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

[Kim Barrett]

> On Jul 18, 2019, at 10:26 AM, Thomas Schatzl <thomas.schatzl at oracle.com> 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.

Before I review this, I have a high-level comment that strongly affects this.

I suspect that we don't really want the number of buffers at all, but
only the number of cards.  I think the number of buffers is just being
used to estimate the number of cards.  Rather than adding a second
counter, perhaps consider replacement of one by the other?

DirtyCardQueueSet::completed_buffers_num() could be retained (at
least for now) as an estimate based on the card count.  That would
probably simplify the replacement effort.