Lesson 0: Hello World
As all good programming instructions begin, we will begin with the treasured program "HelloWorld".//HelloWorld.c
#include <stdio.h>
#include <shmem.h>
int main(){
int my_pe, num_pe; //declare variables for both pe id of processor and the number of pes
shmem_init();
num_pe = shmem_n_pes(); //obtain the number of pes that can be used
my_pe = shmem_my_pe(); //obtain the pe id number
printf("Hello from %d of %d\n", my_pe, num_pe);
shmem_finalize();
return 0;
}
If you are new to HPCs, I will be the first to say that thinking in parallel is difficult; however, it does get easier as you do it. So, lets look at this step by step for the time being.
What does each step of the above do?
- #include < stdio.h > -- including the standard io c library
- #include < shmem.h > -- including the header file for shmem; allows us to use the shmem commands
- int my_pe, num_pe -- declaring variables of type int for both the processor id and number of processors
- shmem_init() -- initializes shmem. A communication world is created. We now have the number of nodes in the communication world and the id of each processing element (pe) available.
- num_pe = shmem_n_pes() -- obtain the number of pes that can be used. Note this is a private variable.
- my_pe = shmem_my_pe() -- obtain the id of pe. Again, note that this is a private variable. It is not shared with all the processes.
- shmem_finalize() -- releases the shmem resources; the proverbial final curtain to shmem commands
-
The variable my_pe may not make sense initially. For example, say there are 10 servers
that you can use. Each one of these servers will be given a unique id (0-9). This unique id
allows you to control the pes individually -- they can be programmed to do different functions!
To compile the above code, we use the openshmem cc (oshcc) compiler wrapper:
oshcc HelloWorld.c -o HelloWorld
To read more about compilation, look at Annex B.
To run your treasured code, shmem was kind enough to provide the wrapper, oshrun:
oshrun -np 10 HelloWorld
where 10 is the number of processors we want to use. Note: the max number of processes you may use on your home computer is the number of cores you have. To determine this, simply type in your terminal lscpu | grep Core.
The output should be:
Hello from 0 of 10 Hello from 1 of 10 Hello from 2 of 10 ... Hello from 9 of 10 |