739K, capacity 4486K, committed 4864K, reserved 1056768K
  class space    used 304K, capacity 386K, committed 512K, reserved 1048576K

總結

本篇對PS、CMS、G1的日誌進行了較為詳細的分析，基本可以參照著看懂垃圾回收的的過程。

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