Haskell for multicores
This site attempts to document all our available information on exploiting such hardware with Haskell.
Throughout, we focus on exploiting shared-memory SMP systems, with aim of lowering absolute wall clock times. The machines we target are typical 2x to 32x desktop multicore machine, on which vanilla GHC will run.
To get an idea of what we aim to do -- reduce running times by exploiting more cores -- here's a naive "hello, world" of parallel programs: parallel, naive fib. It simply tells us whether or not the SMP runtime is working:
import Control.Parallel import Control.Monad import Text.Printf cutoff = 35 fib' :: Int -> Integer fib' 0 = 0 fib' 1 = 1 fib' n = fib' (n-1) + fib' (n-2) fib :: Int -> Integer fib n | n < cutoff = fib' n | otherwise = r `par` (l `pseq` l + r) where l = fib (n-1) r = fib (n-2) main = forM_ [0..45] $ \i -> printf "n=%d => %d\n" i (fib i)
We compile it with the `-threaded` flag:
$ ghc -O2 -threaded --make fib.hs [1 of 1] Compiling Main ( fib.hs, fib.o ) Linking fib ...
And run it with:
where 'x' is the number of cores you have (or a slightly higher value). Here, on a quad core linux system:
./fib +RTS -N4 76.81s user 0.75s system 351% cpu 22.059 total
So we were able to use 3.5/4 of the available cpu time. And this is typical, most problems aren't easily scalable, and we must trade off work on more cores, for more overhead with communication.
1.1 Further reading
- GHC's multiprocessor guide
- runtime options to enable SMP parallelism
- API documentation for paralell strategies
- Real World Haskell: Concurrent and Parallel Programming
2 Thread primitives
3 Synchronisation with locks
4 Message passing channels
5 Lock-free synchronisation
6 Asynchronous messages
- Async exceptions
7 Parallelism strategies
- Parallel, pure strategies
8 Data parallel arrays
9 Foreign languages calls and concurrency
Non-blocking foreign calls in concurrent threads.
10 Profiling and measurement