RFR (S): 8067341: Modify PLAB sizing algorithm to waste less

[Thomas Schatzl]

Hi all,

  I tried to get some more internal comments on the question of the
final change to improve PLAB sizes.

On Wed, 2015-08-26 at 11:54 +0200, Thomas Schatzl wrote:
> Hi Jon (and Eric),
[...]
> > When _region_end_waste is high, waste_used_for_calculation under 
> > estimates the
> > amount used so under estimates the amount of waste (since the waste
> > is a percentage of waste_used_for_calculation ) and so lowers the size
> > of the next PLAB.  I can see that it scales reasonably.
> > 
> > Why G1ExpectedAveragePLABOccupancyPercent that can be different than 50?
> > 
> > If it is so that the user can scale the PLAB's up or down, what about 
> 
> That is the intent.
> 
> > scale factor called G1PLABPercent?
> 
> That name seems to me a bit too unspecific, I chose
> G1PLABOccupancyFactor for this revision. Feel free to comment on this.

Others commented on the name and the value range; mainly that a
percentage from 1-100 (or something similar) seems more natural or at
least common, also the use as a divisor in the calculation.

Using a double instead of an integer allows more flexibility in the
choice of values.

As for the name, the argument came up that the G1PLABPercent suggestion
is somewhat too undescriptive, and it is better to use a longer, more
appropriate name like G1LastPLABAverageOccupancy (note that added "Last"
in the name). Since we do not expect it to be used a lot, except experts
that measured that there is an issue there previously, a longer name
does not seem to be a big issue.

http://cr.openjdk.java.net/~tschatzl/8067341/webrev.2
http://cr.openjdk.java.net/~tschatzl/8067341/webrev.0_to_2 (change from baseline!)

Thanks,
  Thomas