[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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