[Haskell-cafe] GHCi timing statistics

David Rush kumoyuki at gmail.com
Thu Mar 5 22:23:54 UTC 2015

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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