[Haskell-beginners] Performance of parallel mergesort

Jon Harrop jon at ffconsultancy.com
Thu Dec 24 19:43:31 EST 2009

On Thursday 24 December 2009 13:24:30 you wrote:
> ** As you pointed the initial code he chose doesn't parallelize very
> well basically

I Googled for "parallel mergesort haskell" and found that code in lecture 
notes by an associate university professor who has pages of published papers 
on his specialist topic of parallel programming in Haskell. I had no idea it 
was bad code.

> ** He tried to compile it with GHC 6.8 which was an important step
> towards better internals to handle parallelism in GHC but was also a
> time of big upheaval in this field and so not as stable as one could
> wish.

I didn't know that.

> ** He then installed GHC 6.10 and did not even clean up his previous
> compilations attempts before declaring that it didn't work (the error
> message he cited is usually observed when GHC encounters an interface
> file created by a previous version of GHC or on another architecture)
> at all without testing on some other clean sources.

Looks like 6.12 fixed that problem.

> ** He then apparently decided that using a binary package for the 6.12
> version was beneath its dignity and went on to install it from source
> (generally not recommended except for GHC developers that needs to
> work on the code or for port to unusual architectures)

I had no idea that building GHC from source was such a big deal. I'll use the 

> and claimed 
> that there was a "circularity problem" since its 6.10 could not
> compile it (which it obviously could, as he discovered and qualified
> as "miraculous", not even considering that he had a working 6.8
> before).

The circularity problem is that the GHC and Haskell Platform download pages 
both say that you must install the other first:

  "Stop! For most users, we recommend installing the Haskell Platform instead 
of GHC." -

  "You only need GHC installed to get started:" -

I wasn't sure how to proceed so I guess and apparently got it wrong.

Dr Jon Harrop, Flying Frog Consultancy Ltd.

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