[Ns-developers] Questions about Packet, Buffers, and zero payloads

[George Riley]

Hi All,
I am nearing completion on the MPI-based distributed simulation for  
ns-3.
The code to do time synchronization is working perfectly, as is the code
to detect transient messages.  I have implemented  the code to send  
packets
using MPI sends and packet serializatino, but am puzzled by a couple  
of issues
and could use some advise

After quite a bit of trying various methods, the code ended up looking  
like this:

  // Serialize the packet
   i->buffer = p->Serialize();
   // Add the dest node and dest device
   i->buffer.AddAtStart(sizeof(node) + sizeof(dev));
   Buffer::Iterator it = i->buffer.Begin();
   it.WriteU32(node);
   it.WriteU32(dev);

   // Find the system id for the destination node
   Ptr<Node> destNode = NodeList::GetNode(node);
   uint32_t nodeSysId = destNode->GetSystemId();
   MPI_Isend((void*)i->buffer.PeekData(), i->buffer.GetSize(),  
MPI_CHAR, nodeSysId,
             0, MPI_COMM_WORLD, &(i->request));
   cout << "MPI Packet sent" << endl;

This seems to work; but one question is does the "AddAtStart" after  
the serialization
cause a buffer re-alloc and copy?  If so this is not acceptable.   
Second question is
that the i->buffer.GetSize() on the MPI_Isend returns the size of the  
buffer including
the zero-byte (and non-existent) payload.  Shouldn't we only send the  
actual packet data
without the zero payload?  How do I get the zero payload back on the  
receiving end?
I did not see where this is handled in Packet::Serialize() and  
Packet::DeSerialize().

Any advise appreciated.

George

--------------------------------------------------
George Riley
Associate Professor
Georgia Tech
Electrical and Computer Engineering
riley at ece.gatech.edu
404-894-4767
Klaus Advanced Computing Building
Room 3360
266 Ferst Drive
Atlanta, Georgia 30332-0765

ECE4110 Web page:
http://users.ece.gatech.edu/~riley/ece4110/

ECE4112 Web page:
http://users.ece.gatech.edu/~riley/ece4112/