At 05:37 AM 2/13/2007, Devesh Sharma wrote: >On 2/12/07, Devesh Sharma <[EMAIL PROTECTED]> wrote: >>On 2/10/07, Tang, Changqing <[EMAIL PROTECTED]> 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.
Seems orthogonal. Any time h/w is shared, multiple flows will have an impact on one another. That is why we have the different arbitration mechanisms to enable one to control that impact. >> > 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! Keep in mind the timeout can be infinite. RNR NAK are not expected to be frequent so their performance impact was considered reasonable. Mike >> > >> > Thank you. >> > >> > --CQ >> > >> > >> > > >> > > Mike >> > > >> > > >> > > >> > >> > _______________________________________________ >> > openib-general mailing list >> > [email protected] >> > http://openib.org/mailman/listinfo/openib-general >> > >> > To unsubscribe, please visit >> http://openib.org/mailman/listinfo/openib-general >> > >> > _______________________________________________ openib-general mailing list [email protected] http://openib.org/mailman/listinfo/openib-general To unsubscribe, please visit http://openib.org/mailman/listinfo/openib-general
