[ceph-users] ceph cluster iops low

I have my ceph IOPS very low with over 48 SSD backed on NVMs for DB/WAL on four physical servers. The whole cluster has only about 20K IO total. Looks the IOs are suppressed over bottleneck somewhere. Dstat shows a lots csw and interrupts over 150K, while I am using FIO bench 4K 128QD test. I check SSD throughput only about 40M at 250 ios each. Network are total 20G and not full of traffic. CPU are around 50% idle on 2*E5 2950v2 each node. Is it normal to get that high and how to reduce it? where else could be the bottleneck?