Current Status

[Jon Olson]

On Thu, 18 Feb 1999, Jerry Kramskoy wrote:
>Even more of a consideration with using a handle to access an object is the
>increase in RAM occupancy ... which may well be of concern to
>embedded system manufacturers.  But it certainly makes GC compaction
>easier.
>
>
Actually, with a compacting collector you'd normally save alot more
RAM than single reference word would cost.  This is because memory
in conservative non-copying collectors is allocated in blocks of discrete units
each having a given size.  Hence wastage comes in three forms:

  1) Memory in conservative non-copying collectors gets allocated in blocks
      (often 4Kb or so) and chopped up into as many blocks of a given size
       that will fit.  Thus, a 4K block contains 256 16-byte blocks.  If you're
      only using 128 16-byte blocks, the remainder is wasted and cannot
      be used for other size allocations.
  2) Memory blocks only get allocated in certain discrete allocation quantities.
      Thus, if the collector only supports 64 and 96 byte allocations, and you
      allocate 65 bytes, the remaining 31 bytes are wasted.
  3) Less so but still significant, conservative non-copying collectors have
      alot of general bookkeeping overhead to keep track of their colors, block
      lists, etc.

With a compacting collector all the wastage of types 1) and 2) go away.
Plus, the overhead for 3) can often be smaller since you're managing an
array of fixed size objects which each point to a single entry in a packed
heap.

--
Jon Olson, Modular Mining Systems
	   3289 E. Hemisphere Loop
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