Are good colors more common than bad colors?

[raell at web.de]

Dear all,

on slide 35 of the presentation about ZGC by Per Lidén and Stefan Karlsson [1] it 
is stated that most object references will have the good color. I tried to analyze 
the probability of a non-root reference that is loaded for the first time for 
having a good color: 

A complete cycle has the three phases 

1. remapping/marking 
2. relocation 
3. no gc action (till the next marking/remapping starts) 

In phase 1 'good' means that the right marked bit is set. At the beginning of the phase 
all non-root references are bad, at the end of the phase all are good. So, if a non-root 
reference is selected randomly, it is good with a probability of about 50%. 

In phases 2, 3 'good' means that the remapped bit is set. Since in these phases no remapping 
is done (except by the load barrier), all non-root references are bad. So, if a non-root 
reference is selected randomly, it is good with a probability of 0%. 

Altogether, it seems to me, that in most parts of a cycle a non-root reference will have 
the bad color. 

Of course, I my be missing something. Therefore, I would be interested in an argument, why 
most object references are expected to have a good color. 

Thank you very much!

Ralph 


[1] http://cr.openjdk.java.net/~pliden/slides/ZGC-Jfokus-2018.pdf