RFR (S): 8227719: G1 pending cards estimation too conservative for cost prediction

[Thomas Schatzl]

Hi,

On Thu, 2019-07-18 at 16:26 +0200, Thomas Schatzl wrote:
> Hi all,
> 
>   can I have reviews for this change that instead of using a very
> conservative estimate of the number of pending cards in the DCQS,
> that is assuming that all completed buffers are full, simply
> calculates it as buffers are completed, i.e. giving the exact value.
> 
> Having a better input values to the algorithms seems to be desirable
> :)
> 
> Anecdotally, in my tests this makes a significant difference in cases
> where G1 has formerly been unable to utilize the allowed pause time
> goal fully, i.e. use significantly larger eden, decreasing total
> pause time and number of GCs. There is also the risk that the results
> are less conservative now, leading to more overshooting though.
> 
> There are imho a lot issues with the prediction heuristics remaining,
> but at least one input value is a bit better now.

I forgot to mention that this change is based on JDK-8162929 also out
for review.

Thomas

> 
> CR:
> https://bugs.openjdk.java.net/browse/JDK-8227719
> Webrev:
> http://cr.openjdk.java.net/~tschatzl/8227719/webrev/
> Testing:
> hs-tier1-3, self verification, (manual) perf testing
> 
> Thanks,
>   Thomas
>