Re: Reference Counting (was Re: Searching Method for Incremental Garbage Collection)

[beckwb@ois.com (R. William Beckwith)]

Kevin Warne (kevinw@whistler.com) wrote:

: Depending on the number of inc/decs, the cost of RCing can be very
: significant.  A single inc + dec that hits on-chip cache would 
: take 6 cycles.  Multiply by the avg. number of inc/decs taken
: per object and the cost and percentage of cache misses and I think
: you'll find the incrementing and decrementing operations are 
: consequential after all.

An incr/decr only occurs when there is a change to the graph of
objects.  If the graph does not change, then there is not need
to incr/decr the counts.

Paul Wilson's RT, generational, incremental GC'tor causes work
when the graph of objects is changed (write barrier).  Surely,
the color changes required with a graph change can be as fast
as a simple incr/decr operation.

Maybe my perspective is skewed because I have been RC'ing in Ada 9X.
I realize that C++ calls a copy constructor if you pass an object
as an argument by value.  But I was under the assumption that every
C++ RC'ing implementation worth its salt would require that the
reference objects are passed by reference (&).

The other trick about RC'ing is that you don't have to keep the
counts in the objects, you can keep them all in the same data
structure so that you pages are unnecessarily dirtied.

... Bill

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