[openib-general] Immediate data question

[Devesh Sharma]

On 2/12/07, Devesh Sharma <devesh28 at gmail.com> wrote:
> 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,
post ordering accross QP is not guaranteed hence presence of same CQ
or different CQ will not affect any thing.
> > rather B and C are doing a long time RDMA_WRITE,or send/recv. But B
If RDMA WRITE _on_ B, no effect on performance. If RDMA WRITE _on_ C,
_may_ affect the performance, since load is on same HCA. In case of
Send/Recv again _may_ affect the performance, with the same reason.
> > must sends RNR periodically to A, right?. So does the pending message
> > from A affects B's overall performance  between B and C ?
But RNR NAK is not for very long time.....possibly this performance
hit you will not be able to observe even. The moment rnr_counter
expires connection will be broken!
> >
> >         Thank you.
> >
> > --CQ
> >
> >
> > >
> > > Mike
> > >
> > >
> > >
> >
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