[rvm-core] Immix

Hi all,

I'd like to finally make the shift to GenImmix as our production  
collector.  See rationale below.

My plan is to make the switch in the next 24 hrs unless I hear  
objections.   I suggest we revisit the decision in a week, having let  
things run for a while.

It is trivial to back out, so if things are looking ugly and I'm  
unresponsive, you should feel free to pull the plug.

Cheers,

--Steve


Rationale:

1. GenImmix is now as stable as production (as far as we can tell with  
the tests we're currently running---we'll have a better picture once  
it is run as production).

2. GenImmix provides a modest performance advantage over GenMS  
(currently used by production) in large heaps and can be significantly  
faster in small heaps. It improves both mutator (a little) and GC  
(somewhat more).
        . Here are numbers comparing GenImmix and production on a 3 X heap on  
a Core 2 Quad:
                http://cs.anu.edu.au/~Steve.Blackburn/private/results/immix-production-2009/c2q/
                (with links for total time ("time"), mutator and gc times).
                (Note that there's a small bug in my scripts which doesn't deal  
gracefully with gc time for mpegaudio, where GenImmix does zero GCs at  
this heap size, while production does a non-zero amount).

3. GenImmix is a bit more algorithmically robust than GenMS since it  
includes defragmentation.   Whether or not the actual implementation  
is currently more robust is something that only more aggressive  
testing will tell.


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