[SPDK] iSCSI Sequential Read with BS 128K Performance

Hi, all:
I just tested SPDK 20.10.x iSCSI performance and I got
following performance data: (All Testing with FIO QD 32)
[OK] (100%RndR_4K,   100%RndW_4K)   : (read: IOPS=155k, BW=605MiB/s
(634MB/s),   write: IOPS=140k, BW=547MiB/s (573MB/s))
[OK] (100%RndR_8K,   100%RndW_8K)   : (read: IOPS=147k, BW=1152MiB/s
(1208MB/s), write: IOPS=128k, BW=1003MiB/s (1051MB/s))
[NOK] (100%SeqR_128K, 100%SeqW_128K) : (read: IOPS=210, BW=26.3MiB/s
(27.5MB/s),  write: IOPS=14.6k, BW=1831MiB/s (1920MB/s))
=> Read Bad, Write OK
[NOK] (100%SeqR_32K,  100%SeqR_16K)  : (read: IOPS=9418, BW=294MiB/s
(309MB/s),   read: IOPS=105k, BW=1641MiB/s (1721MB/s))
=> [NOK] BS_32K + QD_32
=> [OK]  BS_16K + QD_32
[OK] (100%SeqR_8K,   100%SeqR_4K)   : (read: IOPS=149k, BW=1160MiB/s
(1217MB/s), read: IOPS=157k, BW=612MiB/s (642MB/s))

Focus on BS 128K
[OK] QD1: read: IOPS=5543, BW=693MiB/s (727MB/s)
[OK] QD8: read: IOPS=21.1k, BW=2634MiB/s (2762MB/s)
[NOK] QD16: read: IOPS=301, BW=37.7MiB/s (39.5MB/s)

FIO Configuration:
ioengine=libaio
direct=1
numjobs=1

I also check with document:
https://ci.spdk.io/download/performance-reports/SPDK_tcp_perf_report_2007.pdf
Inside this document, it also suggest Sequential Read test with BS=128K +
QD8

I think the low performance with BS=128K + QD32 should not relate to iSCSI,
but can anyone share experience about tuning iSCSI sequential read
performance? It's weird that performance drop with high QD. Any suggestion
are welcome.

Thanks

Following are my test configuration
1. Network bandwidth: 40GB
2. TCP Setting at both target and client
    tcp_timestamps: "1"
    tcp_sack: "0",
    tcp_rmem: "4096 87380 134217728"
    tcp_wmem: "4096 87380 134217728",
    tcp_mem: "4096 87380 134217728",
    rmem_default: "524287",
    wmem_default: "524287",
    rmem_max: "268435456",
    wmem_max: "268435456",
    optmem_max: "268435456",
    netdev_max_backlog: "300000"}
3. number of CPU cores at target and client: 48 vcores
     Intel(R) Xeon(R) Silver 4215 CPU @ 2.50GHz * 2
4. disable irqbalance / enable CPU power government
5. run SPDK with: ./iscsi_tgt -m 0x08007C08007C