[rds-devel] Re: RDS IB transport software flow control?

Richard Frank richard.frank at oracle.com
Tue Nov 6 08:31:56 PST 2007


<<
Its not so complex, the SDP flow control defined in the IB spec can be 
used as a starting point.
 >>

One consideration - is that we not discuss / reference SDP in any RDS 
design / development discussions.

Or Gerlitz wrote:
> Richard Frank wrote:
>> Yes - it relies on IB RNRs - reasoning was simply to keep the RDS 
>> wire protocol to a minimum.
>
> Thinking on the matter, on the one hand the current RDS/IB code does 
> not have software flow control but it does do explicit ACK-ing on each 
> message.
>
> It might be a win/win to have software flow control protocol and which 
> indirectly provide ACK-ing functionality, since IB RC keep the order 
> of messages, if you know how many credits you have on the remote side, 
> you know what was the last messages the accepted and the credit update 
> is sent after the message placed into memory. The flow control info 
> can be piggybacked on messages going the other direction and below 
> some water mark, we would need a "credit-update" message. Its not so 
> complex, the SDP flow control defined in the IB spec can be used as a 
> starting point.
>
> The win/win here, comes from not doing ACK on every packet as been 
> done now (and I hear that ways are being searched re how to optimize 
> this, eg with RDMA, etc), not doing ACK for protocol based on IB RC, 
> something which creates doubts and confusion for users / reviewers / 
> implementors and more important, not loading the IB network with RNR 
> NAK messages where you have for some reason an imbalanced setup and 
> not having the HCA to arm and act on RNR timers, etc.
>
> Or.
>



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