Re: Garbage collection (was a spinoff of a spinoff of a GA diatribe)

[Hans-Juergen Boehm <boehm@mti.sgi.com>]

Robert I. Eachus wrote:
> 
> In article <JSA.96Oct22152111@alexandria> jsa@alexandria (Jon S Anthony) writes:
> 
>    > Pro: ...
>    >    Faster - allocations can be _much_ faster and collections can be
>    >    much more efficient than good ol' "free".
> 
>    >    Works across the board (no need for special casing various situations)
> 
>    You must have been smoking some good stuff.  Precise collectors can
> do the later, but never the former. Think for a second, any reclamation
> the garbage collector can do, the free routine can do as well, without
> the memory scan for live references.
> 
This is wrong for many reasons:

1. Deallocating many objects at once is much faster than deallocating
them one at a time in sequence.  Free list headers are already in
registers, etc.

2. In a multithreaded environment with a single heap, the collector has
to perform one lack acquisition/release per roundtrip.  Explicit
deallocation needs 2.

3. "Precise" collectors can arrange to allocate from contiguous memory
which may be much cheaper.

4. It ignores all bookkeeping costs (e.g. reference counts) required for
manual deallocation.  These can easily dominate.

5. Certain algorithms require SUBSTANTIALLY more locking without GC.

In fact even our conservative collector often beats C malloc/free for
very small objects.  Please feel free to try the experiment, especially
before posting remarks like the above.

May I also suggest a scan of the recent literature?  If you're
interested in conservative collection, you might start with

http://reality.sgi.com/employees/boehm_mti/gc.html

It discusses some of these issues and points to some more general
standard references.


-- 
Standard disclaimer ...
Hans-Juergen Boehm
boehm@mti.sgi.com