Density of duplicate references in the heap
Jon Harrop
I took an odd design decision with HLVM and made references a struct of
run-time type, metadata (e.g. array length), pointer to mark state and
pointer to data. So every reference consumes 4x32=128 bits rather than the
usual 32 bits but heap-allocated values no longer require a header.
My performance results really surprised me. For example, the "gc" benchmark
in the HLVM test suite fills a hash table that is represented by an array
spine containing references to array buckets. Despite having (fat)
references everywhere, HLVM is 2.2x faster than OCaml on x86.
The main disadvantage of HLVM's approach is probably that every duplicate
reference now duplicates the header information, wasting 96 bits. However,
I do not believe references are duplicated in the heap very often. Both
trees and hash tables contain many references but none are duplicated.
So I'm wondering if anyone has studied the makeup of heaps and could point
me to data on the proportion of the heap typically consumed by *duplicate*
references, i.e. how much space is HLVM likely be wasting?
--
Dr Jon D Harrop, Flying Frog Consultancy Ltd.
http://www.ffconsultancy.com/?u