[Mpi-forum] MPI One-Sided Communication

[Scott Atchley]

On Apr 27, 2009, at 9:14 AM, Underwood, Keith D wrote:

> For many (most?) science and engineering codes, the trade-off  
> between matching for two sided and synchronization for one sided is  
> at best a wash, and often would fall in favor of two sided. There  
> is, of course, an exception....
>
> If one sided delivered truly stellar performance in terms of message  
> rate and latency, you could (in some cases) eliminate the cost of  
> the copy at the sender that is tyically done to send long messages  
> to cover the overhead. The hardware to deliver that level of  
> performance is truly rare...
>
> Keith

"cost of the copy at the sender that is tyically done to send long  
messages to cover the overhead" ?

Typically, any HPC NIC uses zero-copy for large messages. Copy is only  
used for small messages (where small depends on the fabric/NIC/ 
software stack).

It is even possible to avoid copy on send for large messages over  
standard Ethernet using sendfile() and other options.

Scott