[openib-general] Immediate data question

[Michael Krause]

At 05:37 AM 2/13/2007, Devesh Sharma wrote:
>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.

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
>> > >
>> > >
>> > >
>> >
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