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

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