While investigating throughput gaps for refworkload wls_webapp_atomic JSP_HTTPS_POST_3 runs with jdk8b115, between g1 and parallelgc, I noticed the following:

g1 lacks of parallelism.  G1 cpu utilization is about 50%, parallelgc cpu is 60%.  Parallelgc has more throughput.  One of the reasons could be g1 734,518 objects allocated out side tlabs, while parallelgc has only 19 for 20s jfr duration.  Also parallelgc allocate much less TLAB than g1.  g1 dev menntioned when g1 allocates new tlab, the memory is not alined properly.

I tried to increase g1 TLAB by -XX:G1HeapRegionSize=8m -XX:TLABSize=4m.  That makes cpu utilization to ~57%, throughput to 47161.176.  So that helps.  But G1 Avg TLABSize is 1.6M.  So I tried to increase it further by -XX:G1HeapRegionSize=16m -XX:TLABSize=8m, that did not increase the throughput further.

Here is the summary
gc and parameters	avg TLAB size	tlab allocations	total mem for TLABs	obj allocated out size TLAB	throughput	
parallelgc 	4m	11,696	51.67G	19	50187.008	
g1 	85kb	333,507	27.04GB	734,518	46022.863	
g1 -XX:G1HeapRegionSize=4m -XX:TLABSize=4m 	1.63m	30,726	49.01G	7,058	47161.176	
g1 -XX:G1HeapRegionSize=16m -XX:TLABSize=4m 	3.65M	13,828	49.26G	1,967	46556.9	

We need to investigate 2 things:
1. g1 new_tlab allocates from AllocRegion memory alignment
2. TLAB size.  Currently the upper bound for TLAB is humongous object size, which is 50% of RegionSize.  But this might be too strict.