[Haskell-cafe] GHCi timing statistics

[Eric Seidel]

I believe `:set +s` displays cpu time, not wall-clock time.

On Thu, Mar 5, 2015, at 14:23, David Rush wrote:
> Hi all - 
> 
> Is it my imagination, or are the statistics printed by GHCi when :set +s 
> has been enabled entirely spurious when SMP parallelism is turned on? I'm 
> trying to determine if I am actually getting any improvement, and - both 
> by the wall clock and by the reported statistics I don't seem to be; 
> however, the reported statistics don't seem to line up very well with the 
> wall clock time. 
> 
> An additional oddity that I've noted is that my algorithm's reference 
> implementation - which simply can't run in parallel due to a foolishly 
> introduced data dependency - still seems to kick 16 CPUs well into the 
> 90%+ utilization range when I have +RTS -N. This code has no parallelism 
> primitives in it whatsoever. 
> 
> I realize that I'm not being terribly rigorous in my description here,
> but 
> that is partly because I am not sure how to report on this yet. I think
> my 
> next step is going to be to build a standalone executable to check using 
> the Unix time command. 
> 
> For reference I am running Haskell platform from Debian Jessie on a 16-
> core amd64 system with 29GiB or RAM available. This would appear to be
> GHC 
> 7.6.3 :(
> 
> - d 
> 
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