RFR (XS): 8033601: G1: Make array chunking use the same length field as the other young GCs

[Bengt Rutisson]

Hi everyone,

I'm sponsoring the following fix from Tony:

http://cr.openjdk.java.net/~brutisso/8033601/webrev.00/

https://bugs.openjdk.java.net/browse/JDK-8033601

 From a previous email conversation on this email list:

Talking about array chunking in G1, we noticed a small difference in 
that code compared to what the other GCs (ParNew / PS) do. G1 uses the 
to-space length field to encode how much of the array has been scanned 
(i.e., the from-space length field is always correct). ParNew and PS use 
the from-space length field (and the to-space length field is always 
correct). I came across that issue when I was working on a change to 
periodically scan PLABs (for some profiling stuff I was working on) and 
in G1 some PLABs were unparseable because the length field of arrays 
could be incorrect (even if the PLAB is retired, it's possible that the 
length field is still incorrect given that other threads might still be 
working on that array).

The patch to change G1 to do what ParNew / PS also do is very small (and 
we've had it for a few months in our repo without any issues). Any 
chance of convincing you to also take it? I should be clear that the 
patch doesn't fix a correctness issue in G1; the code is correct as is. 
But it will bring G1 in line with the other two GCs and allow PLAB 
scanning during GC, if you want to do that...


Thanks,
Bengt