Haskell for multicores

From HaskellWiki

Revision as of 01:29, 3 September 2008

GHC Haskell comes with a large set of libraries and tools for building programs that exploit multicore architectures.

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.

1 Introduction

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:

   +RTS -Nx

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 Examples

• Riemann's zeta function

1.2 Further reading

• GHC's multiprocessor guide
• runtime options to enable SMP parallelism
• API documentation for paralell strategies
• Real World Haskell: Concurrent and Parallel Programming
• Blog posts about parallelism

2 Thread primitives

Control.Concurrent Control.Concurrent

• forkIO

TODO - finish

For explicit concurrency and/or parallelism, Haskell implementations have a light-weight thread system that schedules logical threads on the available operating system threads. These light and cheap threads can be created with forkIO. Full OS threads will not be discussed here beyond saying they pose a significantly higher overhead, but you create them using forkOS if truly needed.

forkIO :: IO () -> IO ThreadId

Lets take a simple Haskell application that hashes two files and prints the result:

import Data.Digest.Pure.MD5 (md5)
    import qualified Data.ByteString.Lazy as L
    import System.Environment (getArgs)
 
    main = do
        [fileA, fileB] <- getArgs
        hashAndPrint fileA
        hashAndPrint fileB
 
    hashAndPrint f = L.readFile f >>= return . md5 >>= \h -> putStrLn (f ++ ": " ++ show h)

This is a straight forward solution that hashs the files one at a time printing the resulting hash to the screen. What if we wanted to use more than one processor to hash the files in parallel?

One solution is to start a new thread, hash in parallel, and print the answers as they are computed:

import Control.Concurrent (forkIO)
    import Data.Digest.Pure.MD5 (md5)
    import qualified Data.ByteString.Lazy as L
    import System.Environment (getArgs)
 
    main = do
        [fileA,fileB] <- getArgs
        forkIO $ hashAndPrint fileA
        hashAndPrint fileB
 
    hashAndPrint f = L.readFile f >>= return . md5 >>= \h -> putStrLn (f ++ ": " ++ show h)

Now we have a rough program with reasonable great performance boost, which is expected given the trivially parallel computation.

But wait! You say there is a bug? Two, actually. One is that if the main thread is finished hashing fileB first, the program will exit before the child thread is done with fileA. The second is a potential for garbled output due to two threads writing to stdout. Both these problems can be solved using some inter-thread communication - we'll pick this example up in the MVar section.

2.1 Further reading

• A concurrent port scanner
• Research papers on concurrency in Haskell
• [http://haskell.org/haskellwiki/Research_papers/Parallelism_and_concurrency#Parallel_Haskell Research papes on parallel Haskell

3 Synchronisation with locks

Control.Concurrent.MVar

• MVar

Previously in the forkIO example we developed a program to hash two files in parallel and ended with a couple small bugs because the program terminated prematurely (the main thread would exit when done). A second issue was that threads can conflict with eachothers use of stdout.

Locking mutable variables (MVars) can be used to great effect not only for communicating values (such as the resulting string for a single function to print) but it is also common for programmers to use their locking features as a signaling mechanism.

MVars are a polymorphic mutable variables that might or might not contain a value at any given time. This example will only use the following four functions:

newEmptyMVar :: IO (MVar a)
    takeMVar :: MVar a -> IO a
    putMVar :: MVar a -> a -> IO ()

While they are fairly self-explanitory it should be noted that takeMVar will block until the MVar is non-empty and putMVar will block until the current MVar is empty. Taking an MVar will leave the MVar empty when returning the value.

Lets now generalize our forkIO program to operate on any number of files, block until the hashing is complete, printing all the results as they are computed but from one function so no stdout garbling occurs.

import Data.Digest.Pure.MD5
    import qualified Data.ByteString.Lazy as L
    import System.Environment
    import Control.Concurrent
 
    main = do
        files <- getArgs
        str <- newEmptyMVar
        mapM_ (forkIO . hashAndPrint str) files
        printNrResults (length files) str
 
        printNrResults 0 _ = return ()
            printNrResults i var = do
            s <- takeMVar var
            putStrLn s
            printNrResults (i - 1) var
 
    hashAndPrint str f = do
        bs <- L.readFile f
        putMVar str (f ++ ": " ++ show (md5 bs))

3.1 Further reading

4 Message passing channels

Control.Concurrent.Chan

• Chan

Todo

4.1 Examples

• Implementing a chat server
• Two reader threads

4.2 Further reading

5 Lock-free synchronisation

Software Transactional Memory

• STM

Todo

5.1 Further reading

• Research papers on transactional memory

6 Asynchronous messages

Control.Exception:asynchronous

• Async exceptions

Todo

6.1 Examples

• Graceful exit

6.2 Further reading

7 Parallelism strategies

Control.Parallel

• Parallel, pure strategies

Todo

7.1 Further reading

• Parallel binary trees benchmark

8 Data parallel arrays

Data Parallel Arrays

Todo

8.1 Further reading

9 Foreign languages calls and concurrency

Non-blocking foreign calls in concurrent threads.

10 Profiling and measurement

   +RTS -sstderr

10.1 Further reading

Todo

Category: Parallelism