G1垃圾回收器

作者: 后来丶_a24d | 来源:发表于2020-03-30 16:52 被阅读0次

G1 的主要关注点在于达到可控的停顿时间，在这个基础上尽可能提高吞吐量
G1 没有 CMS回收器的碎片化问题（或者说不那么严重），同时提供了更加可控的停顿时间
如果你的应用使用了较大的堆，而且还要求有较低的垃圾收集停顿时间（如 0.5 秒），那么 G1 非常适合

发展历史

Serial（串行）收集器： Serial收集器是一个单线程的收集器，但它的“单线程”的意义并不仅仅是说明它只会使用一个CPU或一条收集线程去完成垃圾收集工作，更重要的是在它进行垃圾收集时，必须暂停其他所有的工作线程，直到它收集结束。
Parallel收集器 : 相比Serial收集器，Parallel最主要的优势在于使用多线程去完成垃圾清理工作，这样可以充分利用多核的特性，大幅降低gc时间。
CMS收集器：在Minor GC时会暂停所有的应用线程，并以多线程的方式进行垃圾回收。在Full GC时不再暂停应用线程，而是使用若干个后台线程定期的对老年代空间进行扫描，及时回收其中不再使用的对象。
G1收集器（或者垃圾优先收集器）的设计初衷是为了尽量缩短处理超大堆（大于4GB）时产生的停顿

G1对比CMS回收器总览

CMS分代收集器将整个堆分为年轻代、老年代和永久代，每个代的空间是确定的

CMS.png
而 G1 将整个堆划分为一个个大小相等的小块（每一块称为一个 region），每一块的内存是连续的。和分代算法一样，G1 中每个块也会充当 Eden、Survivor、Old 三种角色，但是它们不是固定的，这使得内存使用更加地灵活, 当然还有H(Humongous, 表示这些Region存储的是巨大对象（humongous object，H-obj，H-obj直接分配到了old gen），即大小大于等于region一半的对象)图中未展示

G1.png
G1堆内存中一个区域(Region)的大小可以通过-XX:G1HeapRegionSize参数指定，大小区间最小1M、最大32M，总之是2的幂次方。默认把堆内存按照2048份均分。

JVM垃圾收集器种类

新生代

Serial (第一代)
PraNew (第二代)
Parallel Scavenge (第三代)
G1收集器(第四代)

老年代

Serial Old (第一代)
Parallel Old (第二代)
CMS (第三代)
G1收集器 (第四代)

G1回收流程

基础

Remembered Sets：每个区块都有一个 RSet，用于记录进入该区块的对象引用（如区块 A 中的对象引用了区块 B，区块 B 的 Rset 需要记录这个信息），它用于实现收集过程的并行化以及使得区块能进行独立收集。总体上 Remembered Sets 消耗的内存小于 5%。
Collection Sets：将要被回收的区块集合。GC 时，在这些区块中的对象会被复制到其他区块中，总体上 Collection Sets 消耗的内存小于 1%。

年轻代YoungGC收集

当所有eden region使用达到最大阀值并且无法申请足够内存时，会触发一次YoungGC。每次younggc会回收所有Eden以及Survivor区，并且将存活对象复制到Old区以及另一部分的Survivor区
回收过程

根扫描,跟CMS类似，Stop the world，扫描GC Roots对象。
处理Dirty card,更新RSet.
扫描RSet, 扫描RSet中所有old区对扫描到的young区或者survivor去的引用。
拷贝扫描出的存活的对象到survivor2/old区
处理引用队列，软引用，弱引用，虚引用

Old GC /并发标记周期

主要完成了垃圾定位的工作，定位出了哪些分区是垃圾最多的，因为整堆一般比较大，所以这个周期应该会比较长，中间可能会被多次 stop-the-world 的 Young GC 打断。

初始标记：stop-the-world，它伴随着一次普通的 Young GC 发生，然后对 Survivor 区（root region）进行标记，因为该区可能存在对老年代的引用。因为 Young GC 是需要 stop-the-world 的，所以并发周期直接重用这个阶段，虽然会增加 CPU 开销，但是停顿时间只是增加了一小部分。
并发标记：寻找整个堆的存活对象，该阶段可以被 Young GC 中断
重新标记：stop-the-world，完成最后的存活对象标记
清理：清理阶段真正回收的内存很少。

在分配H-obj之前先检查是否超过 initiating heap occupancy percent和the marking threshold, 如果超过的话，就启动global concurrent marking，为的是提早回收，

mixed混合GC

并发周期结束后是混合垃圾回收周期，不仅进行年轻代垃圾收集，而且回收之前标记出来的老年代的垃圾最多的部分区块。混合垃圾回收周期会持续进行，直到几乎所有的被标记出来的分区（垃圾占比大的分区）都得到回收，然后恢复到常规的年轻代垃圾收集，最终再次启动并发周期。
当越来越多的对象晋升到老年代old region时，为了避免堆内存被耗尽，虚拟机会触发一个混合的垃圾收集器，即mixed gc，该算法并不是一个old gc，除了回收整个young region，还会回收一部分的old region。

Full GC

G1没有fullGC概念，需要fullGC时，调用serialOldGC进行全堆扫描, 尽量避免
导致full gc的因素

concurrent mode failure：并发模式失败，CMS 收集器也有同样的概念。G1 并发标记期间，如果在标记结束前，老年代被填满，G1 会放弃标记。这个时候说明
一. 堆需要增加了
二. 或者需要调整并发周期，如增加并发标记的线程数量，让并发标记尽快结束
三. 或者就是更早地进行并发周期，默认是整堆内存的 45% 被占用就开始进行并发周期。
晋升失败：并发周期结束后，是混合垃圾回收周期，伴随着年轻代垃圾收集，进行清理老年代空间，如果这个时候清理的速度小于消耗的速度，导致老年代不够用，那么会发生晋升失败。说明
一. 混合垃圾回收需要更迅速完成垃圾收集，也就是说在混合回收阶段，每次年轻代的收集应该处理更多的老年代已标记区块。
疏散失败：年轻代垃圾收集的时候，如果 Survivor 和 Old 区没有足够的空间容纳所有的存活对象。这种情况肯定是非常致命的，因为基本上已经没有多少空间可以用了，这个时候会触发 Full GC 也是很合理的。说明:
一. 最简单的就是增加堆大小
大对象分配失败，我们应该尽可能地不创建大对象，尤其是大于一个区块大小的那种对象。
MetaSpace满了时
System.gc()也有可能

参考: Full GC触发条件总结以及解决策略

GC日志解析

线上日志


2020-04-01T02:22:37.259+0800: 41886.299: [GC pause (G1 Evacuation Pause) (young), 0.0511506 secs]
   [Parallel Time: 49.6 ms, GC Workers: 2]
      [GC Worker Start (ms): Min: 41886299.2, Avg: 41886299.2, Max: 41886299.2, Diff: 0.0]
      [Ext Root Scanning (ms): Min: 2.6, Avg: 2.6, Max: 2.6, Diff: 0.0, Sum: 5.3]
      [Update RS (ms): Min: 1.4, Avg: 1.5, Max: 1.5, Diff: 0.0, Sum: 2.9]
         [Processed Buffers: Min: 27, Avg: 27.5, Max: 28, Diff: 1, Sum: 55]
      [Scan RS (ms): Min: 0.1, Avg: 0.1, Max: 0.1, Diff: 0.0, Sum: 0.1]
      [Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Object Copy (ms): Min: 45.3, Avg: 45.3, Max: 45.3, Diff: 0.0, Sum: 90.6]
      [Termination (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
         [Termination Attempts: Min: 1, Avg: 1.0, Max: 1, Diff: 0, Sum: 2]
      [GC Worker Other (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [GC Worker Total (ms): Min: 49.5, Avg: 49.5, Max: 49.5, Diff: 0.0, Sum: 99.0]
      [GC Worker End (ms): Min: 41886348.7, Avg: 41886348.7, Max: 41886348.7, Diff: 0.0]
   [Code Root Fixup: 0.0 ms]
   [Code Root Purge: 0.0 ms]
   [Clear CT: 0.1 ms]
   [Other: 1.5 ms]
      [Choose CSet: 0.0 ms]
      [Ref Proc: 0.1 ms]
      [Ref Enq: 0.0 ms]
      [Redirty Cards: 0.2 ms]
      [Humongous Register: 0.2 ms]
      [Humongous Reclaim: 0.2 ms]
      [Free CSet: 0.3 ms]
   [Eden: 119.0M(119.0M)->0.0B(1421.0M) Survivors: 34.0M->2048.0K Heap: 1431.1M(3072.0M)->1314.9M(3072.0M)]
 [Times: user=0.10 sys=0.00, real=0.05 secs] 
2020-04-01T02:23:15.724+0800: 41924.764: [GC pause (G1 Humongous Allocation) (young) (initial-mark), 0.0792630 secs]
   [Parallel Time: 77.2 ms, GC Workers: 2]
      [GC Worker Start (ms): Min: 41924764.6, Avg: 41924764.7, Max: 41924764.7, Diff: 0.0]
      [Ext Root Scanning (ms): Min: 3.7, Avg: 3.9, Max: 4.1, Diff: 0.3, Sum: 7.8]
      [Update RS (ms): Min: 43.2, Avg: 43.2, Max: 43.3, Diff: 0.1, Sum: 86.5]
         [Processed Buffers: Min: 253, Avg: 266.5, Max: 280, Diff: 27, Sum: 533]
      [Scan RS (ms): Min: 0.2, Avg: 0.2, Max: 0.2, Diff: 0.0, Sum: 0.4]
      [Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Object Copy (ms): Min: 29.6, Avg: 29.7, Max: 29.9, Diff: 0.3, Sum: 59.4]
      [Termination (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
         [Termination Attempts: Min: 1, Avg: 1.0, Max: 1, Diff: 0, Sum: 2]
      [GC Worker Other (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [GC Worker Total (ms): Min: 77.0, Avg: 77.1, Max: 77.1, Diff: 0.0, Sum: 154.1]
      [GC Worker End (ms): Min: 41924841.7, Avg: 41924841.7, Max: 41924841.7, Diff: 0.0]
   [Code Root Fixup: 0.0 ms]
   [Code Root Purge: 0.0 ms]
   [Clear CT: 0.2 ms]
   [Other: 1.8 ms]
      [Choose CSet: 0.0 ms]
      [Ref Proc: 0.1 ms]
      [Ref Enq: 0.0 ms]
      [Redirty Cards: 0.0 ms]
      [Humongous Register: 0.1 ms]
      [Humongous Reclaim: 0.2 ms]
      [Free CSet: 0.4 ms]
   [Eden: 565.0M(1421.0M)->0.0B(234.0M) Survivors: 2048.0K->26.0M Heap: 1918.6M(3072.0M)->1342.0M(3072.0M)]
 [Times: user=0.16 sys=0.00, real=0.08 secs] 
2020-04-01T02:23:15.804+0800: 41924.843: [GC concurrent-root-region-scan-start]
2020-04-01T02:23:15.823+0800: 41924.863: [GC concurrent-root-region-scan-end, 0.0195871 secs]
2020-04-01T02:23:15.823+0800: 41924.863: [GC concurrent-mark-start]
2020-04-01T02:23:16.076+0800: 41925.116: [GC concurrent-mark-end, 0.2529475 secs]
2020-04-01T02:23:16.077+0800: 41925.116: [GC remark 2020-04-01T02:23:16.077+0800: 41925.116: [Finalize Marking, 0.0002514 secs] 2020-04-01T02:23:16.077+0800: 41925.117: [GC ref-proc, 0.0065589 secs] 2020-04-01T02:23:16.083+0800: 41925.123: [Unloading, 0.0521240 secs], 0.0613578 secs]
 [Times: user=0.09 sys=0.02, real=0.06 secs] 
2020-04-01T02:23:16.139+0800: 41925.178: [GC cleanup 1413M->600M(3072M), 0.0063161 secs]
 [Times: user=0.01 sys=0.00, real=0.00 secs] 
2020-04-01T02:23:16.145+0800: 41925.185: [GC concurrent-cleanup-start]
2020-04-01T02:23:16.147+0800: 41925.187: [GC concurrent-cleanup-end, 0.0020016 secs]
2020-04-01T02:23:33.263+0800: 41942.303: [GC pause (G1 Evacuation Pause) (young), 0.0364350 secs]
   [Parallel Time: 35.0 ms, GC Workers: 2]
      [GC Worker Start (ms): Min: 41942303.5, Avg: 41942303.5, Max: 41942303.5, Diff: 0.0]
      [Ext Root Scanning (ms): Min: 2.3, Avg: 2.4, Max: 2.4, Diff: 0.1, Sum: 4.7]
      [Update RS (ms): Min: 10.3, Avg: 10.3, Max: 10.3, Diff: 0.0, Sum: 20.7]
         [Processed Buffers: Min: 139, Avg: 147.5, Max: 156, Diff: 17, Sum: 295]
      [Scan RS (ms): Min: 0.1, Avg: 0.1, Max: 0.1, Diff: 0.0, Sum: 0.2]
      [Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Object Copy (ms): Min: 22.0, Avg: 22.1, Max: 22.1, Diff: 0.1, Sum: 44.1]
      [Termination (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
         [Termination Attempts: Min: 1, Avg: 1.0, Max: 1, Diff: 0, Sum: 2]
      [GC Worker Other (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [GC Worker Total (ms): Min: 34.9, Avg: 34.9, Max: 34.9, Diff: 0.0, Sum: 69.7]
      [GC Worker End (ms): Min: 41942338.3, Avg: 41942338.3, Max: 41942338.3, Diff: 0.0]
   [Code Root Fixup: 0.0 ms]
   [Code Root Purge: 0.0 ms]
   [Clear CT: 0.1 ms]
   [Other: 1.3 ms]
      [Choose CSet: 0.0 ms]
      [Ref Proc: 0.1 ms]
      [Ref Enq: 0.0 ms]
      [Redirty Cards: 0.1 ms]
      [Humongous Register: 0.1 ms]
      [Humongous Reclaim: 0.2 ms]
      [Free CSet: 0.2 ms]
   [Eden: 234.0M(234.0M)->0.0B(143.0M) Survivors: 26.0M->10.0M Heap: 792.7M(3072.0M)->520.3M(3072.0M)]
 [Times: user=0.07 sys=0.00, real=0.04 secs] 
2020-04-01T02:23:46.011+0800: 41955.051: [GC pause (G1 Evacuation Pause) (mixed), 0.0347943 secs]
   [Parallel Time: 33.3 ms, GC Workers: 2]
      [GC Worker Start (ms): Min: 41955051.1, Avg: 41955051.1, Max: 41955051.1, Diff: 0.0]
      [Ext Root Scanning (ms): Min: 2.3, Avg: 2.3, Max: 2.3, Diff: 0.0, Sum: 4.6]
      [Update RS (ms): Min: 14.4, Avg: 14.5, Max: 14.5, Diff: 0.0, Sum: 28.9]
         [Processed Buffers: Min: 49, Avg: 66.0, Max: 83, Diff: 34, Sum: 132]
      [Scan RS (ms): Min: 0.9, Avg: 1.0, Max: 1.1, Diff: 0.1, Sum: 2.0]
      [Code Root Scanning (ms): Min: 0.2, Avg: 0.3, Max: 0.4, Diff: 0.1, Sum: 0.6]
      [Object Copy (ms): Min: 15.2, Avg: 15.2, Max: 15.2, Diff: 0.0, Sum: 30.4]
      [Termination (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
         [Termination Attempts: Min: 1, Avg: 1.0, Max: 1, Diff: 0, Sum: 2]
      [GC Worker Other (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [GC Worker Total (ms): Min: 33.3, Avg: 33.3, Max: 33.3, Diff: 0.0, Sum: 66.6]
      [GC Worker End (ms): Min: 41955084.4, Avg: 41955084.4, Max: 41955084.4, Diff: 0.0]
   [Code Root Fixup: 0.0 ms]
   [Code Root Purge: 0.0 ms]
   [Clear CT: 0.2 ms]
   [Other: 1.3 ms]
      [Choose CSet: 0.1 ms]
      [Ref Proc: 0.1 ms]
      [Ref Enq: 0.0 ms]
      [Redirty Cards: 0.1 ms]
      [Humongous Register: 0.1 ms]
      [Humongous Reclaim: 0.1 ms]
      [Free CSet: 0.4 ms]
   [Eden: 143.0M(143.0M)->0.0B(142.0M) Survivors: 10.0M->11.0M Heap: 663.3M(3072.0M)->446.8M(3072.0M)]
 [Times: user=0.06 sys=0.00, real=0.04 secs] 
2020-04-01T02:23:58.566+0800: 41967.606: [GC pause (G1 Evacuation Pause) (mixed), 0.0273826 secs]
   [Parallel Time: 25.9 ms, GC Workers: 2]
      [GC Worker Start (ms): Min: 41967606.0, Avg: 41967606.0, Max: 41967606.0, Diff: 0.0]
      [Ext Root Scanning (ms): Min: 2.3, Avg: 2.3, Max: 2.3, Diff: 0.0, Sum: 4.6]
      [Update RS (ms): Min: 8.3, Avg: 8.3, Max: 8.3, Diff: 0.0, Sum: 16.5]
         [Processed Buffers: Min: 53, Avg: 57.0, Max: 61, Diff: 8, Sum: 114]
      [Scan RS (ms): Min: 1.5, Avg: 1.7, Max: 1.8, Diff: 0.3, Sum: 3.3]
      [Code Root Scanning (ms): Min: 0.1, Avg: 0.3, Max: 0.4, Diff: 0.3, Sum: 0.6]
      [Object Copy (ms): Min: 13.4, Avg: 13.4, Max: 13.4, Diff: 0.0, Sum: 26.7]
      [Termination (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
         [Termination Attempts: Min: 1, Avg: 1.0, Max: 1, Diff: 0, Sum: 2]
      [GC Worker Other (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.1]
      [GC Worker Total (ms): Min: 25.9, Avg: 25.9, Max: 25.9, Diff: 0.0, Sum: 51.8]
      [GC Worker End (ms): Min: 41967631.8, Avg: 41967631.8, Max: 41967631.8, Diff: 0.0]
   [Code Root Fixup: 0.0 ms]
   [Code Root Purge: 0.0 ms]
   [Clear CT: 0.2 ms]
   [Other: 1.2 ms]
      [Choose CSet: 0.1 ms]
      [Ref Proc: 0.1 ms]
      [Ref Enq: 0.0 ms]
      [Redirty Cards: 0.1 ms]
      [Humongous Register: 0.1 ms]
      [Humongous Reclaim: 0.1 ms]
      [Free CSet: 0.3 ms]
   [Eden: 142.0M(142.0M)->0.0B(142.0M) Survivors: 11.0M->11.0M Heap: 588.8M(3072.0M)->391.3M(3072.0M)]
 [Times: user=0.05 sys=0.01, real=0.02 secs] 
2020-04-01T02:24:11.056+0800: 41980.096: [GC pause (G1 Evacuation Pause) (mixed), 0.0271698 secs]
   [Parallel Time: 25.9 ms, GC Workers: 2]
      [GC Worker Start (ms): Min: 41980095.8, Avg: 41980095.8, Max: 41980095.8, Diff: 0.0]
      [Ext Root Scanning (ms): Min: 2.3, Avg: 2.3, Max: 2.3, Diff: 0.0, Sum: 4.7]
      [Update RS (ms): Min: 8.8, Avg: 8.9, Max: 9.0, Diff: 0.2, Sum: 17.8]
         [Processed Buffers: Min: 53, Avg: 55.5, Max: 58, Diff: 5, Sum: 111]
      [Scan RS (ms): Min: 0.4, Avg: 0.5, Max: 0.5, Diff: 0.1, Sum: 0.9]
      [Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Object Copy (ms): Min: 14.1, Avg: 14.1, Max: 14.2, Diff: 0.1, Sum: 28.3]
      [Termination (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
         [Termination Attempts: Min: 1, Avg: 1.0, Max: 1, Diff: 0, Sum: 2]
      [GC Worker Other (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.1]
      [GC Worker Total (ms): Min: 25.9, Avg: 25.9, Max: 25.9, Diff: 0.0, Sum: 51.8]
      [GC Worker End (ms): Min: 41980121.7, Avg: 41980121.7, Max: 41980121.7, Diff: 0.0]
   [Code Root Fixup: 0.0 ms]
   [Code Root Purge: 0.0 ms]
   [Clear CT: 0.1 ms]
   [Other: 1.1 ms]
      [Choose CSet: 0.0 ms]
      [Ref Proc: 0.1 ms]
      [Ref Enq: 0.0 ms]
      [Redirty Cards: 0.1 ms]
      [Humongous Register: 0.1 ms]
      [Humongous Reclaim: 0.1 ms]
      [Free CSet: 0.2 ms]
   [Eden: 142.0M(142.0M)->0.0B(1841.0M) Survivors: 11.0M->2048.0K Heap: 534.3M(3072.0M)->383.8M(3072.0M)]
 [Times: user=0.05 sys=0.00, real=0.02 secs]

具体字段含义【译】深入理解G1的GC日志（一）这篇文章分析的比较透彻

线上参数

 -Xss256k 栈大小
 -XX:MaxGCPauseMillis=200 GC预期最大停顿时间，JVM会尽可能满足这个目标，太小每次收集的数据就小
 -XX:+UseG1GC 使用G1 GC
 -XX:-OmitStackTraceInFastThrow 堆栈异常快速抛出
 -XX:CICompilerCount=2 编译时的编译器线程数
 -Xmx3072m 服务器是4g 最大堆 调整优化过
 -Xms3072m 最小堆 调整优化过
 -XX:MetaspaceSize=256m 元数据空间大小，调整优化过
 -XX:MaxMetaspaceSize=256m 最大元数据空间大小 调整优化过
 -XX:MinMetaspaceFreeRatio=0 空闲堆空间的最小百分比，HeapFreeRatio < MinHeapFreeRatio，则需要进行堆扩容，扩容的时机应该在每次垃圾回收之后
 -XX:MaxMetaspaceFreeRatio=100 空闲堆空间的最大百分比，如果HeapFreeRatio > MaxHeapFreeRatio，则需要进行堆缩容
 -XX:+PreserveFramePointer 显示完整的stack，优化过
 -XX:+PrintGC 打印日志
 -XX:+PrintGCDetails 打印日志
 -XX:+PrintGCDateStamps 打印日志
 -XX:+HeapDumpOnOutOfMemoryError oom报错
 -XX:ParallelGCThreads=2 并行配置GC线程数，与cpu核数相同
 -XX:ConcGCThreads=2 并发收集时线程数与cpu核数相同
 -XX:+PrintAdaptiveSizePolicy 分析优化gc策略，排查问题好帮手

调优案例

案例1

每隔一两分钟就会Full GC, 从GC日志看，有频繁的大对象分配,GC cleanup 没有回收掉一点内存，说明老年代的对象都是可达

GC pause (G1 Humongous Allocation)

jmap -histo:live pid查看String对象贼大
使用MAT发现EvictingQueue对象非常多，而EvictingQueue中大量持有String对象。EvictingQueue在使用时已经设置了长度为100，但是在多线程环境下add会超过100.add源码

 public boolean add(E e) {
        checkNotNull(e);
        if (maxSize == 0) {
            return true;
        }
        if (size() == maxSize) {
            delegate.remove();
        }
        delegate.add(e);
        return true;
    }

线程1和线程2同时进入add方法，判断size() == maxSize均为false，都执行了delegate.add方法并且都成功了，此时size()=101。那么在线程3执行add时，size() == maxSize仍为false，所以就会一直累加元素，直到OOM。size() >= maxSize避免击穿.

案例2

full gc频繁，查看日志, 看了下生产Metadata 可能是mateSpace 空间不够导致OOM

Full GC (Metadata GC Threshold)

看了下生产jvm参数配置Metadata 过小导致的，增大之后问题解决。

案例3

服务器卡顿，打开-XX:+PrintAdaptiveSizePolicy查看日志发现

predicted time is too high

这说明设置的预期停顿时间内无法做到扫描很多区域。后面几次只是扫描了几个region就结束了，如果这些region都是昂贵的，自然需要很长的时间进行mixed gc
解决的方案有：放宽预期停顿时间，降低-XX:G1OldCSetRegionThresholdPercent比例以降低预期时间，增加-XX:G1MixedGCCountTarget，减少每轮需要回收的region数

案例4 元数据空间引起的fullgc，最终是反射导致的

假笨说-从一起GC血案谈到反射原理

G1垃圾回收器

目录

基础知识

目的

发展历史

G1对比CMS回收器总览

JVM垃圾收集器种类

G1回收流程

基础

年轻代YoungGC收集

Old GC /并发标记周期

mixed混合GC

Full GC

GC日志解析

线上参数

调优案例

案例1

案例2

案例3

案例4 元数据空间引起的fullgc，最终是反射导致的

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