Re: [tipc-discussion] [net-next v2 1/1] tipc: add smart nagle feature

[Ying X. <yin...@wi...>]

On 10/25/19 12:28 AM, Jon Maloy wrote:
> 1) TIPC_NODELAY might be a good option, although I fear people might misuse it in the belief that TIPC nagle has the same disadvantages as TCP nagle, which it doesn't.
> But ok, I'll add it.
> 
> 2) CONN_PROBE/CONN_PROBE_REPLY are not considered simply because they are so rare (once per hour) that they won't make any difference.
> 
> 3) We don't really have any tools to measure this. The latency measurement in our benchmark tool never trigs nagle mode, so we won't notice any difference.
>      When nagle is enabled, we can only measure latency per direction, not round-trip delay (since there is no return message), but logically it works as follows:
> 
> Scenario 1: 
>      1) Socket goes to nagle mode. The message trigging this is not bundled, but just sent out with the 'response_req' bit set.
>      2) A number of messages and possible skbs are added to the queue.
>      3) The ACK_MSG (response on msg 1) arrives after 1 RTT, and the accumulated messages are sent. So, the first message, probably added just after the 'resp-req' message was sent might have a delay of up to one RTT. The remaining messages in the queue will have a lower delay, and notably a message added just before the ACK_MSG arrives will have almost no delay.
> 
> Scenario 2:
>      1) Socket is in nagle mode, and a number of messages are being accumulated. The last message in the queue always have the resp_req bit set.
>      2) The queue surpasses 64 k, or a larger message than 'maxnagle'is being sent. So the whole send queue is sent out. 
>      3) Obviously we didn't wait for the expected MSG_ACK in this case, so the delay for all messages is less than 1 RTT.
> 
> Remains to know the size of RTT, but in the type of clusters we are running this is rarely more than a few milliseconds, at most. This in contrast to TCP, where the delay may be several hundred milliseconds.
> 

Thank you for your clear explanation. It makes me fully understood why
you stated the delay was no more than one RTT after nagle was enabled.