[mpich-discuss] Can ch3:nemesis:mx use both MX and TCP networks ?

[Scott Atchley]

On Oct 13, 2009, at 1:05 PM, Guillaume Mercier wrote:

>> I always thought (but never tested) that the ch3:nemesis:mx  
>> cmehannel has the ability to independently select either MX or TCP  
>> to communicate between any pair of MPI processes when shared memory  
>> communication isn’t possible (e.g. the two process run on different  
>> nodes).
>>
>
> When Nemesis:Mx is selected, only MX can be used.

Not only it is a runtime selection, but it is a compile time selection.

You could compile your application against a shared MPICH2 Nemesis/TCP  
lib. You could then run that binary and use LD_LIBRARY_PATH to select  
either a Nem/TCP or a Nem/MX libmpich.so.

>> However after carefully reading the README file, I fear mpich2  
>> doesn’t have this flexibility that is, network communication with  
>> the ch3:nemesis:mx channel is always done using MX (of course  
>> network communication occurs when shared memory communication isn’t  
>> possible).
>>
>
> Your interpretation is right.
>>

Guillaume, does Nemesis handle shmem directly or does it pass it to  
Nem/MX to let MX handle the shmem?

>> When shared memory communication isn’t possible, does the  
>> ch3:nemesis:mx channel has the ability to independently select  
>> either MX or TCP to communicate between two MPI processes ?
>>
> The Mx module does not offer this kind of possibility. Could you  
> explain to me when shared memory is not possible within
> a node ? I don't see the case.

I believe he is asking for selection of Nem/TCP or Nem/MX for non- 
shmem traffic.

Scott