Question

我有一个关于g1gc的问题。

这些是堆使用情况图。

上面是-Xms4g -Xmx4g。
底部是-Xms8g -Xmx8g。

我不知道8g选项为何导致g1gc发生更多。其他选项均为默认设置。

服务器规范是40个逻辑过程。

ps。适当的调整选项是什么？

其他问题

由于内存大小越大->区域大小越大，内存分配能否更快？

gc.log

4G gc.log

2019-05-07T21:03:42.093+0900: 10.280: [GC pause (G1 Evacuation Pause) (young), 0.1785373 secs]
   [Parallel Time: 43.4 ms, GC Workers: 28]
      [GC Worker Start (ms): Min: 10280.0, Avg: 10280.1, Max: 10280.6, Diff: 0.6]
      [Ext Root Scanning (ms): Min: 0.0, Avg: 0.4, Max: 0.8, Diff: 0.8, Sum: 12.0]
      [Update RS (ms): Min: 0.8, Avg: 1.1, Max: 1.6, Diff: 0.8, Sum: 31.5]
         [Processed Buffers: Min: 0, Avg: 2.0, Max: 3, Diff: 3, Sum: 56]
      [Scan RS (ms): Min: 0.0, Avg: 0.4, Max: 0.7, Diff: 0.7, Sum: 10.9]
      [Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Object Copy (ms): Min: 38.0, Avg: 38.5, Max: 39.9, Diff: 1.9, Sum: 1079.0]
      [Termination (ms): Min: 1.3, Avg: 2.6, Max: 3.2, Diff: 1.9, Sum: 74.1]
         [Termination Attempts: Min: 413, Avg: 769.6, Max: 855, Diff: 442, Sum: 21549]
      [GC Worker Other (ms): Min: 0.0, Avg: 0.1, Max: 0.2, Diff: 0.1, Sum: 2.0]
      [GC Worker Total (ms): Min: 42.7, Avg: 43.2, Max: 43.4, Diff: 0.7, Sum: 1209.5]
      [GC Worker End (ms): Min: 10323.3, Avg: 10323.3, Max: 10323.4, Diff: 0.1]
   [Code Root Fixup: 0.0 ms]
   [Code Root Purge: 0.0 ms]
   [Clear CT: 0.4 ms]
   [Other: 134.7 ms]
      [Choose CSet: 0.0 ms]
      [Ref Proc: 132.4 ms]
      [Ref Enq: 0.9 ms]
      [Redirty Cards: 0.3 ms]
      [Humongous Register: 0.1 ms]
      [Humongous Reclaim: 0.0 ms]
      [Free CSet: 0.7 ms]
   [Eden: 928.0M(928.0M)->0.0B(828.0M) Survivors: 26.0M->120.0M Heap: 1193.0M(4096.0M)->409.0M(4096.0M)]
Heap after GC invocations=8 (full 0):
 garbage-first heap   total 4194304K, used 418816K [0x00000006c0000000, 0x00000006c0204000, 0x00000007c0000000)
  region size 2048K, 60 young (122880K), 60 survivors (122880K)
 Metaspace       used 28525K, capacity 30824K, committed 31104K, reserved 1077248K
  class space    used 3583K, capacity 4166K, committed 4224K, reserved 1048576K
}
 [Times: user=1.21 sys=0.08, real=0.18 secs]
{Heap before GC invocations=8 (full 0):
 garbage-first heap   total 4194304K, used 744448K [0x00000006c0000000, 0x00000006c0204000, 0x00000007c0000000)
  region size 2048K, 219 young (448512K), 60 survivors (122880K)
 Metaspace       used 28525K, capacity 30824K, committed 31104K, reserved 1077248K
  class space    used 3583K, capacity 4166K, committed 4224K, reserved 1048576K
2019-05-07T21:03:42.895+0900: 11.082: [GC pause (G1 Evacuation Pause) (young), 0.0505563 secs]
   [Parallel Time: 11.6 ms, GC Workers: 28]
      [GC Worker Start (ms): Min: 11082.3, Avg: 11082.6, Max: 11083.6, Diff: 1.3]
      [Ext Root Scanning (ms): Min: 0.0, Avg: 0.4, Max: 0.8, Diff: 0.8, Sum: 9.9]
      [Update RS (ms): Min: 0.4, Avg: 1.0, Max: 1.5, Diff: 1.1, Sum: 29.4]
         [Processed Buffers: Min: 1, Avg: 1.8, Max: 3, Diff: 2, Sum: 50]
      [Scan RS (ms): Min: 0.8, Avg: 1.2, Max: 1.4, Diff: 0.6, Sum: 32.4]
      [Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
      [Object Copy (ms): Min: 8.3, Avg: 8.4, Max: 8.6, Diff: 0.2, Sum: 236.3]
      [Termination (ms): Min: 0.0, Avg: 0.1, Max: 0.1, Diff: 0.1, Sum: 2.8]
         [Termination Attempts: Min: 1, Avg: 42.7, Max: 52, Diff: 51, Sum: 1195]
      [GC Worker Other (ms): Min: 0.0, Avg: 0.1, Max: 0.2, Diff: 0.1, Sum: 2.0]
      [GC Worker Total (ms): Min: 10.2, Avg: 11.2, Max: 11.5, Diff: 1.3, Sum: 312.9]
      [GC Worker End (ms): Min: 11093.7, Avg: 11093.8, Max: 11093.8, Diff: 0.1]
   [Code Root Fixup: 0.0 ms]
   [Code Root Purge: 0.0 ms]
   [Clear CT: 0.3 ms]
   [Other: 38.6 ms]
      [Choose CSet: 0.0 ms]
      [Ref Proc: 37.0 ms]
      [Ref Enq: 0.5 ms]
      [Redirty Cards: 0.3 ms]
      [Humongous Register: 0.1 ms]
      [Humongous Reclaim: 0.0 ms]
      [Free CSet: 0.5 ms]
   [Eden: 318.0M(252.0M)->0.0B(1052.0M) Survivors: 120.0M->48.0M Heap: 727.0M(4096.0M)->397.0M(4096.0M)]
Heap after GC invocations=9 (full 0):
 garbage-first heap   total 4194304K, used 406528K [0x00000006c0000000, 0x00000006c0204000, 0x00000007c0000000)
  region size 2048K, 24 young (49152K), 24 survivors (49152K)
 Metaspace       used 28525K, capacity 30824K, committed 31104K, reserved 1077248K
  class space    used 3583K, capacity 4166K, committed 4224K, reserved 1048576K
}
 [Times: user=0.34 sys=0.02, real=0.05 secs]
{Heap before GC invocations=9 (full 0):
 garbage-first heap   total 4194304K, used 912384K [0x00000006c0000000, 0x00000006c0204000, 0x00000007c0000000)
  region size 2048K, 271 young (555008K), 24 survivors (49152K)
 Metaspace       used 29461K, capacity 31868K, committed 32256K, reserved 1077248K
  class space    used 3681K, capacity 4237K, committed 4352K, reserved 1048576K

8G gc.log

2019-05-05T02:39:16.996+0900: 201016.724: [GC pause (G1 Evacuation Pause) (young), 0.0336675 secs]
   [Parallel Time: 12.9 ms, GC Workers: 28]
      [GC Worker Start (ms): Min: 201016724.7, Avg: 201016724.9, Max: 201016725.0, Diff: 0.2]
      [Ext Root Scanning (ms): Min: 0.8, Avg: 1.2, Max: 5.2, Diff: 4.4, Sum: 32.4]
      [Update RS (ms): Min: 0.0, Avg: 3.1, Max: 3.5, Diff: 3.5, Sum: 87.7]
         [Processed Buffers: Min: 0, Avg: 11.1, Max: 30, Diff: 30, Sum: 310]
      [Scan RS (ms): Min: 0.1, Avg: 0.3, Max: 0.3, Diff: 0.2, Sum: 7.3]
      [Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.1]
      [Object Copy (ms): Min: 6.9, Avg: 7.5, Max: 7.7, Diff: 0.8, Sum: 211.2]
      [Termination (ms): Min: 0.2, Avg: 0.3, Max: 0.4, Diff: 0.2, Sum: 9.0]
         [Termination Attempts: Min: 105, Avg: 124.7, Max: 146, Diff: 41, Sum: 3491]
      [GC Worker Other (ms): Min: 0.0, Avg: 0.1, Max: 0.2, Diff: 0.2, Sum: 3.2]
      [GC Worker Total (ms): Min: 12.4, Avg: 12.5, Max: 12.7, Diff: 0.4, Sum: 350.8]
      [GC Worker End (ms): Min: 201016737.3, Avg: 201016737.4, Max: 201016737.5, Diff: 0.2]
   [Code Root Fixup: 0.0 ms]
   [Code Root Purge: 0.0 ms]
   [Clear CT: 0.7 ms]
   [Other: 20.0 ms]
      [Choose CSet: 0.0 ms]
      [Ref Proc: 17.2 ms]
      [Ref Enq: 0.2 ms]
      [Redirty Cards: 0.3 ms]
      [Humongous Register: 0.1 ms]
      [Humongous Reclaim: 0.0 ms]
      [Free CSet: 1.7 ms]
   [Eden: 4864.0M(4864.0M)->0.0B(4868.0M) Survivors: 48.0M->44.0M Heap: 5968.1M(8192.0M)->1091.2M(8192.0M)]
Heap after GC invocations=19405 (full 0):
 garbage-first heap   total 8388608K, used 1117388K [0x00000005c0000000, 0x00000005c0404000, 0x00000007c0000000)
  region size 4096K, 11 young (45056K), 11 survivors (45056K)
 Metaspace       used 187853K, capacity 205120K, committed 210176K, reserved 1232896K
  class space    used 22574K, capacity 25471K, committed 26368K, reserved 1048576K
}
 [Times: user=0.39 sys=0.00, real=0.04 secs]
{Heap before GC invocations=19405 (full 0):
 garbage-first heap   total 8388608K, used 6106497K [0x00000005c0000000, 0x00000005c0404000, 0x00000007c0000000)
  region size 4096K, 1228 young (5029888K), 11 survivors (45056K)
 Metaspace       used 187853K, capacity 205120K, committed 210176K, reserved 1232896K
  class space    used 22574K, capacity 25471K, committed 26368K, reserved 1048576K
2019-05-05T02:39:33.830+0900: 201033.558: [GC pause (G1 Evacuation Pause) (young), 0.0373282 secs]
2019-05-05T02:39:33.830+0900: 201033.558: [GC pause (G1 Evacuation Pause) (young), 0.0373282 secs]
   [Parallel Time: 13.9 ms, GC Workers: 28]
      [GC Worker Start (ms): Min: 201033558.4, Avg: 201033558.5, Max: 201033558.6, Diff: 0.2]
      [Ext Root Scanning (ms): Min: 0.8, Avg: 1.2, Max: 4.5, Diff: 3.7, Sum: 32.5]
      [Update RS (ms): Min: 0.0, Avg: 2.8, Max: 3.1, Diff: 3.1, Sum: 77.4]
         [Processed Buffers: Min: 0, Avg: 10.3, Max: 31, Diff: 31, Sum: 289]
      [Scan RS (ms): Min: 0.1, Avg: 0.3, Max: 0.3, Diff: 0.3, Sum: 7.1]
      [Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.1]
      [Object Copy (ms): Min: 8.5, Avg: 8.8, Max: 8.9, Diff: 0.4, Sum: 246.0]
      [Termination (ms): Min: 0.3, Avg: 0.4, Max: 0.5, Diff: 0.2, Sum: 12.0]
         [Termination Attempts: Min: 135, Avg: 156.5, Max: 175, Diff: 40, Sum: 4382]
      [GC Worker Other (ms): Min: 0.0, Avg: 0.1, Max: 0.2, Diff: 0.2, Sum: 3.3]
      [GC Worker Total (ms): Min: 13.3, Avg: 13.5, Max: 13.7, Diff: 0.3, Sum: 378.4]
      [GC Worker End (ms): Min: 201033571.9, Avg: 201033572.0, Max: 201033572.1, Diff: 0.2]
   [Code Root Fixup: 0.0 ms]
   [Code Root Purge: 0.0 ms]
   [Clear CT: 0.8 ms]
   [Other: 22.6 ms]
      [Choose CSet: 0.0 ms]
      [Ref Proc: 18.5 ms]
      [Ref Enq: 0.3 ms]
      [Redirty Cards: 1.0 ms]
      [Humongous Register: 0.1 ms]
      [Humongous Reclaim: 0.0 ms]
      [Free CSet: 2.1 ms]
   [Eden: 4868.0M(4868.0M)->0.0B(4880.0M) Survivors: 44.0M->32.0M Heap: 5963.4M(8192.0M)->1082.0M(8192.0M)]
Heap after GC invocations=19406 (full 0):
 garbage-first heap   total 8388608K, used 1107927K [0x00000005c0000000, 0x00000005c0404000, 0x00000007c0000000)
  region size 4096K, 8 young (32768K), 8 survivors (32768K)
 Metaspace       used 187853K, capacity 205120K, committed 210176K, reserved 1232896K
  class space    used 22574K, capacity 25471K, committed 26368K, reserved 1048576K
}
 [Times: user=0.41 sys=0.00, real=0.04 secs]
{Heap before GC invocations=19406 (full 0):
 garbage-first heap   total 8388608K, used 6122963K [0x00000005c0000000, 0x00000005c0404000, 0x00000007c0000000)
  region size 4096K, 1228 young (5029888K), 8 survivors (32768K)
 Metaspace       used 187853K, capacity 205120K, committed 210176K, reserved 1232896K
  class space    used 22574K, capacity 25471K, committed 26368K, reserved 1048576K

Answer 1

使用 G1GC 设置的JVM将通过构造一个名为 region 的内存块来启动，而无需区分新物理内存/旧物理内存/旧物理内存。从逻辑上讲，有“新的” /“幸存者” /“旧的”，但实际上并没有按地址分开。

在任何区域创建对象，并将对象的引用者信息存储在“记忆”集中（在整个堆中使用5％的级别）。记忆集是一种数据结构，可以轻松知道哪个区域分配了带有参考的对象。（将参考文献跟踪到该地区）

如果要创建一个大于区域大小的对象，它将在多个区域上创建一个对象，这组区域称为Humongous。此信息也存储在记忆集中。

根据堆大小，区域大小可以从1 MB到32 MB不等。下表显示了如果未明确设置区域大小，将根据最小堆大小选择区域大小。

|---------------------|------------------|
|    Min Heap Size    |   Region Size    |
|---------------------|------------------|
|     heap < 4GB      |       1MB        |
|---------------------|------------------|
|  4GB <= heap < 8GB  |       2MB        |
|---------------------|------------------|
|  8GB <= heap < 16GB |       4MB        |
|---------------------|------------------|
| 16GB <= heap < 32GB |       8MB        |
|---------------------|------------------|
| 32GB <= heap < 64GB |      16MB        |
|---------------------|------------------|
|     64GB < heap     |      32MB        |
|---------------------|------------------|

因此，在您的情况下，区域的大小将以不同的方式计算。另外，内存分配模式可能会因您的应用程序而异。为了找到更准确的原因，我们需要一个垃圾收集日志。

您可以设置InitiatingheapOccupancyPercent来让后台线程开始计时。堆使用率与堆总大小的比率。减小该值可使您快速启动后台线程。默认值为45。但是，如果该值太小，则垃圾收集将运行得太频繁。这会占用CPU周期，并可能影响应用程序本身的性能。

您知道，电锯图是健康的应用程序。因此，即使不进行其他设置也没有大问题。

额外：错误报告，可帮助您解决问题

Bug-8151176中的描述是出于实际计算IHOP的目的而计算旧发电量/总堆大小

这意味着完全忽略了年轻一代的职业。也就是说，如果年轻一代的分数大于IHOP，则并发周期将无法开始。

原因是，如果旧发电机占用率超过当前堆容量的固定百分比，则静态IHOP将启动。如果用户或人机工程学上的任何一项决定旧代不能大于触发并发标记的堆容量的一部分，则标记将永远不会开始。

Answer 2

我想知道是否可能实际上发生了更频繁的垃圾回收，因为您能够更快地分配内存。

如果您查看图表及其比例，第一个图表表明，在GC不运行的情况下，应用程序可以在大约12秒内分配大约2GB的空间。相比之下，第二张图显示在GC不运行的情况下，大约6秒钟内大约有2.5GB。

如果应用程序能够以两倍以上的速率进行分配，则意味着生成的垃圾量是两倍以上。

接下来，比较两个锯齿图的上斜率和下斜率。在第一个中，上坡是不平坦的，比下坡更浅。在第二个坡度中，坡度较平缓，向上坡度比向下坡度陡。

这是一个理论...

在第一种情况（较小的堆）中，当收集器处于空闲状态或标记时，某些事情会更缓慢地降低分配速率。

在第二种情况下（较大的堆），分配速率更快，这使GC要做更多的工作。更多的工作意味着它需要更频繁地运行。如果我们假设这种更快的分配速率也对应于应用程序线程更多的CPU使用率，并且在GC删除对象时保持了更快的分配速率+应用程序线程活动，那么这可能是从GC线程窃取周期，从而导致开垦速度要慢一些（参见下坡）。

另一种理论（没有任何证据支持！）是您的应用程序正在广泛使用软/弱引用来实现缓存。更多的内存可能意味着更大的缓存，这可能以“有趣”的方式改变应用程序的行为。例如，它可以导致更快的请求处理，以及（因此）更快地处理与请求相关联的短期对象。

这些理论都很脆弱。确实，我们需要有关您的应用程序的更多信息和更多指标。

Answer 3

根据您的两个图表，看起来您的服务器在较短的时间内分配了更多的内存。正如您在自己的图片上看到的那样，在第一种情况下，当内存使用量达到3G时，将执行混合收集，在第二种情况下，请说5G。但这取决于应用程序的内存分配模式。 G1 gc尝试满足gc最大暂停（由XX：MaxGCPauseMillis =设置，默认为500ms），而不会牺牲吞吐量。

为什么我在增加内存时会更频繁地获得GC？

3 个答案:

额外：错误报告，可帮助您解决问题