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

[Patrick Palka]

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
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