Threadscope reporting that nothing is happening (MUT or GC) for ~2 seconds

Hi, I'm analyzing the performance of my parallel --make branch of GHC ( https://github.com/parcs/ghc/tree/cherry-picking if anyone is interested) by compiling the Cabal repository in parallel, using two capabilities. I'm seeing very strange results from Threadscope: at some point during the compile, absolutely no work is being performed on any HEC for about 2 seconds. How do I interpret this kind of behavior? What does it mean? Relevant screenshot: http://i.imgur.com/uDQk9zG.png Patrick