[mpiwg-rma] [EXTERNAL] Re: making dynamic windows suck less

[Jeff Hammond]

PAMI can do RDMA against both registered memory segments and virtual
addresses today, but the performance difference is noticeable on BGQ.
Other implementations have different performance characteristics, so
it's a solved problem in some sense.  I just do not like having to
assume the existence of the necessary hardware features that enable
virtual-to-physical translation in the network.

I'm not too worried about Cray solving these problems in the future,
so if you're sure that OFED and Portals4 are now or will soon be able
to deal with the issues created by dynamic windows, then I am less
concerned about this issue.

I think Pavan's proposal is the right one to focus on at this point
since it's a simple concept and addresses the common case anyways.

Best,

Jeff

On Mon, Dec 2, 2013 at 9:50 AM, Barrett, Brian W <bwbarre at sandia.gov> wrote:
> The idea behind dynamic windows was to make them possible to implement today
> and push the vendors to support a more rational programming model tomorrow.
> Like Jim, I'm not sure things are bleak enough that we should change.
>
> In OFED, I think it would be very difficult today.  But with Mellanox's work
> to support dynamic registration cache in hardware, I think things are
> trending in the right direction there.
>
> For Portals 4, if an implementation supports the BIND_INACCESSIBLE feature,
> and I think most implementations will, supporting dynamic windows is
> straight-forward.
>
> PAMI and DMAPP are probably more difficult, but a firehose-like setup should
> be possible until hardware catches up.
>
> Brian
>
> --
>   Brian W. Barrett
>   Scalable System Software Group
>   Sandia National Laboratories
> ________________________________
> From: mpiwg-rma [mpiwg-rma-bounces at lists.mpi-forum.org] on behalf of Jeff
> Hammond [jeff.science at gmail.com]
> Sent: Sunday, December 01, 2013 10:16 AM
> To: MPI WG Remote Memory Access working group
> Cc: MPI WG Remote Memory Access working group
> Subject: [EXTERNAL] Re: [mpiwg-rma] making dynamic windows suck less
>
> How do you do pure (i.e. active-message and rendezvous-free) RDMA with just
> a virtual address (MPI_Aint) in (1) PAMI, (2) OFED, (3) DMAPP, (4) Portals4?
>
> Jeff
>
> Sent from my iPhone
>
> On Dec 1, 2013, at 10:40 AM, Jim Dinan <james.dinan at gmail.com> wrote:
>
> Can the MPI_Win_memobj be an MPI_Aint?
>
> I don't share this pessimism with respect to dynamic windows; I think we
> need to prove that these issues exist before we try to fix them.
>
>  ~Jim.
>
>
> On Wed, Nov 27, 2013 at 5:46 PM, Jeff Hammond <jeff.science at gmail.com>
> wrote:
>>
>> Lots of people think - I take position on their correctness - that
>> dynamic windows are doomed to substandard performance.  I can
>> certainly see some potential for this on networks that require memory
>> registration.
>>
>> Can we try to add an opaque object that can encapsulate memory
>> registration such that dynamic windows 2.0 would not have so many
>> problems?  If one were to send-recv not just the virtual address but
>> MPI_Win_memobj (to be named properly later), then maybe we could deal
>> with the shortcomings of IB, etc. w.r.t. RDMA and memory registration.
>>
>> This is really just a thought.  I have no concrete proposal.  Maybe
>> people will have ideas in December.
>>
>> Jeff
>>
>> --
>> Jeff Hammond
>> jeff.science at gmail.com
>> _______________________________________________
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>> mpiwg-rma at lists.mpi-forum.org
>> http://lists.mpi-forum.org/mailman/listinfo.cgi/mpiwg-rma
>
>
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-- 
Jeff Hammond
jeff.science at gmail.com