The Code Root Scan Phase during garbage collection finds references to Java objects in compiled code. To speed up this process, G1 maintains a remembered set for compiled code that contains references into the Java heap. That is, every region contains a set of compiled code that contains references into it.

Assuming that such references are few, previous code used a single thread per region to iterate over a particular region's references, which poses a scalability bottleneck if the distribution of these references is very unbalanced.

G1 now distributes this code root scan work across multiple threads within regions, removing this bottleneck.