Why does the G1GC reduce the younger generation to the start of mixed collections?

Question

When G1 decides that he needs to start making mixed collections, he aggressively reduces our Eden space from 10g to about 1g.

{Heap before GC invocations=294 (full 0): garbage-first heap total 20480000K, used 18304860K [0x00000002de000000, 0x00000002de804e20, 0x00000007c0000000) region size 8192K, 1363 young (11165696K), 11 survivors (90112K) Metaspace used 37327K, capacity 37826K, committed 38096K, reserved 1083392K class space used 3935K, capacity 4081K, committed 4096K, reserved 1048576K 2016-03-31T20:57:31.002+0000: 7196.427: [GC pause (G1 Evacuation Pause) (young) Desired survivor size 717225984 bytes, new threshold 1 (max 1) - age 1: 41346816 bytes, 41346816 total 7196.427: [G1Ergonomics (CSet Construction) start choosing CSet, _pending_cards: 144693, predicted base time: 48.88 ms, remaining time: 951.12 ms, target pause time: 1000.00 ms] 7196.427: [G1Ergonomics (CSet Construction) add young regions to CSet, eden: 1352 regions, survivors: 11 regions, predicted young region time: 20.72 ms] 7196.427: [G1Ergonomics (CSet Construction) finish choosing CSet, eden: 1352 regions, survivors: 11 regions, old: 0 regions, predicted pause time: 69.60 ms, target pause time: 1000.00 ms] 7196.494: [G1Ergonomics (Mixed GCs) start mixed GCs, reason: candidate old regions available, candidate old regions: 789 regions, reclaimable: 4703761904 bytes (22.43 %), threshold: 5.00 %] , 0.0673540 secs] [Parallel Time: 60.1 ms, GC Workers: 18] [GC Worker Start (ms): Min: 7196427.8, Avg: 7196428.1, Max: 7196428.2, Diff: 0.4] [Ext Root Scanning (ms): Min: 7.3, Avg: 7.5, Max: 7.7, Diff: 0.4, Sum: 134.2] [Update RS (ms): Min: 28.2, Avg: 28.8, Max: 29.9, Diff: 1.7, Sum: 518.4] [Processed Buffers: Min: 41, Avg: 57.7, Max: 80, Diff: 39, Sum: 1039] [Scan RS (ms): Min: 0.1, Avg: 0.2, Max: 0.5, Diff: 0.4, Sum: 3.7] [Code Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.1] [Object Copy (ms): Min: 22.1, Avg: 23.1, Max: 23.4, Diff: 1.3, Sum: 416.2] [Termination (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.1] [Termination Attempts: Min: 1, Avg: 1.0, Max: 1, Diff: 0, Sum: 18] [GC Worker Other (ms): Min: 0.0, Avg: 0.1, Max: 0.2, Diff: 0.2, Sum: 2.5] [GC Worker Total (ms): Min: 59.5, Avg: 59.7, Max: 60.0, Diff: 0.5, Sum: 1075.1] [GC Worker End (ms): Min: 7196487.7, Avg: 7196487.8, Max: 7196487.9, Diff: 0.2] [Code Root Fixup: 0.2 ms] [Code Root Purge: 0.0 ms] [Clear CT: 1.9 ms] [Other: 5.2 ms] [Choose CSet: 0.0 ms] [Ref Proc: 0.5 ms] [Ref Enq: 0.0 ms] [Redirty Cards: 0.5 ms] [Humongous Register: 0.2 ms] [Humongous Reclaim: 0.1 ms] [Free CSet: 2.3 ms] [Eden: 10.6G(10.6G)->0.0B(848.0M) Survivors: 88.0M->152.0M Heap: 17.5G(19.5G)->7128.3M(19.5G)] Heap after GC invocations=295 (full 0): garbage-first heap total 20480000K, used 7299344K [0x00000002de000000, 0x00000002de804e20, 0x00000007c0000000) region size 8192K, 19 young (155648K), 19 survivors (155648K) Metaspace used 37327K, capacity 37826K, committed 38096K, reserved 1083392K class space used 3935K, capacity 4081K, committed 4096K, reserved 1048576K } [Times: user=1.09 sys=0.00, real=0.07 secs] 2016-03-31T20:57:31.070+0000: 7196.495: Total time for which application threads were stopped: 0.0699324 seconds, Stopping threads took: 0.0003462 seconds 

This is after it was launched with 10-11 g of Eden for 60 or more collections.

Below are the relevant JVM GC parameters that we run with

 -Xms20000m -Xmx20000m -XX:+UseG1GC -XX:G1RSetUpdatingPauseTimePercent=5 -XX:MaxGCPauseMillis=1000 -XX:GCTimeRatio=99 -XX:InitiatingHeapOccupancyPercent=35 -XX:MaxTenuringThreshold=1 -XX:G1ConcRefinementThreads=6 -XX:ConcGCThreads=18 -XX:ParallelGCThreads=18 -XX:+PrintGCDetails" -XX:+PrintGCDateStamps" -XX:+PrintHeapAtGC" -XX:+PrintTenuringDistribution" -XX:+PrintGCApplicationStoppedTime" -XX:+PrintPromotionFailure" -XX:+PrintAdaptiveSizePolicy" 

According to page 55 of this presentation , you need to resize the Eden so that the maximum pause points to the whole pile, and not just to the new generation. Why is the collector so aggressive?

The average pause time of the young generation is from 50 to 150 ms with a heap size of 10 g. If the presentation is correct (I did not find anything to support the statement), I would expect a halving (20 g of heap), not 10x.

Answer 1

You can find the answer for your request in slide no: 56

The younger generation was reduced by 20 times

Thus, the reduction from factory 10X is not a surprise.

From the infoQ article by Monica Beckwith on G1GC tuning tips:

A full GC could be avoided by letting the child / young gene shrink to the minimum default value (5% of the total Java heap)

Since you did not explicitly set the younger generation size, the default value

 -XX:G1NewSizePercent=5 

Sets the percentage of heap to use for at least the size of the younger generation.

So, to respect your temporary pause goal

 -XX:MaxGCPauseMillis=1000 

a young gene can reduce up to 5% of the total heap.

I found one good google group article on G1GC at https://groups.google.com/a/jclarity.com/forum/#!msg/friends/hsZiz6HTm9M/klrRjBclCwAJ

If the predicted pause point in G1 is greater than the target pause target, then reduce the younger generation, but not more than G1NewSizePercent of the current Java heap size (not the maximum size). Again, the overall Java heap will grow (or decrease) based on the value calculated by the GC time ratio compared to the GCTimeRatio value.

Note. G1NewSizePercent and G1MaxNewSizePercent should not be confused with NewSize or MaxNewSize.

G1NewSizePercent and G1MaxNewSizePercent place the lower and upper bounds, respectively, regarding how small or how large the young gene can be given by G1.

On the other hand, you have configured a lot of options, which may be optional. The G1GC sine is working fine if most of the parameters are set to default values. Refer to this SE question for more details.

Java 7 (JDK 7) garbage collection and collection in G1

In conclusion . Depending on the purpose of the pause time, the young gene will be reduced. If you are really concerned about reducing the younger generation to a low value, configure -XX:G1NewSizePercent . But I will not recommend it if -XX:MaxGCPauseMillis was met

EDIT:

From the G1GC ergonomics page,

Typically, heap size will fluctuate as the garbage collector tries to meet competing goals. This is true even if the application has reached a steady state . Pressure to achieve throughput (which may require a larger heap) competes with the goals of maximum pause time and minimum footprint (which may require a small heap).