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

[Chao Leng <lengchao@huawei.com>]

On 2022/8/21 14:20, Christoph Hellwig wrote:
> On Fri, Aug 19, 2022 at 03:58:25PM +0800, Chao Leng wrote:
>> Now the ack timeout of RoCE is 2 second(2^(18+1)*4us=2 second). In the
>> case of low concurrency, if some packets lost due to network abnormal
>> such as network rerouting, Optical fiber signal interference, etc,
>> it will wait 2 second to try retransmitting the lost packets.
>> As a result, the I/O latency is greater than 2 seconds.
>> The I/O latency is so long for real-time transaction service. Indeed we
>> do not have to wait so long time to make sure that packets are lost.
>> Setting the ack timeout to 262ms(2^(15+1)*4us=262ms) is sufficient.
> 
> I'll leave people more familar with RoCE to judge the merits of this
> change, but I really want a comment explaining the choice in the
> source code.
Now the TCP retransmission timeout interval is 250ms, and this setting
has been maintained for many years.
The network quality of rdma is better than that of common Ethernet.
That is the reason to set 262ms as the default ack timeout.
Adding a module parameter may be a better option.
> 
> .
>