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Re: [fwd] Avoiding inter-patch dependencies

From: Karl Fogel <kfogel_at_newton.ch.collab.net>
Date: 2002-01-31 18:46:09 CET

Zack Weinberg <zack@codesourcery.com> writes:
> Buried in the cesspool of another l-k flamewar, we have an interesting
> idea. Not for 1.0, and not to be done without thinking through the
> implications real carefully, but still.
>
> [some followups were interesting too - see
> http://www.uwsg.indiana.edu/hypermail/linux/kernel/0201.3/1758.html]

Wow. That looks very interesting. Post-1.0, but his point about
distributing the work among the clients is something to keep in mind.

-K

> --- Forwarded message ---
>
> Subject: Re: A modest proposal -- We need a patch penguin
> From: Linus Torvalds (torvalds@transmeta.com)
> Date: Wed Jan 30 2002 - 13:23:01 EST
>
> On Wed, 30 Jan 2002, Larry McVoy wrote:
> >
> > There is a way to do this in BK that would work just fine. It pushes some
> > work back onto the developer, but if you are willing to do it, we have no
> > problem doing what you want with BK in its current form and I suspect that
> > the work is very similar to what you are already doing.
>
> The thing is, bk should be able to do this on its own.
>
> The rule on check-in should be: if the resultant changeset can be
> automatically merged with an earlier changeset, it should be _parallel_ to
> that changeset, not linear.
>
> And note the "automatically merged" part. That still guarantees your
> database consistency at all levels.
>
> Let us assume that you have a tree that looks like
>
> a -> b -> c
>
> together with modifications "d". Now, "bk ci" (or, because this is more
> expensive than a plain "ci", you can call it "bk backmerge" or something,
> and all sane people will just stop using "ci") should instead of doing a
> plain
>
> a -> b -> c -> d
>
> it would see how far back it can go with an automatic merge and add "d" at
> the _furthest_ point possible. So let's assume that "d" really cannot be
> merged with "b" but doesn't clash with "c", so what you'd create with "bk
> backmerge" is the "maximally parallel version:
>
> a -> b -> c
> -> d
>
> Now, you'll say that this is exponentially expensive to do, and I agree.
> But CPU-time is cheap, and notice that this "backmerge" would be done not
> in one centralized location, but in parallel at different developers.
>
> (Yeah, my example may look "cheap", but if you do exclusively backmerging,
> then after a while your trees will have lots of very wide development, and
> the more interesting backmerge is when you already have
>
> a -> b -> c -> f
> -> d
> -> e
>
> and you're adding "g", and it doesn't merge with "c" but not with "d" and
> "e", so you have to test every path backwards to see where you can push
> it. In this case you'd end up with
>
> a -> b -> c -> f
> -> g
> -> d
> -> e
>
> kind of tree.)
>
> Just how expensive would that be? I'd be willing to make my machine sweat
> a bit, if it would just automatically generate the most parallel
> changesets possible.. And I bet you could do _this_ fairly easily if you
> didn't care too much about trying to be too efficient.
>
> You're saying your merges are perfect. USE that knowledge, and make it do
> "bk backmerge".
>
> (Once you do backmerge, the false dependencies no longer exist, AND
> suddenly "bk" actually gives people information that they didn't have
> before: the revision history actually tells you the _true_ dependencies in
> the tree, not some stupid "this is the order in which things were checked
> in" stuff that doesn't add any value that can't be seen from the changeset
> directly)
>
> Larry, give me that, and I'll promise to use bk exclusively for two months
> to shake it out..
>
> Linus
>
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Received on Sat Oct 21 14:37:01 2006

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