RFR: JDK-8148759: G1AllocRegion::_count inconsistently used if more than one context is active

[Thomas Schatzl]

Hi Bengt,

On Thu, 2016-03-17 at 18:04 +0100, Bengt Rutisson wrote:
> Hi everyone,
> 
> Could I have a couple of reviews for this change?
> 
> http://cr.openjdk.java.net/~brutisso/8148759/webrev.00
> https://bugs.openjdk.java.net/browse/JDK-8148759
> 
> The _count value is per G1GCAllocRegion, so if we have more than one 
> survivor alloc region we will notice too late that we have exceeded 
> the maximum number of regions we had decided to use for survivors.
> 
> The proposed patch uses young_list()->survivor_length() instead, 
> which is a "global" value for all alloc regions. This value used to 
> be updated when we retired a region, but to make proper use of it it 
> needs to be updated when we pick a new survivor region. Thus, I had 
> to move the call to young_list()->add_survivor_region(). I've gone 
> through and checked other users of the values updated by 
> add_survivor_region() and I haven't found anyone that is using this 
> value inside of the actual evacuation path, which is where the
> difference would be noticeable.

  we talked about this offline a bit and I can kind of understand the
reasoning of others to not have this check in G1CollectorPolicy.

I would prefer to move the check into an extra method like
generation_is_full(InCSetState dest) that is checked instead of
inlining it (so that it can be easily identified later to be moved in
some kind of heap sizing/heap shaping or even evacuation policy).

Thanks,
  Thomas