Rare very big pause

[Aleksey Shipilev]

On 12/19/2017 04:12 PM, Kirill A. Korinsky wrote:
> May you suggest any way to understand where and how much it try to allocate?
> 
> Because this application shouldn't allocate so big memory and how I can see it from line before it had 245Mb free memory when GC cycle happened.
>> Concurrent marking triggered. Free: 245M, Free Threshold: 245M; Allocated: 245M, Alloc Threshold: 0M

I don't think this is application's fault. Your config runs with 4 GB heap. Default settings for
"adaptive" heuristics target to maintain 3% of free heap at all times during the cycle. That means
120M. Single application thread may allocate this much in around 20 ms. And concurrent mark takes
around a second. So, we have only a razor-thin room for error here. With a small heap like that,
even a trivial allocation spike may blow up with Allocation Failure.

I guess Shenandoah's heuristics needs to reserve more on smaller heaps, so that some absolute amount
of space was always available. We have patches in queue to make this better. Meanwhile, you may want
to adjust these:

  experimental(uintx, ShenandoahInitFreeThreshold, 30,                      \
               "Initial remaining free threshold for adaptive heuristics")  \
          range(0,100)                                                      \
                                                                            \
  experimental(uintx, ShenandoahMinFreeThreshold, 3,                        \
               "Minimum remaining free threshold for adaptive heuristics")  \
          range(0,100)                                                      \
                                                                            \
  experimental(uintx, ShenandoahMaxFreeThreshold, 70,                       \
               "Maximum remaining free threshold for adaptive heuristics")  \
          range(0,100)                                                      \
                                                                            \

Say, -XX:ShenandoahMinFreeThreshold=20 for your case.

Thanks,
-Aleksey