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

[Thomas Schatzl]

Hi Jon (and Eric),

  thanks for your reviews.

On Tue, 2015-08-25 at 16:09 -0700, Jon Masamitsu wrote:
> http://cr.openjdk.java.net/~tschatzl/8067341/webrev/src/share/vm/gc/g1/g1_globals.hpp.frames.html
> 
>    85   experimental(size_t, G1ExpectedAveragePLABOccupancyPercent, 50,           \
>    86                "Expected average occupancy of PLABs after GC in percent.")  \
>    87                range(0, 100)                                                \
> 
> Range should be (1, 100) since you divide by 
> G1ExpectedAveragePLABOccupancyPercent
> in the calculations?
> 

Okay, good catch. I introduced that variable very late.

> http://cr.openjdk.java.net/~tschatzl/8067341/webrev/src/share/vm/gc/g1/g1EvacStats.cpp.frames.html
> 
> 77     size_t const waste_used_for_calculation = used() > _region_end_waste ? used() - _region_end_waste : 0;
> 
> "waste_used_for_calculation" is a measure of "used" during the GC
> so maybe a better name would be "used_for_waste_calculation" or
> "used_for_PLAB_calculation"?

Okay.

> 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.

> 
>    80     size_t const total_waste_allowed = waste_used_for_calculation * (TargetPLABWastePct / 100);
>    81     size_t const cur_plab_sz = total_waste_allowed * (G1PLABPercent / 100)
> 
> 
> G1PLABPercent would be easier to explain.  It's effect would be linear 
> with its
> value (as opposed to inversely proportional) and is a shorter name :-).

In that case to me a simple scale factor seems more appropriate - I
created a new webrev to look at:

http://cr.openjdk.java.net/~tschatzl/8067341/webrev.1
http://cr.openjdk.java.net/~tschatzl/8067341/webrev.0_to_1

I also tried to improve the documentation.

Thanks,
  Thomas