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

[Luo, Ziyi]

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.

Best,
Ziyi

On 6/18/20, 11:18 AM, Thomas Schatzl wrote:

    Hi all,

       can I have reviews for this change that tones down the g1 adaptive
    ihop allocation rate prediction to not follow the most recent allocation
    rate that much?

    Instead of 0.95, use the default 0.7.

    CR:
    https://bugs.openjdk.java.net/browse/JDK-8247748
    Webrev:
    http://cr.openjdk.java.net/~tschatzl/8247748/webrev/
    Testing:
    tier1-3

    Thanks,
       Thomas