Re: Rgerading RDMA/SDP and traditional socket apps

On Oct 4, 2004, at 5:04 AM, Mohan Parthasarathy wrote:

>  Hi Mike/Caitlin,
>
>    Thanks for the answers. So, what I understand is that my assumption
>    that existing socket apps might not have ownership of the buffer 
> when
>    the send completes, was wrong.
>    But, doesn't this mean that send calls will take more time to 
> complete,
>    if the RNIC has to DMA the buffers before control to the buffer is
>    returned to the caller. Are there any potential performance issues 
> here
>    for existing synchronous sockets ?
>
>  Regards,
>  Mohan


SDP defines multiple transfer procedures precisely to avoid
this problem.  SDP should be configured so that zero-copy
transfers are only done when doing so will improve performance.

For example,  when the receive  is posted long enough before
the send for the data sink buffer to have been advertised the
data source can RDMA Write to the final target buffer without
any delay.

SDP inherently involves more wire traffic, but depending on
the timing of sends and receives this extra traffic may have no
impact on latency.  The benefits of direct placement should
overcome any penalty for the wire traffic.  But that penalty is
there, which is why local-only optimizations that achieve direct
placement without wire traffic might be more efficient, and why
full adaptation to RDMA-aware application logic will almost
always be more efficient.