Re: [PATCH] nvme-rdma: set ack timeout of RoCE to 262ms

[Chao Leng <lengchao@huawei.com>]

Hi, Max
     Can you give some comment? Thank you.

On 2022/8/29 21:15, Chao Leng wrote:
> 
> 
> On 2022/8/29 17:06, Sagi Grimberg wrote:
>>
>>>>>> If so, which devices did you use ?
>>>>> The host HBA is Mellanox Technologies MT27800 Family [ConnectX-5];
>>>>> The switch and storage are huawei equipments.
>>>>> In principle, switches and storage devices from other vendors
>>>>> have the same problem.
>>>>> If you think it is necessary, we can test the other vendor switchs
>>>>> and linux target.
>>>>
>>>> Why is the 2s default chosen, what is the downside for a 250ms seconds ack timeout? and why is nvme-rdma different than all other kernel rdma
>>> The downside is redundant retransmit if the packets delay more than
>>> 250ms in the networks and finally reaches the receiver.
>>> Only in extreme scenarios, the packet delay may exceed 250 ms.
>>
>> Sounds like the default needs to be changed if it only addresses the
>> extreme scenarios...
>>
>>>> consumers that it needs to set this explicitly?
>>> The real-time transaction services are sensitive to the delay.
>>> nvme-rdma will be used in real-time transactions.
>>> The real-time transaction services do not allow that the packets
>>> delay more than 250ms in the networks.
>>> So we need to set the ack timeout to 262ms.
>>
>> While I don't disagree with the change itself, I do disagree why this
>> needs to be driven by nvme-rdma locally. If all kernel rdma consumers
>> need this (and if not, I'd like to understand why), this needs to be set in the rdma core.Changing the default set in the rdma core is another option.
> But it will affect all application based on RDMA.
> Max, what do you think? Thank you.
>> .
> 
> .