Currently after remark (and also after all other related phases at the end of marking) G1 prints out the amount of used memory (i.e. not available for allocation) in regular logging output (PrintGCDetails etc.).

It would be interesting to get to know the actual live data as detected by marking. Using -XX:+PrintRegionLivenessInfo you can get this information, but only in addition to per-region information, which is often not desirable.

Add information about summary liveness information to the GC output at a more regular logging level (e.g. -XX:G1LogLevel=finest).

Also consider output of brief summary information of per-region liveness (e.g. on character per region).