[openib-general] Immediate data question

[Devesh Sharma]

On 2/10/07, Tang, Changqing <changquing.tang at hp.com> wrote:
> > >
> > >Not for the receiver, but the sender will be severely slowed down by
> > >having to wait for the RNR timeouts.
> >
> > RNR = Receiver Not Ready so by definition, the data flow
> > isn't going to
> > progress until the receiver is ready to receive data.   If a
> > receive QP
> > enters RNR for a RC, then it is likely not progressing as
> > desired.   RNR
> > was initially put in place to enable a receiver to create
> > back pressure to the sender without causing a fatal error
> > condition.  It should rarely be entered and therefore should
> > have negligible impact on overall performance however when a
> > RNR occurs, no forward progress will occur so performance is
> > essentially zero.
>
> Mike:
>         I still do not quite understand this issue. I have two
> situations that have RNR triggered.
>
> 1. process A and process B is connected with QP. A first post a send to
> B, B does not post receive. Then A and B are doing a long time
> RDMA_WRITE each other, A and B just check memory for the RDMA_WRITE
> message. Finally B will post a receive. Does the first pending send in A
> block all the later RDMA_WRITE ?
According to IBTA spec HCA will process WR entries in strict order in
which they are posted so the send will block all WR posted after this
send, Until-unless HCA has multiple processing elements, I think even
then processing order will be maintained by HCA
 If not, since RNR is triggered
> periodically till B post receive, does it affect the RDMA_WRITE
> performance between A and B ?
>
> 2. extend above to three processes, A connect to B, B connect to C, so B
> has two QPs, but one CQ. A posts a send to B, B does not post receive,
> rather B and C are doing a long time RDMA_WRITE, or send/recv. But B
> must sends RNR periodically to A, right?. So does the pending message
> from A affects B's overall performance  between B and C ?
>
>         Thank you.
>
> --CQ
>
>
> >
> > Mike
> >
> >
> >
>
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