Code review comment for lp:~jameinel/bzr/2.1.0b4-convert-kg-heads

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Robert Collins wrote:
> Review: Approve
> I'm not sure that 100 is the right figure, could you do some timing data
> on e.g. emacs and put forward a change to that figure if its wrong?
>
> This branch can land now however.
>
> review: approve
> merge: approve
>

So I don't think there is going to be an ideal value that is always correct.

The emacs history is going to be very different from mysql, for example.
(I've got about 70k mysql revs, but the mainline is only 2k-4k long
depending on the branch.)

However, repo.revisions.get_known_graph_ancestry([tip])

Takes somewhere between 0.5 to 1.0s. Conversions I've done are around 3
revs per second. This heads is about 2x faster, so if we assume

3r/s * 1.0s / 0.5 = 6revs.

So somewhere around 6 revs it will be faster to just load the whole
revision graph.

As mentioned, there are lots of variables. (I think Emacs heads() is
going to generally be cheaper because it is more linear, though it may
be more expensive because then you have to walk more revs?)

I could drop it to 10 revs if you would prefer. Adding 1.0s is pretty
trivial, and we only do this on 'local' access. (IDS only works on
file=>file, and it is being done on the 'source' side for smart streaming.)

Were you thinking the number should be bigger or smaller?

John
=:->
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