OpenMP
'Parallel: Using more than one processor at a time'
Introduction
I'd rather have a computer that not. They're handy for email and buying stuff from Amazon. Definitely. Indeed, for people of a certain mindset--people like you and me--we can do all sorts of interesting things like simulating the natural world, and in the process look at questions like, "will Greenland melt?" and "what would happen if it did?".
Sometimes it's handy to have more than one computer. Let's say that we have a new whizz-bang weather model that takes 26 hours to work out what the weather will do tomorrow. "All very well", you say, "but about as much use as a chocolate teapot." In order for the model to be of any use, we need it to run faster. We need to divide up the work it does and run it over two, or more computers. We need to enter the world of parallel programming.
"Hippee!" we cry, but a word of caution. Getting models to work in parallel is a lot, I say it again, a lot harder than getting them to work on a single processor. Before setting out down the road, it is well worth checking that you really do need your model to run faster, and that you've explored all avenues in that regard.
You still with us? OK, let's get stuck in.
OpenMP
There are a number of different ways to create parallel programs, and we're going to start with one approach, called OpenMP. There a number of reasons for this:
- It's pretty widely available
- It's good for the muli-core processors that we find in a lot of computers today
- It's fairly easy to use
- and it's based upon the not-so-minde-bending concept of threads
At this point, we could launch overselves into a long and detailed discussion of threads, the OpenMP runtime environment, pre-processor macro statements and the like. But we won't. Because it's less fun. Let's just try an example instead.
OK, to get the examples, login to a Linux box and cut & paste the below onto the command line:
svn co http://source.ggy.bris.ac.uk/subversion-open/parallel ./parallel
Hello, world
A time-honoured starting point, except this time we'll get many hellos!