Description

Directions
• Each problem must be self-contained in a file, named properly, and must compile using the command mpicc filename.c
Problems
Level:Basic
1. Create a function my scatter with declaration
void my_scatter(int* sendbuf, int sendcount, int* recvbuf, int recvcount, int root)
2. Create a function my broadcast with declaration
void my_broadcast(int* buffer, int count)
3. Create a function my allgather with declaration

Figure 1: A simple scatter algorithm
void my_allgather(int* sendbuf, int sendcount, int* recvbuf)
that reproduces some of the functionality of MPI Allgather, assuming that the send type is MPI INT, that send count and receive count match, and that the communicator is
Level: Advanced
1. Create a function my gather with declaration
void my_gather(int* recvbuf, int recvcount, int* sendbuf, int sendcount, int root)
that improves the my gather function we wrote in class so that it does not assume that the number of processors is a power of 2, and it does not assume that the root is rank 0. Your algorithm should follow the “fan-in” algorithm discussed in class, but treating root as rank 0, root+1 as rank 1, and so on. Name the file my gather.c
2. Create a function my allplus with declaration
int my_allplus(int* sendbuf, int* recvbuf, int count)
that puts the sum of all the data held in each processors sendbuf array on every process.
For example, if there are four processes p0,p1,p2,p3 and pi holds data di,0,di,1,di,2 then when the processes complete the call to my allplus each processor should find
d0,0 + d0,1 + d0,2 + d1,0 + d1,1 + d1,2 + d2,0 + d2,1 + d2,2 + d3,0 + d3,1 + d3,2 available in recvbuf[0].
No processor should do more than dlogpe receives and dlogpe sends, and no send or receive should be longer than one integer.
Note: This is not (quite) the algorithm that we sketched out in class!
Errata
What does the testing code do, and how to interpret the results
The testing code compiles your function (my all gather, my scatter or whatever), and compiles them along with a main function that I have written. It calls your function with a number of different inputs, and makes sure that your code is behaving correctly. The command that I’m using to compile your code is shown first. For example:
mpicc –std=c99 -g -lm -Wl,-wrap,MPI_Recv -Wl,-wrap,MPI_Send
-Wl,-wrap,main my_alltoall.c test_helpers.c my_alltoall_test_driver.c mpi_send_wrapper.c -o my_alltoall_exe
You can see that I’m calling mpicc to compile your code into a bunch of code that I’ve written. You can compile your code exactly the same way I am, if you want. Of course, then you would be using my main rather than the one you wrote.
You can see the arguments that my main is giving your functions in each test case. For example, here is the output of one test case for my alltoall
Using the command: mpirun -np 2 ./my_alltoall_exe –len=10 Rank 1 calls my_alltoall with: sendbuff:[1,346,381,486,301] sendcount:5 recvbuff:[-1939952792,32728,26006624,0,26004688,0,25434272,0,1,0] recvcount:5
Rank 0 calls my_alltoall with: sendbuff:[0,243,272,259,420] sendcount:5 recvbuff:[-222065816,32616,13234448,0,13233520,0,12691280,0,1,0] recvcount:5
Rank 0 finishes my_alltoall with:
recv_buff: [0,243,272,259,420,1,346,381,486,301]
Rank 1 finishes my_alltoall with: recv_buff: [0,243,272,259,420,1,346,381,486,301]
We can see that rank 0 has an initial sendbuf of [1,346,381,486,301] and rank 1 has an initial sendbuf of [0,243,272,259,420]. The code completes, and after calling my alltoall both ranks have [0,243,272,259,420,1,346,381,486,301] in their recv buff.
Once the executable runs, my code examines some logs to make sure that your code executed in a reasonable fashion (the correct number of sends, receives, etc).
If you wish to debug a certain test case, you can just hard-code in the inputs into your own main, and call your function with the same inputs that my code is using.
Using the standard code
When you compile your code, you will need to add in the object files provided in the obj directory.
For example, you would switch from compiling my alltoall like this:
mpicc my_alltoall.c -o my_alltoall
to like this:
mpicc my_alltoall.c ./objs/my_alltoall_standard.x86_64.o -o my_alltoall
The compiler will warn you about “implicit declaration” when you compile, but the overall compilation will succeed.