[MPICH] Can I ask MPI to not copy the data to shared-mem ?

[Darius Buntinas]

On 10/08/2007 01:38 PM, chong tan wrote:
> I am interested in the case that the send and recieve buffer are the 
> same buffer allocated from shared-memory, in which case there is no need 
> for the data copying, send and recieve just only do the sync.

In this case, why not use a zero-byte message?  The sender fills in the 
shared buffer, does a write barrier, then calls MPI_Send with a 
zero-byte message.  The receiver calls MPI_Recv, to receive zero bytes, 
then when it returns, it does a read barrier and reads the data from the 
shared buffer.

> A side question,  when MPI_Send is called, does MPICH copy the data into 
> a intermediate global buffer ? and have the data copy from this same 
> buffer on MPI_Recv call ?

It depends on the channel and what you're sending.  For non-contiguous 
data, the data may be packed into a contiguous buffer before sending and 
unpacked from a contiguous buffer when receiving.  Otherwise, if a 
channel uses sockets, an intermediate buffer is not used by MPICH2, but 
the OS will perform intermediate copies.  For a channel that uses a 
high-performance network, an intermediate buffer is used for small 
messages, but for large messages the data is transferred directly from 
or into the user's buffer.

-d

> tan
> 
> 
> */Darius Buntinas <buntinas at mcs.anl.gov>/* wrote:
> 
> 
>     Well, you specify one buffer in the call to MPI_Send and another buffer
>     in MPI_Recv, so there would have to be at least one copy. You can't do
>     it without any copies (otherwise, how would the data get from the send
>     buffer to the receive buffer?).
> 
>     MPICH2 does support communication over shared memory (as opposed to
>     over
>     a network) which improves performance for intranode communication.
>     Configure with --with-device=ch3:ssm or --with-device=ch3:nemesis.
> 
>     Darius
> 
> 
>     On 10/08/2007 12:22 PM, chong tan wrote:
>      > If I the pointer passed to MPI_Send and MPI_Recieve are already
>     shared
>      > memory, and need not be copied at all ?
>      >
>      > If so, How ?
>      >
>      > thanks
>      > tan
>      >
>      >
>      >
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