The Haskell Concurrency Primitive Shootout

I took a look at System.Timeout and it doesn’t appear to utilize a global lock, but it does use two global IORefs and atomicModifyIORef. If you dig down deep enough into atomicModifyIORef‘s implementation, it does lead to some locking. Maybe that is causing contention (spoiler probably not) ? To see if I could cause contention I wrote the following benchmark: main = do ref <- newIORef maxCap <- ge