Setting one side’s gid to 0 in RDMA will result in a very slow speed, BW: 0.048323 GB/s, and will return to normal if both parties’ gid is set to other,BW: 6.003038 GB/s. I tested the speed of different gids with perftest and found that there is not much difference, please ask what may be the cause.

set the gid to 0 in perftest

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                    RDMA_Read BW Test

 Dual-port       : OFF         Device         : mlx5_bond_0
 Number of qps   : 1           Transport type : IB
 Connection type : RC          Using SRQ      : OFF
 PCIe relax order: ON
 ibv_wr* API     : ON
 CQ Moderation   : 100
 Mtu             : 1024[B]
 Link type       : Ethernet
 GID index       : 0
 Outstand reads  : 16
 rdma_cm QPs     : OFF
 Data ex. method : Ethernet
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 local address: LID 0000 QPN 0x1642 PSN 0xf6d81d OUT 0x10 RKey 0x203d00 VAddr 0x007f9d25c4c000
 GID: 254:128:00:00:00:00:00:00:176:252:78:255:254:179:17:18
 remote address: LID 0000 QPN 0x1c6f PSN 0x9d149 OUT 0x10 RKey 0x012800 VAddr 0x007f20cd130000
 GID: 254:128:00:00:00:00:00:00:100:155:154:255:254:172:09:41
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 #bytes     #iterations    BW peak[MiB/sec]    BW average[MiB/sec]   MsgRate[Mpps]
 8388608    1000             10918.53            10918.52           0.001365
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set the gid to 3 in perftest

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                    RDMA_Read BW Test
RX depth:               1
post_list:              1
inline_size:            0
 Dual-port       : OFF         Device         : mlx5_bond_0
 Number of qps   : 1           Transport type : IB
 Connection type : RC          Using SRQ      : OFF
 PCIe relax order: ON
 ibv_wr* API     : ON
 CQ Moderation   : 100
 Mtu             : 1024[B]
 Link type       : Ethernet
 GID index       : 3
 Outstand reads  : 16
 rdma_cm QPs     : OFF
 Data ex. method : Ethernet
---------------------------------------------------------------------------------------
 local address: LID 0000 QPN 0x1641 PSN 0xe2e08b OUT 0x10 RKey 0x203d00 VAddr 0x007fe60e845000
 GID: 00:00:00:00:00:00:00:00:00:00:255:255:10:119:46:61
 remote address: LID 0000 QPN 0x1c6e PSN 0x80035d OUT 0x10 RKey 0x012800 VAddr 0x007fde75aa3000
 GID: 00:00:00:00:00:00:00:00:00:00:255:255:10:119:46:62
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 #bytes     #iterations    BW peak[MiB/sec]    BW average[MiB/sec]   MsgRate[Mpps]
 8388608    1000             11035.96            11035.96           0.001379
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It can be found that the bandwidth of the two is about the same.
But in my code, the gid setting to 0 is very slow, and the transfer size is 3999268864byte and takes 75.seconds.
I would like to ask what could be causing it.