RFR: 8228657: ZGC: ZObjectAllocator::used() should take undone allocations into account

[Erik Österlund]

Hi Per,

Looks good.

Thanks,
/Erik

On 2019-07-26 15:11, Per Liden wrote:
> In ZObjectAllocator, when we allocate a page we increment _used. 
> However, we might later undo that page allocation, but we then leave 
> _used as is. This results in a sometimes slightly incorrect value 
> being reported by ZCollectedHeap::tlab_used(), which is fed into the 
> TLAB heuristics. This incorrectness is not that big of a deal, but we 
> should still fix this.
>
> We can't safely decrement _used (because we might be executing on a 
> different CPU now), but we can track the amount of undos we've done, 
> and later subtract that from _used.
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8228657
> Webrev: http://cr.openjdk.java.net/~pliden/8228657/webrev.0
>
> /Per