RFR (XXS): 8247748: Use default alpha for G1 adaptive IHOP allocation rate calculation

[Thomas Schatzl]

Hi,

On 18.06.20 20:45, Luo, Ziyi wrote:
> Hi Thomas,
> 
> I am a little bit confused here.
> 
> According to the TruncatedSeq constructor, the alpha is used to construct
> AbsSeq:
> http://hg.openjdk.java.net/jdk/jdk/file/08211be640e9/src/hotspot/share/utilities/numberSeq.cpp#l31
> 
> When adding a new value to AbsSeq, the weight of the new val is (1.0 - _alpha):
> http://hg.openjdk.java.net/jdk/jdk/file/08211be640e9/src/hotspot/share/utilities/numberSeq.cpp#l146
> http://hg.openjdk.java.net/jdk/jdk/file/08211be640e9/src/hotspot/share/utilities/numberSeq.cpp#l44
> 
> Right now, in Adaptive IHOP, the weight of the new value is (1-0.95) =
> 0.05. After changing alpha to the default 0.7, the weight is increased to 0.3,
> which actually emphasizes the most recent allocation rate and will potentially
> make the prediction spikier.
> 

   you are right. I mostly went with my memory  which apparently has 
been the complete opposite of what it does/did, which led to the CR 
title/description :(

In my tests I could not see an actual difference in behavior so I 
figured it would be better to not deviate from the other, existing 
predictors instead of some other number as we do not have any 
explanation for the default value (of 0.7) either. I.e. something like 
"one magic number is better than two".

For this (very weak) reason I would still like to make this change, but 
I am open to just retract this change.

Thanks for making me aware of this!

Thanks,
   Thomas