Lush2 performance

Hey Lushies,

If you have played around with Lush 2 a bit you will
have noticed that it runs quite a bit slower than
Lush 1.x. The reason is that much has changed under
the hood, Lush 2 is powered by a mark-and-sweep garbage
collector, whereas Lush 1.x uses reference counting
internally. So a *lot* has changed in the interpreter
and getting the interpreter to work again after taking
it apart and putting it back together was the main
concern.

The reason we changed to a mark-and-sweep collector is
that it is now much easier to re-write the compiler so
that compiled code behaves the same as interpreted code
in terms of memory management (no more memory pools for
dynamically allocated objects and no more "hidden
arguments" problems...). Alas, we are not there yet and
won't be for the 2.0 release.

The collector currently spawns a child process to do the
mark-and-sweep, so it is working "concurrently" in a
modest sense. I was doing a lot of experiments and found
that this does not really speed things up significantly
on a multicore box. Apparently the cost of spawning a
child process is large enough (on a linux box at least)
to offset the concurrency gains.

I finally decided to simplify the collector and to rip out
all the code that has to do with creating the child process
and with communication. After this, the interpreter was a
fast as before, but it opened the possibilities for some
other optimizations in the interpreter. After all this Lush2
is within a factor of two to Lush 1.x in terms of execution
speed on my machine (if you want to check it out, the code
is currently in the branch lush2-mmrev, I will merge it
back into trunk within a few days).

This simplification should also eliminate some potential
portability problems (to be looked at this summer)!

Cheers,
Ralf


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