[SPDK] Single iSCSI max performance with SPDK NULL Block Device

Hi,

Currently, I am using SPDK 19.04 for evaluating SPDK iSCSI Performance. As
I know, current SPDK iSCSI only uses 1 reactor to handle 1 target's iSCSI
IO.
When I try with SPDK NULL Block Device, the max performance with
100%RandWrite / 40G NIC / BS = 4K is about 155K.
I try to finish IO immediately at different layer : SPDK BDev Layer, SCSI
Layer.
It looks like iSCSI protocol handling become the bottleneck of SPDK NULL
Block Device.
Is there any way in SPDK to let multiple reactors handle 1 iSCSI target IOs?

Current test topology as following:

Client: FIO <--> /dev/sdb (Test 1: with 1 thread + with/without blk-mq,
Test 2: with 4 threads  + with/without blk-mq)
Target: 1 SPDK iSCSI target <--> 1 LUN <--> SPDK NULL Block Device

Thanks