Request for code review - 8130265: gctests/LargeObjects/large001 fails with OutOfMemoryError: Java heap space

[Thomas Schatzl]

Hi Alex,

On Mon, 2015-08-31 at 11:38 -0400, Alexander Harlap wrote:
> 
> On 8/31/2015 4:40 AM, Thomas Schatzl wrote:
> > Hi,
[...]
> >>>>>>
> >>>>>> I also have a question about the order of
> >>>>>> attempt_allocation_at_safepoint() and the expand_and_allocate(), and
> >>>>>> could not find this answered before: so full gc might shrink the heap
> >>>>>> too much after full gc, and we should expand the heap.
> >>>>>> If I were to do this change, there would always (unconditionally) be an
> >>>>>> expansion after full gc, because the current change only expands if the
> >>>>>> allocation after gc fails. Also the 02 change.
> >>>>>>
> >>>>>> The review thread so far suggests that full gc might shrink too much,
> >>>>>> but this still seems to be the case here in some cases.
> >>> Could you clarify this question? To me, this looks like a potential bug
> >>> that has been overlooked so far in this change.
> >> Yes, full GC in some situation may over-shrink heap such way that heap
> >> expansion will be needed. It was reason for
> >> gctests/LargeObjects/large001 failure.  Still I prefer to do expansion
> >> only after allocation attempt failed. Why should we do unconditional
> >> expansion ( even if it is not needed )?
> > To keep the Min/MaxHeapFreeRatio constraints the heap should observe.
> >
> >>   In the existing code we have
> >> call to expand_and_allocate() only after
> >> attempt_allocation_at_safepoint() failure. What potential bug do you mean?
> > That the Min/MaxHeapFreeRatio values are not observed, causing
> > performance degradation, in the worst case like in this situation.
> >
> > The descriptions of the flags do not indicate an exception like "do not
> > care if the GC thinks it is not required".
> >
> > Thanks,
> >    Thomas
> >
> 
> Min/MaxHeapFreeRatio already were observed in calculation of shrink value. I do
> not see reason for expanding heap immediately after do_collection before doing
> attempt to allocate.

Okay, thanks for clarification, that has been the information I was
missing.

Thanks,
  Thomas