As part of debugging 6659663, we have turned off parallel updating and scanning of RSets during G1 evacuation pauses (6677707) while we're ironing out the bugs. We would like to put that decision on two flags: -XX:-/+G1EnableParallelRSetUpdating and -XX:-/+G1EnableParallelRSetScanning. The flags will be experimental because we might ultimately remove them. We'll set them to off by default. When we want to re-enable parallel updating and scanning (as part of 6677708), we'll simply set the default of those flags to on.