Comments on: The MPI C++ bindings are gone: what does it mean to you?

By: Mark Hoemmen

Mark Hoemmen — Mon, 22 Oct 2012 16:37:11 +0000

Thanks Jeff for taking a look at the Boost innards for me

It seems like the Boost folks could just expose deserialization. Boost.MPI already depends explicitly on Boost’s serialization facility. Once you synch up the window, there’s a perfectly valid buffer of MPI_PACKED waiting for you to deserialize. There’s nothing terribly exotic about that interface, though it would force them to distinguish between types that require serialization and types that don’t.

Another thing they could do would be to register a callback on the window for synch operations. The callback automatically deserializes when the user requests a synch. For C++ types T that require serialization, this would use a hidden array of char that would be the actual window. This would be tricky because you would have to know the offset into the window on both sides.

Should I go talk to the Boost.MPI folks?

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By: Jeff Hammond

Jeff Hammond — Mon, 22 Oct 2012 14:08:25 +0000

My understanding is that serialization must be symmetric with deserialization when used. In MPI send-recv, this is not a problem, since the Boost.MPI recv call can call the deserialization after MPI_Recv. However, in RMA, this is not possible because the target is passive.

Looking at http://www.boost.org/doc/libs/1_46_1/boost/mpi/communicator.hpp, I see the following:

/**
* INTERNAL ONLY
*
* We’re sending a type that does not have an associated MPI
* datatype, so it must be serialized then sent as MPI_PACKED data,
* to be deserialized on the receiver side.
*/
template
void send_impl(int dest, int tag, const T& value, mpl::false_) const;

The “so it must be serialized then sent as MPI_PACKED data , to be deserialized on the receiver side” is what can’t happen with RMA. There is no way to call MPI_UNPACK

Perhaps there is a way to do this all with MPI datatypes. I don’t know though because I am not a C++ guru nor do my applications require exotic types.

I don’t see any problem with your restricted usage of RMA (MPI datatypes but not Boost.Serialization objects) but I’m not sure the Boost.MPI community is. If there are to be new MPI C++ bindings in the standard some day in the future, I have to assume they will be conservative regarding types such that they can be implemented in a straightforward way on top of the MPI communication features available at the time.

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By: Mark Hoemmen

Mark Hoemmen — Sat, 20 Oct 2012 19:09:52 +0000

Hi Jeff — Could you clarify what you said about active messages being needed if Boost.MPI wanted to support one-sided communication? My understanding is that one-sided communication works as long as you aren’t using custom MPI_Op. Thus, MPI_Put and MPI_Get should be fine; it’s just MPI_Accumulate and ilk that pose issues. Is that correct?

I actually wouldn’t mind if one-sided communication were only supported for certain C++ types, as long as there is a traits class that tells me whether it’s supported.

I’m sad that we made the decision a while back not to use Boost.MPI. We had our reasons at the time (dealing with a certain compiler’s inability to build Boost) but I do think we could have handled that better.

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