Odd G1GC behavior on 8u91

[Vitaly Davidovich]

On Wed, Aug 24, 2016 at 6:29 PM, Poonam Bajaj Parhar <
poonam.bajaj at oracle.com> wrote:

> Also, do you see entries like "*[G1Ergonomics (Mixed GCs) do not start
> mixed GCs, reason:" *in the GC logs which mean that the mixed GCs are not
> happening due to some reason. What is the reason listed with these log
> entries?
>
Hi Poonam,

Yes, I do see a few those, but only very early in the process lifetime, and
nowhere near the Full GCs.

2016-08-24T10:33:04.733+0000: 8.460: [SoftReference, 0 refs, 0.0010108
secs]2016-08-24T10:33:04.734+0000: 8.461: [WeakReference, 383 refs,
0.0006608 secs]2016-08-24T10:33:04.735+0000: 8.462: [FinalReference, 4533
refs, 0.0020491 secs]2016-08-24T10:33:04.737+0000: 8.464:
[PhantomReference, 0 refs, 15 refs, 0.0011945
secs]2016-08-24T10:33:04.738+0000: 8.465: [JNI Weak Reference, 0.0000360
secs] 8.467: [G1Ergonomics (Mixed GCs) do not start mixed GCs, reason:
concurrent cycle is about to start]
2016-08-24T10:35:22.846+0000: 146.574: [SoftReference, 0 refs, 0.0011450
secs]2016-08-24T10:35:22.847+0000: 146.575: [WeakReference, 440 refs,
0.0006071 secs]2016-08-24T10:35:22.848+0000: 146.575: [FinalReference, 7100
refs, 0.0018074 secs]2016-08-24T10:35:22.850+0000: 146.577:
[PhantomReference, 0 refs, 76 refs, 0.0013148
secs]2016-08-24T10:35:22.851+0000: 146.579: [JNI Weak Reference, 0.0000443
secs] 146.584: [G1Ergonomics (Mixed GCs) do not start mixed GCs, reason:
concurrent cycle is about to start]
2016-08-24T10:35:56.507+0000: 180.234: [SoftReference, 0 refs, 0.0010184
secs]2016-08-24T10:35:56.508+0000: 180.235: [WeakReference, 138 refs,
0.0006883 secs]2016-08-24T10:35:56.508+0000: 180.236: [FinalReference, 3682
refs, 0.0023152 secs]2016-08-24T10:35:56.511+0000: 180.238:
[PhantomReference, 0 refs, 45 refs, 0.0012558
secs]2016-08-24T10:35:56.512+0000: 180.239: [JNI Weak Reference, 0.0000197
secs] 180.247: [G1Ergonomics (Mixed GCs) do not start mixed GCs, reason:
concurrent cycle is about to start]
2016-08-24T10:37:33.387+0000: 277.114: [SoftReference, 0 refs, 0.0010965
secs]2016-08-24T10:37:33.388+0000: 277.115: [WeakReference, 5 refs,
0.0006378 secs]2016-08-24T10:37:33.388+0000: 277.116: [FinalReference, 3440
refs, 0.0028640 secs]2016-08-24T10:37:33.391+0000: 277.119:
[PhantomReference, 0 refs, 0 refs, 0.0011392
secs]2016-08-24T10:37:33.392+0000: 277.120: [JNI Weak Reference, 0.0000148
secs] 277.130: [G1Ergonomics (Mixed GCs) do not start mixed GCs, reason:
candidate old regions not available]

Does that tell you anything?


>
> Thanks,
> Poonam
>
> On 8/24/2016 3:18 PM, Jenny Zhang wrote:
>
> More comments about the questions
>
> Thanks
> Jenny
>
> On 8/24/2016 11:43 AM, Vitaly Davidovich wrote:
>
> Right before the Full GC, ergonomics report a failure to expand the heap
> due to an allocation request of 32 bytes.  Is this implying that a mutator
> tried to allocate 32 bytes but couldn't? How do I reconcile that with
> Eden+Survivor occupancy reported right above that?
>
> Yes, it means the mutator tries to allocate 32byte but can not get it.
> Heap won't expand as it already reaches max heap.
>
> Do you see any humongous objects allocatoin?
>
>
> Young gen is sized to 30GB, total heap is 96GB.  Allocation rate of the
> application is roughly 1GB/s.  Am I correct in assuming that allocation is
> outpacing concurrent marking, based on the above? What tunable(s) would you
> advise to tweak to get G1 to keep up with the allocation rate? I'm ok
> taking some throughput hit to mitigate 90s+ pauses.
>
> The entire log might give a better picture. Especially if the marking
> cycle is triggered, how well the mixed gc cleans up the heap.
>
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