Coarrays
| Images and this_image | |
| Coarray syntax | |
| SYNC ALL | |
| Run on images |
Images and this_image
A coarray program runs as several copies called images, and each image executes the same code with its own number.
The this_image function returns the number of the current image, while num_images returns the total count.
The program below greets the user once per image with its own number attached.
program demo
implicit none
print *, "hello from image", this_image()
print *, "of", num_images()
end program demo
Coarray syntax
A coarray declaration adds square brackets to a normal declaration, giving every image its own copy of the variable.
An undecorated name means the local copy, while a name with brackets in square form reads the copy on another image.
Here each image stores its own number and image one collects the value from image two.
program demo
implicit none
integer :: me[*]
me = this_image()
sync all
if (this_image() == 1) print *, me[2]
end program demo
SYNC ALL
Images run at their own pace, so a sync all statement is the meeting point where every image waits for all the others.
Place it after remote writes and before remote reads, so no image reads data that a slower image has not stored yet.
The sketch writes a value for the neighbor, synchronizes, and only then reads the neighbor value.
program demo
implicit none
integer :: slot[*]
slot = this_image() * 10
sync all
if (this_image() == 1) print *, slot[num_images()]
sync all
end program demo
Run on images
A normal run starts a single image, while a coarray launcher starts several images of the same program at once.
The launcher name and flags depend on the compiler setup, so check local docs before running multi-image jobs on a cluster.
The text below shows the idea with four images and the output it would produce.
cafrun -n 4 ./a.out
hello from image 1 of 4
Next: OpenMP basics
Article author: Arthur Isaev
