[Haskell-cafe] When are MVars better than STM?

[Yuras Shumovich]

On Sun, 2016-01-24 at 17:46 +1100, Thomas Koster wrote:
> 
> 2. When given two capabilities (+RTS -N2), MVars are suddenly an
> order
> of magnitude slower than with just one capability. Why?

One possible explanation is closure locking which is not performed when
there is only one capability. In my quick measurements it gives 40%
speedup: https://ghc.haskell.org/trac/ghc/ticket/693#comment:9