RFR: JDK-8211955: GC abstraction for LAB reserve

Roman Kennke rkennke at redhat.com
Thu Oct 11 12:41:35 UTC 2018


Hi Per,

> On 10/11/2018 12:54 PM, Roman Kennke wrote:
> [...]
>>> For example, it's not obvious to me if you consider the Brooks pointer
>>> to be an extension of the object header, and hence part of the object
>>> and included in obj->size(), of if it's a separate thing. If it's a
>>> separate thing, then a cell concept might prove useful. But if it's a
>>> separate thing, then JDK-8211270 now incorrectly made
>>> JvmtiEnv::GetObjectSize() and WB_GetObjectSize() return the cell size
>>> instead of the object size.
>>>
>>> I assume you thought these things through? What's the story for
>>> Shenandoah here?
>>
>> We never made the distinction between cell and object size. Practically
>> speaking, we treated the fwd pointer as part of the object, but made it
>> invisible to the rest of the runtime, i.e. it's GC specific. We wanted
>> to be honest about reporting the space that each object occupies in the
>> heap, hence the changes to WB and JVMTI.
> 
> Were these two functions the only places where this was an issue, or do
> you have more patches lined up affecting shared code with similar
> problems? It would help to know to better understand if this is a
> small-ish problem that we can live with for now or if we need to do
> something bigger here, like start thinking about if we need the cell
> concept or something else.
> 
> I can't help but feel that we're piece by piece building a mess here,
> that will take a long time to clean up, instead of having a clear and
> well thought out plan from the beginning. I hope I'm wrong, of course.

Those are the only functions where this matters. In-fact, the LAB
reserve abstraction is the last one I have lined up. The remaining stuff
that Shenandoah needs is a bunch of extra utility classes and C2 changes.

>>>> and this could be called by JVMT and whitebox (JDK-8211270) with
>>>> obj->size(), and by min_fill_size() with the static minimum obj size,
>>>> and the GC would translate it to the cell size. The obj_size() from
>>>> JDK-8211270 would then be removed and we'd have a single abstraction
>>>> for
>>>> this. Want me to make that change?
>>>>
>>>> Do you agree with the rest of this change? Because there's some
>>>> significant changes how TLAB and PLAB calculate their alloc reserves in
>>>> there too.
>>>
>>> I think we need to think through the basics above first.
>>>
>>> Generally, having PLAB call Universe::heap()->tlab_alloc_reserve() looks
>>> wrong to me, since a PLAB is not a TLAB. If we really want to share this
>>> code I think we should try to find some other way. Maybe just give
>>> tlab_alloc_reserve() a better name, but at the same time I'm not even
>>> sure this belongs in CollectedHeap.
>>
>> We can change it to lab_alloc_reserve() instead. I think it should be
>> shared between TLAB and PLAB because the concept is the same thing, and
>> relies on the same properties: we need some reserve at the end in order
>> to be able to fill it completely, and with Shenandoah this is GC
>> specific. And since all the other code that deals with filling TLABs and
>> PLABs resides in CollectedHeap, this naturally belongs there too, right
>> next to CollectedHeap::fill_with_dummy_object(..) which is also used by
>> both TLAB and PLAB.
> 
> Maybe a stupid question, but why do you even need a Brooks pointer in
> your filler objects? Is not having that maybe a solution here?

Filler objects are objects. Laying them out differently would make
parsing the heap impossible.

Thanks,
Roman

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