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Re: JWZ on s/Java/Perl/

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Ken Fox
February 9, 2001 09:02
Re: JWZ on s/Java/Perl/
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Branden wrote:
> I actually don't understand how traversing a graph can be faster than
> incrementing/decrementing/testing for zero on a refcount.

There are two main reasons advanced garbage collectors are fast:

 1. Cheap allocations. Most fast collectors have a one or two
    instruction malloc. In C it looks like this:

      void *malloc(size) { void *obj = heap; heap += size; return obj; }

    It's easier to do alignments in a macro layer above the allocator
    so the allocator doesn't have to constantly re-align to address
    boundaries. There is basically no difference between the performance
    of heap and stack allocations with a good collector.

 2. Work proportional to live data, not total data. This is hard to
    believe for a C programmer, but good garbage collectors don't have
    to "free" every allocation -- they just have to preserve the live,
    or reachable, data. Some researchers have estimated that 90% or
    more of all allocated data dies (becomes unreachable) before the
    next collection. A ref count system has to work on every object,
    but smarter collectors only work on 10% of the objects.

- Ken

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