4 ms·

66-way merge: “Christ, that's not an octopus, that's a Cthulhu merge”

40 points by jjuhl 8y ago

shakna 8y ago
> Yep, sorry, it's my fault that I overlooked Medusa there. I usually check with gitk, but at this time, I checked the results only via --no-merges option for concentrating on the real changes. > Now I looked at it, and I was turned into sto..n...e...... Takashi [0] [0] https://lkml.org/lkml/2014/1/23/80 https://lkml.org/lkml/2014/1/23/80
amelius 8y ago
I wasn't even aware that Git is capable of merging from more than N=2 parents (technically a three-way merge) ... Is this a relatively new feature? And what is the use-case for N>2?
- mehrdadn 8y ago
  It's been there for a long time AFAIK... not sure what the use case is though. (I guess if they're all non-conflicting then it's chill?)
  - amelius 8y ago
    Perhaps the use-case is "consensus". I.e., when you have a 66-way merge and 50 of those say "x=0;" and 16 of those say "x=1;", then it can pick the one with the most consensus, i.e. "x=0;". Or something like that ... EDIT: of course using a special flag, otherwise indeed it could lead to problems.
    - mehrdadn 8y ago
      I'd wager it's just to make life a bit easier, not because of any technical advantages... i.e. so that they don't have to sign off on 66+ commits for a 66-commit merge.
    - et2o 8y ago
      What is the likelihood that the commits making up the different versions would be byte-for-byte identical, however?
      - amelius 8y ago
        They don't need to be identical. You could use the same kind of algorithms as bioinformaticians use to merge multiple different DNA sequences (which can be of different length). https://en.wikipedia.org/wiki/Edit_distance https://en.wikipedia.org/wiki/Edit_distance https://en.wikipedia.org/wiki/Consensus_sequence https://en.wikipedia.org/wiki/Consensus_sequence
    - tomsmeding 8y ago
      If git did that automatically, without reporting a merge conflict, that would be a disaster. Suppose there were more Americans on team 1 of the Mars Climate Orbiter and more Europeans on team 2, so team 1 chose imperial units by consensus and team 2 chose metric units by consensus. That isn't going to give a stable system.
  - sbx320 8y ago
    It allows a reduction of (useless) merge commits. Assume you have 8 different branches (without conflicts) that you need to merge. Then you can either have 8 merge commits (merged $number into master), or a single one (merged 1,2,3,4,5,6,7,8 into master). If you have no conflicts, it makes the history much cleaner without losing relevant history.
- fredley 8y ago
  The reason github's mascot is Octocat is because of octopus merges! http://cameronmcefee.com/work/the-octocat/ http://cameronmcefee.com/work/the-octocat/
  - amelius 8y ago
    Interesting, but kind of strange that the mascot is based on (I suppose) one of Git's least used/useful features.
- christoph-heiss 8y ago
  There is a good explanation over here[0]. Also, HN-discussion for this article[1]. [0] https://www.destroyallsoftware.com/blog/2017/the-biggest-and-weirdest-commits-in-linux-kernel-git-history https://www.destroyallsoftware.com/blog/2017/the-biggest-and... [1] https://news.ycombinator.com/item?id=13647927 https://news.ycombinator.com/item?id=13647927
- geertj 8y ago
  Git merges aren't really merges in the sense that they store a delta produced by some kind of N-way merge algorithm. A git commit is essentially a full copy of a tree where the parents are stored as simple metadata attributes. The parent links are used by tools like "git log" to show how code evolved. But that evolution is calculated on the fly from the difference between trees. In the data model of git, a 66-way merge is therefore nothing special. It's simply a tree with 66 parent links.
okket 8y ago
(2014)
Deathmax 8y ago
Commit: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=2cde51fbd0f310c8a2c5f977e665c0ac3945b46d https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/lin...
vidarh 8y ago
This just seems like a challenge for someone to come up with an even more extreme one.
- shakna 8y ago
  Apparently tipbot has merged a commit with 243 parents before. [0] Getting much worse might require some truly pathological circumstances. [0] https://lkml.org/lkml/2014/1/23/611 https://lkml.org/lkml/2014/1/23/611
Operyl 8y ago
Related: https://www.destroyallsoftware.com/blog/2017/the-biggest-and-weirdest-commits-in-linux-kernel-git-history https://www.destroyallsoftware.com/blog/2017/the-biggest-and...
wickedOne 8y ago
as discussed over here: https://news.ycombinator.com/item?id=17265151 https://news.ycombinator.com/item?id=17265151

40 points by jjuhl 8y ago

> Yep, sorry, it's my fault that I overlooked Medusa there. I usually check with gitk, but at this time, I checked the results only via --no-merges option for concentrating on the real changes. > Now I looked at it, and I was turned into sto..n...e...... Takashi [0] [0] https://lkml.org/lkml/2014/1/23/80 https://lkml.org/lkml/2014/1/23/80

amelius 8y ago

I wasn't even aware that Git is capable of merging from more than N=2 parents (technically a three-way merge) ... Is this a relatively new feature? And what is the use-case for N>2?

mehrdadn 8y ago
It's been there for a long time AFAIK... not sure what the use case is though. (I guess if they're all non-conflicting then it's chill?)
- amelius 8y ago
  Perhaps the use-case is "consensus". I.e., when you have a 66-way merge and 50 of those say "x=0;" and 16 of those say "x=1;", then it can pick the one with the most consensus, i.e. "x=0;". Or something like that ... EDIT: of course using a special flag, otherwise indeed it could lead to problems.
  - mehrdadn 8y ago
    I'd wager it's just to make life a bit easier, not because of any technical advantages... i.e. so that they don't have to sign off on 66+ commits for a 66-commit merge.
  - et2o 8y ago
    What is the likelihood that the commits making up the different versions would be byte-for-byte identical, however?
    - amelius 8y ago
      They don't need to be identical. You could use the same kind of algorithms as bioinformaticians use to merge multiple different DNA sequences (which can be of different length). https://en.wikipedia.org/wiki/Edit_distance https://en.wikipedia.org/wiki/Edit_distance https://en.wikipedia.org/wiki/Consensus_sequence https://en.wikipedia.org/wiki/Consensus_sequence
  - tomsmeding 8y ago
    If git did that automatically, without reporting a merge conflict, that would be a disaster. Suppose there were more Americans on team 1 of the Mars Climate Orbiter and more Europeans on team 2, so team 1 chose imperial units by consensus and team 2 chose metric units by consensus. That isn't going to give a stable system.
- sbx320 8y ago
  It allows a reduction of (useless) merge commits. Assume you have 8 different branches (without conflicts) that you need to merge. Then you can either have 8 merge commits (merged $number into master), or a single one (merged 1,2,3,4,5,6,7,8 into master). If you have no conflicts, it makes the history much cleaner without losing relevant history.
fredley 8y ago
The reason github's mascot is Octocat is because of octopus merges! http://cameronmcefee.com/work/the-octocat/ http://cameronmcefee.com/work/the-octocat/
- amelius 8y ago
  Interesting, but kind of strange that the mascot is based on (I suppose) one of Git's least used/useful features.
christoph-heiss 8y ago
There is a good explanation over here[0]. Also, HN-discussion for this article[1]. [0] https://www.destroyallsoftware.com/blog/2017/the-biggest-and-weirdest-commits-in-linux-kernel-git-history https://www.destroyallsoftware.com/blog/2017/the-biggest-and... [1] https://news.ycombinator.com/item?id=13647927 https://news.ycombinator.com/item?id=13647927
geertj 8y ago
Git merges aren't really merges in the sense that they store a delta produced by some kind of N-way merge algorithm. A git commit is essentially a full copy of a tree where the parents are stored as simple metadata attributes. The parent links are used by tools like "git log" to show how code evolved. But that evolution is calculated on the fly from the difference between trees. In the data model of git, a 66-way merge is therefore nothing special. It's simply a tree with 66 parent links.

okket 8y ago

(2014)

Deathmax 8y ago

Commit: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=2cde51fbd0f310c8a2c5f977e665c0ac3945b46d https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/lin...

vidarh 8y ago

This just seems like a challenge for someone to come up with an even more extreme one.

shakna 8y ago
Apparently tipbot has merged a commit with 243 parents before. [0] Getting much worse might require some truly pathological circumstances. [0] https://lkml.org/lkml/2014/1/23/611 https://lkml.org/lkml/2014/1/23/611

Operyl 8y ago

Related: https://www.destroyallsoftware.com/blog/2017/the-biggest-and-weirdest-commits-in-linux-kernel-git-history https://www.destroyallsoftware.com/blog/2017/the-biggest-and...

wickedOne 8y ago

as discussed over here: https://news.ycombinator.com/item?id=17265151 https://news.ycombinator.com/item?id=17265151