Move g1 heapRegionSize from class variable to member variable

[loyispa]

Concurrent GCToolKit instances with different g1 region-size will affect each other because of their static sharing of G1GCForwardReference#heapRegionSize. Consider moving it to a member variable.

[karianna]

@loyispa Interesting, I'd love to learn more about how/why you're running multiple instances of GCToolkit if you're able to share?

[loyispa]

@karianna, I am developing a tool that can analyze multiple streaming logs using this great library. I am currently facing a problem if multiple logs of G1 with different region sizes were analyzed concurrently, there would be incorrect calculations for Eden and Survival occupancy because of the sharing of heapRegionSize.

[kcpeppe]

IMHO, I don't think it's correct to include region size with each G1 event. However, there isn't an alternative context to put this static/never changing for a single GC log. IOWs, we didn't give too much thought to parsing multiple logs at the same time. I was thinking that maybe adding a session object might help but then I was thinking that there are other problems you'd run into for this use case. I'm inclined to merge this with the caveat that it might be deprecated in favour of a better solution. @dsgrieve would you care to offer an opinion here?

[dsgrieve]

IMHO, I don't think it's correct to include region size with each G1 event. However, there isn't an alternative context to put this static/never changing for a single GC log. IOWs, we didn't give too much thought to parsing multiple logs at the same time. I was thinking that maybe adding a session object might help but then I was thinking that there are other problems you'd run into for this use case. I'm inclined to merge this with the caveat that it might be deprecated in favour of a better solution. @dsgrieve would you care to offer an opinion here?

@kcpeppe I'm inclined to accept the change with some minor modifications. This will allow @loyispa to move forward but doesn't prevent a possible alternative in the future.