Are push times linear in the size of the repo?

[Michael O'Connor]

I was looking at a breakdown of where the time was spent in pushing one
empty changeset in a repo with a few hundred thousand changesets.

It looks like there are a few places in the course of a push where some
object is created that is linear in the total size of the repo (or the
number of changesets in common between local and remote).  For example, in
determining which changesets are outgoing (see
http://selenic.com/repo/hg/file/52ff737c63d2/mercurial/setdiscovery.py#l131),
in determining which phase information to push (see
http://selenic.com/repo/hg/file/52ff737c63d2/mercurial/exchange.py#l144),
and in determining information about obsolescence of future heads (see
http://selenic.com/repo/hg/file/52ff737c63d2/mercurial/discovery.py#l276).

In the third case, a patch was recently accepted to only compute this set
when the repo actually has an obsstore, but is there a plan to eventually
make pushing more incremental and not depend on the entire repo, or is it
thought that this won't be a problem?  Or perhaps I've misread the
situation?
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