[Haskell-cafe] Re: Importing Data.Char speeds up ghc around 70%

Richard Kelsall r.kelsall at millstream.com
Thu Jan 3 13:20:48 EST 2008

Simon Marlow wrote:
> I have seen strange artifacts like this before that turned out to be 
> caused by one of two things:
>  - bad cache interactions, e.g. we just happen to have laid out the code in
>    such a way that frequently accessed code sequences push each other out
>    of the cache, or the relative position of the heap and stack have a bad
>    interaction.  This happens less frequently these days with 4-way and
>    8-way associative caches on most CPUs.
>  - alignment issues, such as storing or loading a lot of misaligned Doubles
> in the second case, I've seen the same program run +/- 50% in 
> performance from run to run, just based on random alignment of the 
> stack.  But it's not likely to be the issue here, I'm guessing.  If it 
> is code misalignment, that's something we can easily fix (but I don't 
> *think* we're doing anything wrong in that respect).
> I have an Opteron box here that regularly gives +/- 20% from run to run 
> of the same program with no other load on the machine.  I have no idea 
> why...
This got me wondering how I could test for code misalignment problems.
I expect there's a cleverer way, but how about a single executable
containing several copies of the same code to be tested and a loop
that runs and times the different copies. A consistently higher or
lower runtime from one copy would indicate a misalignment problem.
(I'm assuming the different copies of the code would probably load
at fairly random alignments, random padding could be added.) It might
have to run the copies in a different order each time round the loop
to avoid the possibility of external periodic events affecting a
particular copy.


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