On Mon, 2012-12-24 at 16:16 +0100, timothyhobbs at seznam.cz wrote: > The real question is, does this mean that GHC is stopping the world every > time it puts an MVar? No, GHC rts only locks the MVar itself. See here: http://hackage.haskell.org/trac/ghc/browser/rts/PrimOps.cmm#L1358 Yuras