[Mpi3-hybridpm] Hybrid WG meeting at the Forum

[Douglas Miller]

In order to ensure all nodes get the same memory segment, they all have to
use the same "key". At least if this is implemented on Unix SysV or POSIX
shmem. Then there's the possibility of some nodes allocing on a sub-comm
first, etc. I think its just avoiding a lot of potential complications and
don't see what the downside is.


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Douglas Miller                  BlueGene Messaging Development
IBM Corp., Rochester, MN USA                     Bldg 030-2 A410
dougmill at us.ibm.com               Douglas Miller/Rochester/IBM


                                                                           
             "Ron Brightwell"                                              
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                                       Re: [Mpi3-hybridpm] Hybrid WG       
             12/07/2010 12:11          meeting at the Forum                
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On 12/06/2010 05:59 PM, James Dinan wrote:
> On 12/6/10 5:49 PM, Pavan Balaji wrote:
>> Hi Doug,
>>
>> On 12/06/2010 11:42 AM, Douglas Miller wrote:
>>> Re: SHMEM ALLOC: I thought we were considering adding a "const char
>>> *key" param so that shared memory allocations better match OS (e.g.
>>> POSIX) shmem allocations and also to allow the call to be
>>> non-synchronizing.
>>
>> As I recalled, this was still being discussed. Did we decide on adding
>> the key? Ron?
>>
>> I'll add it in as a discussion item.
>
> I think that the key is primarily helpful as a bootstrap mechanism for
> processes to ensure they connect to the right shared segment.  I think
> the intent was to have shmem alloc be collective so this should be
> extraneous.

Right. I think you lose several important optimizations for shared
memory if the call is not collective. It doesn't have to be synchronous,
but it does have to be collective.

-Ron

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