Re: Rgerading RDMA/SDP and traditional socket apps

At 09:58 PM 9/30/2004, Mohan Parthasarathy wrote:
>Hi,
>
>  I am new to RDMA and would appreciate if my questions are viewed
>  with tolerance.
>  I believe existing socket apps doing a send and reusing the buffer will
>  not work with RDMA since the send might not have completed.
>  Isn't it possible to copy the user buffer to another in the SDP library,
>  to let the user app reuse the buffer ? I guess it might lead to increase
>  in memory usage, but it will be only for legacy apps not using the
>  RDMA capabilities, and will probbaly be no different from today's
>  situation where often the user buffer gets copied to a kernel buffer.
>  Also, are there any new socket APIs in SDP or variations in the current
>  socket calls to distinguish if the caller can handle running over RDMA ?
>  Another way to phrase that is if SDP will be aware of legacy apps
>  versus new ones.

If the sockets call returns, then the ownership of the associated buffer is 
also returned.  SDP does not change this.  Therefore, the application can 
reuse the buffer.  Today's network stacks do not guarantee that a buffer 
has been sent when a sockets call returns.  All that is guaranteed is that 
the underlying implementation will attempt to deliver the buffer to the 
remote.  The same applies to RDMA implementations.  The implementation will 
attempt to deliver the buffer.  If the buffer is returned, then either it 
was successfully sent or there is a local copy within the RNIC or a side 
buffer (e.g. a local copy to host memory) that will be used to send the 
data.  The application does not need to worry about this.  SDP does not 
have to worry about this either.  This is the RI / RNIC's 
responsibility.  A good explanation can be found in the verbs draft 
specification which has been posted to this reflector / website.

Mike

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