[Haskell-beginners] Parallelism?

[Edward Z. Yang]

With regards to your original concurrent code (asynchronous database writes),
if the API given to you truly is asynchronous (i.e. it's a file descriptor that
could be monitored with epoll/kqueue/folks), consider integrating it with
the IO manager, so that you don't need to tie up real OS threads on blocking
FFI calls (though I'm not sure what database or what access mechanism is.)  You really
shouldn't need the threaded runtime for a task like this.

Maybe if you give more details we can give more specific advice.

Edward

Excerpts from Michael Craig's message of Thu Dec 01 00:50:15 -0500 2011:
> I was writing some parallel code (asynchronous database writes for an event
> logger, but that's besides the point), and it seemed like the parallelized
> version (i.e. compiled with -threaded -with-rtsopts=-N2) wasn't running
> fast enough. I boiled it down to a dead-simple test:
> 
>     import Control.Concurrent
>     import Data.Time.Clock.POSIX
>     import System.Environment
> 
>     main :: IO ()
>     main = do
>         n <- getArgs >>= return . read . head
>         t1 <- getPOSIXTime
>         work n
>         t2 <- getPOSIXTime
>         putStrLn $ show $ t2 - t1
>         putStrLn $ show $ (fromIntegral n :: Double)
>                         / (fromRational . toRational $ t2 - t1)
> 
>     work :: Integer -> IO ()
>     work n = do
>       forkIO $ putStrLn $ seq (fact n) "Done"
>       putStrLn $ seq (fact n) "Done"
> 
>     fact :: Integer -> Integer
>     fact 1 = 1
>     fact n = n * fact (n - 1)
> 
> (I know this is not the best way to time things but I think it suffices for
> this test.)
> 
> Compiled with ghc --make -O3 test.hs, ./test 500000 runs for 74 seconds.
> Compiling with ghc --make -O3 -threaded -with-rtsopts=-N, ./test 500000
> runs for 82 seconds (and seems to be using 2 cpu cores instead of just 1,
> on a 4-core machine). What gives?
> 
> Mike S Craig