notes about memory-saving GCs
===============================

XXX go into more detail, discuss which pypy GCs we want to try
XXX [fijal] I think both approaches are worth trying. We need to see
    how they behave or try to think how much memory it'll be needed for
    a GC header. note that deferred refcounting without nursery is needed
    for deferred refcounting with nursery (just because it's easier to have
    this in steps). We also want to consider impacts of fragmentation and
    how underlaying C library handles stuff (ie do we use obmalloc or no.
    does it return memory to the OS or no for small objects etc.)

* squeak-like mark-compact collector

* deferred refcounting strategy

* deferred refcounting with a nursery


Some references
===============

* Garbage collector without paging: http://www.cs.umass.edu/~emery/pubs/f034-hertz.pdf, some discussion: http://lambda-the-ultimate.org/node/2391

NOTE: this gc requires modest (600 LOC) support from operating system
      *very* unlikely we can make such assumptions

* paper about measurments http://www-cs.canisius.edu/~hertzm/thesis.pdf

FAQ
===

Why mark&sweep is consuming so much memory on some benchmarks?
--------------------------------------------------------------

It seems that some allocation schemes (unfortunately ones happen to be
found in pypy during normal operation) create very long free lists for small
objects. Consider example:

  [(i,) for i in range(amount)]

in CPython, any immediate object created during list construction is immediately
discarded. On the other hand, while creating the same thing in RPython, for
each allocated space, it gets stored in free list of obmalloc or C library
until collection happens. This means that since collection happens not
very often (due to high costs), this gets accumulated (also for different
python-level operations, different freelists will be populated, if requested
size is different enough). If we invoke gc.collect
every couple iterations (say 100 or 1000), this effect is gone. On the other
hand it's becoming very very slow as a side effect.