RFR: Reduce waste in humongous allocations

[Aleksey Shipilev]

On 12/13/2016 07:20 PM, Roman Kennke wrote:
> Am Dienstag, den 13.12.2016, 19:09 +0100 schrieb Aleksey Shipilev:
>> On 12/13/2016 06:11 PM, Roman Kennke wrote:
>>> http://cr.openjdk.java.net/~rkennke/fixhumongousalloc/webrev.00/
>> 
>> Ugh. The code got even more confusing than it was before... At this point
>> I wonder if acquiring a lock when claiming free regions is saner than 
>> trying to do this in a lock-free manner. With TLAB allocations, this
>> shouldn't be that painful?
> 
> It's not painful in terms of performance, but painful in terms of 
> implemention. We cannot easily acquire the Heap_lock on allocations because
> the allocation might come out of a write barrier, and that Java thread is
> not-in-VM (they call into the VM via a cheap leaf-call). We could change that
> (and have been there already) to use regular calls like, e.g. allocations do,
> but this opens up a whole new class of other problems. For example, we need
> oopmaps at write-barriers which, iirc, presented us some serious optimization
> problems in C2 land. With Roland's work, those might have gone away though
> (seems like we can well live with control inputs to write barriers now..)

OUCH.

> We have been there, and it might be The Correct Way to do it, but it's not
> trivial at all.

We don't need Heap_lock specifically, right? I wonder if we can dispense with a
very short-lived spinlock only in ShenandoahFreeSet to trim the lock-free
madness down there.

>> Also, does the code claim the regions one-by-one? What if we have two 
>> competing multi-region humongous allocations? Does it guarantee to
>> allocate both (e.g. are they stepping on each other's toes, preventing
>> global progress?)
> 
> I guess it could happen. How else could we do it?
> 
> I know this stuff is a bit nightmarish. Accept that as stop-gap- solution,
> and re-visit locked allocation with non-leaf-write-barriers and all that
> stuff later?

No, because I think those competing multi-region allocs are very real, and will
bite us. Let's push something is not affected by that. So far the cure is worse
than a disease :)

Thanks,
-Aleksey