How does mpi bcast work

The receive buffer arguments of the processes in the second group must be consistent with the send buffer argument of the root process in the first group. Notes This function does not support the in-place option. Errors Almost all MPI routines return an error value; C routines as the value of the function and Fortran routines in the last argument.

Before the error value is returned, the current MPI error handler is called. Does anybody know what's wrong with it?

This is a common source of confusion for people new to MPI. For MPI collective communications, everyone has to particpate; everyone has to call the Bcast, or the Allreduce, or what have you. That's why the Bcast routine has a parameter that specifies the "root", or who is doing the sending; if only the sender called bcast, you wouldn't need this. Everyone calls the broadcast, including the receivers; the receviers don't just post a receive.

The reason for this is that the collective operations can involve everyone in the communication, so that you state what you want to happen everyone gets one processes' data rather than how it happens eg, root processor loops over all other ranks and does a send , so that there is scope for optimizing the communication patterns eg, a tree-based hierarchical communication that takes log P steps rather than P steps for P processes.

There is a post that may be helpful for you, click here. Stack Overflow for Teams — Collaborate and share knowledge with a private group.

Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. Asked 10 years ago. Active 1 year, 5 months ago. Viewed 73k times. David David 1, 5 5 gold badges 26 26 silver badges 40 40 bronze badges. You might wonder the advantages of a broadcasting operations compared to a sequence of send. Aside from the actual elegance of not having an if-statement and having to pick between Send and Receive operations, a Broadcast is a lot more effective than a list of send receives.

It is always a bit tricky to talk about performances and collective operations because efficiency and speed is highly dependant on the implementation. Let's compare two possible algorithms for broadcasting an array of integers elements from the process 0 to 8 other processes.

The first implementation is the P2P version described in the previous lesson. The second implementation is dynamic and relies on a tree structure. Basically, a process that already has received information will send to other processes in turn : Process 0 is going to send its data to process 1.

Then, simultaneously, process 0 is going to transmit to process 2, and process 1 to process 3, etc. The following animation describes the sequence of communications. This animations makes things a bit clearer :.