How long does it take to create 1 million threads in Haskell?

What I understand, Haskell have green threads. But how light weight are they. Is it possible to create 1 million threads?

Or How long would it tak开发者_Go百科e for 100 000 threads?

from here.

import Control.Concurrent
import Control.Monad

n = 100000

main = do
    left  <- newEmptyMVar
    right <- foldM make left [0..n-1]
    putMVar right 0    -- bang!
    x <- takeMVar left -- wait for completion
    print x
 where
    make l n = do
       r <- newEmptyMVar
       forkIO (thread n l r)
       return r

thread :: Int -> MVar Int -> MVar Int -> IO ()
thread _ l r = do
   v <- takeMVar r
   putMVar l $! v+1

on my not quite 2.5gh laptop this takes less than a second.

set n to 1000000 and it becomes hard to write the rest of this post because the OS is paging like crazy. definitely using more than a gig of ram (didn't let it finish). If you have enough RAM it would definitely work in the appropriate 10x the time of the 100000 version.

Well according to here the default stack size is 1k, so I suppose in theory it would be possible to create 1,000,000 threads - the stack would take up around 1Gb of memory.

Using the benchmark here, http://www.reddit.com/r/programming/comments/a4n7s/stackless_python_outperforms_googles_go/c0ftumi

You can improve the performance on a per benchmark-basis by shrinking the thread stack size to one that fits the benchmark. E.g. 1M threads, with a 512 byte stack per thread, takes 2.7s

$ time ./A +RTS -s -k0.5k

For this synthetic test case, spawning hardware threads results in significant overheads. Working just with green threads looks like a preferred option. Note that spawning green threads in Haskell is indeed cheap. I've re-run the above program, with n = 1m on MacBook Pro, i7, 8GB of RAM, using:

$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 7.6.3

Compiled with -threaded and -rtsopts:

$ time ./thr
1000000

 real   0m5.974s
 user   0m3.748s
 sys    0m2.406s

Reducing the stack helps a bit:

$ time ./thr +RTS -k0.5k
1000000

 real   0m4.804s
 user   0m3.090s
 sys    0m1.923s

Then, compiled without -threaded:

$ time ./thr
1000000

 real   0m2.861s
 user   0m2.283s
 sys    0m0.572s

And finally, without -threaded and with reduced stack:

$ time ./thr +RTS -k0.5k
1000000

 real   0m2.606s
 user   0m2.198s
 sys    0m0.404s