Performance question on tags and named branches

[Alistair Bell]

Hi all,

We have used Mercurial for some development over the past 18 months, and 
we've had a lot of success, but we've had a number of performance issues 
with it. A lot of this was to do with the workflow we used, where 
essentially every checkin included a merge of heads.

So I'm planning to switch us from a system where people work on 
anonymous branches to a system where people work on named branches 
(probably branches named automatically on checkout by the script that 
creates a new clone) and we don't bother merging the heads. But we still 
want to be able to tag each successful build from the build system. (The 
build system will be the only user authorized to update the default 
branch -- we'll stick a commit hook in our central repo to enforce this.)

Which leads to the problem: my understanding is that any time tags are 
consulted, Mercurial builds a union of the .hgtags files at the head of 
every named branch. We currently have about 13,000 tags in our repo, and 
tag performance isn't a big deal right now -- but once we have over 100 
named branches, I'm guessing we might start hitting performance issues.

Am I worrying too much? Can Mercurial deal with (say) 20,000 tags in 200 
named branches? (Obviously most of the tags will be identical because 
they will come from a time before the branch diverged, but I assume that 
Mercurial will still need to consult every named branch to know that.) 
What is the procedure if different named branches have the same tag 
referring to different revisions? (For instance, if we have a 
PASSED_REGRESSION tag on the default branch that moves every time we 
pass regression, but of course that won't update a branch's tag file.)

I don't particularly want to move to bookmarks, because half the point 
is being able to share tags automatically between users, but before I 
commit us to a named branch system, I want to see if anyone has looked 
at this in terms of performance.

Thanks

Alistair