While migrating our production services from CMS to G1, we found that G1���s complicated write post-barrier incurs considerable CPU cost. Currently the post-barrier is the following for a write ���p.f = q���:

if ((p xor q) >> LOG_REGION_BITS != 0) {  // if the write crosses region boundary
  if (q != null) {
    card_address = &card_table[addr_to_index(p)]
    if (*card_address != YOUNG) {
      store_load_fence;
      if (*card_address != DIRTY) {
        *card_address = DIRTY;
        T.dirtyCardQueue.enqueue(card_address);
      }
    }
  }
}

And for CMS the write barrier is only:
card_address = &card_table[addr_to_index(p)]
*card_address = DIRTY;

The complexity of G1���s write post-barrier is due to the need to support concurrently refinement threads. However, even if user has set -XX:G1ConcRefinementThreads=0, the write post-barrier remains the same. Ideally the write post-barrier could be much simpler if there is no concurrent refinement.

This RFE proposes to add a mode to G1 that uses a simplified write post-barrier:
if ((p xor q) >> LOG_REGION_BITS != 0) {
  if (q != null) {
    card_address = &card_table[addr_to_index(p)]
    *card_address = DIRTY;
  }
}

In this mode, G1 would disable concurrent refinement and per-Java-thread dirty card queue. G1 would need to process all dirty cards during a collection pause. Thus pause time could become longer, but as long as MaxGCPauseMillis is reasonably large with regard to the heap size, G1���s adaptive heuristics should still be able to adjust the young-gen size to meet the pause time goal.

This new mode would reduce G1���s CPU usage considerably. It will be particularly helpful for certain types of workloads, e.g.:
 - Workloads heavily tuned for CMS to minimize old-gen collections, and sensitive to CPU usage;
 - Workloads that mainly care about throughput and CPU usage;

I have implemented a prototype for this mode, and attached some preliminary results.