ghci and ghc -threaded [slowdown]

[Malcolm Wallace]

> It seems that the problem you have is that moving to the multithreaded
> runtime imposes an overhead on the communication between your two
> threads,  when run on a *single CPU*.  But performance on a single CPU
> is not what  you're interested in - you said you wanted parallelism,
> and for that you  need multiple CPUs, and hence multiple OS threads.

Well, I'm interested in getting an absolute speedup.  If the threaded
performance on a single core is slightly slower than the non-threaded
performance on a single core, that would be OK provided that the
threaded performance using multiple cores was better than the same
non-threaded baseline.

However, it doesn't seem to work like that at all.  In fact, threaded on
multiple cores was _even_slower_ than threaded on a single core!

Here are some figures:

    ghc-6.8.2 -O2  
                 apply   MVar    strict  thr-N2  thr-N1
    silicium      7.30    7.95     7.23   15.25  14.71
    neghip        4.25    4.43     4.18    6.67   6.48
    hydrogen     11.75   10.82    10.99   13.45  12.96
    lobster      55.8    51.5     57.6    76.6   74.5

The first three columns are variations of the program using slightly
different communications mechanisms, including threads/MVars with the
non-threaded RTS.  The final two columns are for the MVar mechanism
with threaded RTS and either 1 or 2 cores.  -N2 is slowest.

> I suspect the underlying problem in your program is that the
> communication  is synchronous.  To get good parallelism you'll need to
> use asynchronous  communication, otherwise even on multiple CPUs
> you'll see little  parallelism.

I tried using Chans instead of MVars, to provide for different speeds of
reader/writer, but the timings were even worse.  (Add another 15-100%.)

When I have time to look at this again (probably in the New Year), I
will try some other strategies for communication that vary in their
synchronous/asynchronous chunk size, to see if I can pin things down
more closely.

Regards,
    Malcolm