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

Question

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 take for 100 000 threads?

Answer 1

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