[MERGE] Repository.get_revision_graph improvements

[Robert Collins]

On Tue, 2008-02-26 at 17:30 -0600, John Arbash Meinel wrote:
> 
> If we instead chose a fixed hash algorithm to say 32-bit integers then
> the combined index can have 1 single hash => (index, offset) map. So
> when looking up revision-foo-bar you hash it 1 time, and get back the
> possible indexes and positions right away.

I think there are two problems in this proposal:
a) the in memory hashtable becomes very big. This is a scaling problem
itself.
b) it will force pack-count reads to start any operations, which is also
a scaling problem (rather than being a missed-lookup problem)

That said I don't see my design prohibiting combining of hashtables as
long as the same hash is chosen before the truncation.

-Rob

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> 
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