[rvm-core] Immix

[Steve Blackburn]

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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