Why do I get GC more often when I raise memory?

Question

I have a question about g1gc.

These are the heap usage graph.

The above is -Xms4g -Xmx4g.
The bottom is -Xms8g -Xmx8g.

I don't know why the 8g option causes g1gc to happen more often. Other options are all default.

And server spec is 40 logical process.

ps. What are the proper tuning options?

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addtional question

Can the memory allocation be faster because the larger the memory size -> the larger the region size?

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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

The JVM set up with G1GC will be started by constructing a memory block called region without any distinction of New / Survivor / Old physical memory. Logically there is New / Survivor / Old, but it is not physically separated by address.

The objects are created in any region, and the referrer information of the object is stored in a Remember set (using 5% level in the whole Heap). The remember set is a data structure that makes it easy to know which region is assigned an object with reference. (track references into the region)

If an object that is larger than the region size is to be created, it will create an Object over several regions, and this set of regions is called Humongous. This information is also stored in the remember set.

The region sizes can vary from 1 MB to 32 MB depending on the heap size. The following table shows the region size that will be chosen based on the minimum heap size should the region size not be explicitly set.

|---------------------|------------------|
|    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        |
|---------------------|------------------|

So, in your case, the size of the region will be calculated differently. Also, the memory allocation pattern can vary depending on your application. To find a more accurate cause, we need a garbage collection log.

You can set InitiatingheapOccupancyPercent to let background threads start time. The ratio of heap usage to the total heap size. Decreasing the value allows you to start a background thread quickly. The default value is 45. However, if the value is too small, the garbage collection will run too often. It takes CPU cycles and may affect application performance itself.

As you know, The chainsaw graphs are healthy applications. Therefore, there is no big problem even if you do not do additional setting.

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EXTRA: Bug reports that can help you troubleshoot issues

The description in Bug-8151176 refers to calculating the old gen occupancy/total heap size for the purpose of actually calculating IHOP

This means that the occupation of the young generation is completely ignored. That is, if the younger generation's fraction is greater than the IHOP, the concurrent cycle cannot start.

The reason is that static IHOP starts if old gen occupancy exceeds a fixed percentage of the current heap capacity. If either the user or ergonomics decide that the old gen cannot be larger than that fraction of the heap capacity that triggers concurrent mark, marking will never start.

Answer 2

I am wondering if the more frequent garbage collection may be actually happening because you are able to allocate memory faster.

If you look at the graphs and their scales, the first one show that the application is able to allocate roughly 2GB in roughly 12 seconds while the GC is not running. By contrast, the second graph shows roughly 2.5GB in roughly 6 seconds while the GC is not running.

If an application is able to allocate at more than twice the rate, that means that more than twice the amount of garbage is generated.

Next, compare the up-slopes and down-slopes of the two sawtooth graphs. In the first one, the up-slopes are uneven and more shallow than the down-slopes. In the second one the slopes are smoother, and the up-slopes are steeper than the down-slopes.

So here is a theory ...

In the first case (smaller heap), something is slowing down the allocation rate more while the collector is idle ... or marking.

In the second case (larger heap), the allocation rate is faster which is making more work for the GC to do. More work means that it needs to run more frequently. If we assume that this faster allocation rate also occurs corresponds to more CPU utilization by application threads, AND that the faster allocation rate + application thread activity is maintained while the GC is deleting objects, then this may be stealing cycles from the GC threads, causing the reclamation rate to be slower (c.f. the down-slopes).

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Another theory (not supported by any evidence!) is that your application is making extensive use of soft / weak references to implement caching. More memory potentially means a larger cache which could alter the applications behavior in "interesting" ways. For instance, it could lead to faster request processing and (hence) faster turn-over of short-lived objects associated with requests.

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These theories are all rather tenuous. Really we need more information about your application, and more metrics.