[openib-general] Immediate data question

[Tang, Changqing]

> >
> >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 ? 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 
> 
> 
>