[llvm-dev] [RFC] One or many git repositories?

Mehdi Amini via llvm-dev
2016-08-09T20:58:51Z

> On Aug 9, 2016, at 1:57 PM, Pete Cooper <peter_cooper at apple.com>
wrote:
>
>
>> On Aug 9, 2016, at 1:55 PM, Mehdi Amini <mehdi.amini at apple.com
<mailto:mehdi.amini at apple.com>> wrote:
>>
>>
>>> On Aug 9, 2016, at 1:38 PM, Pete Cooper via llvm-dev <llvm-dev
at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>>
>>>>
>>>> On Aug 9, 2016, at 11:27 AM, Justin Lebar via llvm-dev
<llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>
wrote:
>>>>
>>>>> (2) If I’m stuck using git-svn I kinda feel like there is
no real point in changing anything.
>>>>
>>>> No real point *for you specifically*.
>>>>
>>>> But the vast majority of people would not be stuck using
git-svn.  And
>>>> in addition the LLVM project would not be stuck using svn, with
all
>>>> the baggage, hosting issues, workflow issues (for people other
than
>>>> you), etc.
>>>>
>>>> The bar by which this proposal should be measured is not
"is it a net
>>>> gain for beanz?"  :)  I think we'd be thrilled with a
"meh" from your
>>>> corner.
>>> Justin, I don’t think this conversation is really going anywhere.
>>
>> I’m not sure what you’re referring to exactly, but in the context of
"this thread isn’t getting anywhere”, I strongly disagree.
> Sorry, I was specially replying to 'I think we'd be thrilled with a
"meh" from your corner.’.  I didn’t feel like that was helping the
conversation along.

OK, I agree with you then :)

>
> I agree with everything else you say about actually talking about the
different proposals.  I hope my point is well received that we really do need to
eventually describe the impact to daily workflow, once the proposals are far
enough along to do so.

I agree with you also on this. I voiced in the past (on IRC toward Justin/David
probably) that the proposal should include examples of workflow and how they
translate to whatever the proposal will be.

Cheers,

—
Mehdi


>
> Pete
>>
>> I believe that the recent workflow tests I performed (see my last
emails in this thread) are proof that this thread has been productive, and I
believe discussing here and hearing concerns from people (Chris and others) are
necessary before getting a proposal fleshed out and having a survey.
>>
>> Having a survey without getting to the end of *what* we want to survey
about is non-sense to me.
>>
>> (That may miss your point, but your point wasn’t clear either…).
>>
>> —
>> Mehdi
>>
>>
>>
>>
>>> Renato already mentioned talking about this at the conference, and
there has also been talk of a survey.  I think we need those to see how the
community actually feel about the proposals here.
>>>
>>> Chris may be the only vocal advocate of an alternative to your
proposal, but then there are people like me who are quiet because we are waiting
for the survey to appear.
>>>  I would have been much more vocal if I thought we were actually
going to adopt the monorepo, but for now I believe it is still only a proposal.
>>>
>>> Full disclosure, I don’t want a monorepo.  I think it optimizes for
the use case where people want to bisect, and I don’t think its reasonable to
push on everyone to have a monorepo for those who want to bisect.  The
submodules repo has already been demonstrated as one potential solution to this
which would allow those who want to bisect to do so, while everyone else can
continue to work more or less as they do today.
>>>
>>> In terms of the proposals, I think you, Mehdi, Chris, and a number
of others have proven that there is almost no technical solution beyond our
reach.  What we do have are proposals which optimize for different use cases.
Given this, I think the most useful thing from my point of view (and hopefully
to others) would be for those advocating each different solution to actual give
short examples of each of the different use cases and how to support them.
>>>
>>> For example:
>>>
>>> Monorepo, pushing a change to compiler-rt:
>>> 1: Git commit …
>>> 2: Git pull --rebase
>>> 3: test
>>> 4 a: Git push /* no commits to any other project so the push works
*/.  Goto 5
>>> 4 b: Git push /* someone committed to some other project in the
monorepo.  Goto 2 */
>>> 5: Done
>>>
>>> I know that this example appears negative in the case where someone
else committed to another project and a rebase is required, but thats exactly
the point.  This is showing that this particular scenario is potentially a
problem compared to today and/or other proposals.  A similar workflow could
(should) be written for the sparse checkout monorepo, GitHub monorepo with svn,
and submodules cases.  The submodules case will likely show that bisecting is
more complex than on the monorepo, while pushing is simpler.
>>>
>>> Similarly, the submodules workflow probably isn’t capable of a
single commit to llvm and clang in the revlock case while the monorepo is, but
we as a community need to decide whether we want to optimize for that or not.  I
don’t have any data to suggest that revlock commits are frequent/infrequent or
even a problem in general, and I don’t think we should optimize for that case
unless its worth doing so.
>>>
>>> Only by actually showing the use cases we care about can the
community make an educated decision about what these proposals actually mean to
our daily workflow.  We can then choose what we are optimizing for.  I
personally want to have a very simple list of repo’s to clone from (or just
one!) and for pushing to be easy, because those are the actions I perform the
most often.  Others will have different use cases they care about and they can
choose the proposal which suits them best.
>>>
>>> Cheers,
>>> Pete
>>>>
>>>> On Tue, Aug 9, 2016 at 11:22 AM, Chris Bieneman <beanz at
apple.com <mailto:beanz at apple.com>> wrote:
>>>>>
>>>>> On Aug 9, 2016, at 10:08 AM, Mehdi Amini <mehdi.amini at
apple.com <mailto:mehdi.amini at apple.com>> wrote:
>>>>>
>>>>>
>>>>> On Aug 8, 2016, at 6:02 PM, Chris Bieneman <beanz at
apple.com <mailto:beanz at apple.com>> wrote:
>>>>>
>>>>>
>>>>>
>>>>> On Aug 8, 2016, at 5:09 PM, Mehdi Amini <mehdi.amini at
apple.com <mailto:mehdi.amini at apple.com>> wrote:
>>>>>
>>>>>
>>>>> On Jul 27, 2016, at 12:50 PM, Chris Bieneman via llvm-dev
>>>>> <llvm-dev at lists.llvm.org <mailto:llvm-dev at
lists.llvm.org>> wrote:
>>>>>
>>>>>
>>>>> On Jul 27, 2016, at 10:21 AM, Justin Lebar <jlebar at
google.com <mailto:jlebar at google.com>> wrote:
>>>>>
>>>>> Thanks for your thoughts, Chris.
>>>>>
>>>>> As supporting evidence of this, I was discussing this
thread yesterday
>>>>> around the office yesterday and had quite a few people
responding something
>>>>> along the lines of “they’re proposing what?”.
>>>>>
>>>>>
>>>>> I hope they'll join us in this thread.
>>>>>
>>>>> Ultimately a survey is going to be strongly biased in favor
of "don't
>>>>> change anything".  There is a strong psychological
bias to weight
>>>>> losses more than gains, so if one doesn't engage with
the issue, it's
>>>>> only natural to conclude "keep it as similar as
possible to what it is
>>>>> today -- that is safe."  But that line of thinking
does not
>>>>> necessarily lead us to the best outcome.
>>>>>
>>>>>
>>>>> I don’t agree with this assertion. I believe that if you
put forth multiple
>>>>> proposals, and have an articulate discussion of the merits
and costs of each
>>>>> solution you can create a survey that can help inform
decision making. I
>>>>> suppose we can agree to disagree.
>>>>>
>>>>>
>>>>> We've heard in thread from a lot of developers about
how a monorepo
>>>>> would improve their workflow.  I would love to hear from
some
>>>>> developers who are actually affected in the way you
describe, rather
>>>>> than just considering the hypothetical.
>>>>>
>>>>> My expectation is that the effect of the monorepo on said
developers
>>>>> would be relatively small -- we're talking about 1gb of
disk space.  I
>>>>> understand that there's a "yuck" factor to
this, but inasmuch as there
>>>>> aren't other concrete effects, this is just change
aversion.  And
>>>>> essentially all of the other effects of the monorepo can be
hidden via
>>>>> sparse checkouts, as we've discussed.
>>>>>
>>>>> Maybe I am wrong.  But I don't think we're going to
get to the bottom
>>>>> of it without actually engaging with people who are
actually affected
>>>>> in the way you posit.
>>>>>
>>>>>
>>>>> Ok, let me describe a few workflows I’ve used in the last
year that are (in
>>>>> my mind) adversely impacted by a mono-repo.
>>>>>
>>>>> Case Study 1 - Simple development on a sub-project
>>>>>
>>>>> I build LLVM + Clang + Compiler-RT using the just-built
Clang to build
>>>>> Compiler-RT. I iterate on some complicated Compiler-RT
changes over a period
>>>>> of a day. Once my Compiler-RT changes are done I rebase the
compiler-rt
>>>>> repo, rebuild compiler-rt then commit.
>>>>>
>>>>> With a mono-repo rebasing the checkout means rebasing the
whole tree. So,
>>>>> either I have to wrangle some crazy git or CMake foo, or
when I run “ninja
>>>>> compiler-rt” after the rebase it will rebuild LLVM and
Clang too. That kinda
>>>>> sucks.
>>>>>
>>>>> What this example illustrates to me is that today we have
loosely coupled
>>>>> projects with an occasional rev lock. Moving to a mono-repo
enforces a tight
>>>>> coupling that isn’t strictly required today.
>>>>>
>>>>> Case Study 2 - Working on a sub-project in isolation across
many platforms
>>>>>
>>>>> I did a lot of work on Compiler-RT last year that had no
direct dependency
>>>>> on any other LLVM project. During the development I was
working with a
>>>>> Compiler-RT checkout and a build directory of just
Compiler-RT. Every once
>>>>> in a while (or every other day as it were) I would make a
change that would
>>>>> break a configuration that I wasn’t directly developing on.
My workflow for
>>>>> handling those cases was:
>>>>>
>>>>> (1) Spin up a VM on a VPS that closely matched the
configuration I broke
>>>>> (2) Checkout Compiler-RT
>>>>> (3) Reproduce, debug, fix the failure
>>>>> (4) Commit the patch from the VM
>>>>>
>>>>> In a mono-repository doing this would require checking out
*all*
>>>>> sub-projects, not just Compiler-RT. I imagine this probably
isn’t a common
>>>>> workflow, but it is one I use that would be adversely
impacted by needing to
>>>>> checkout a full LLVM. Now, you might say I could check out
the sub-project
>>>>> mirror, but then I can’t commit from the VM, which kinda
sucks.
>>>>>
>>>>>
>>>>> So for the “I spin a VM and want to make a commit but don’t
want to download
>>>>> a few hundred MBs with a git clone” story, it turns out
that the github
>>>>> bridge with SVN helps to optimize with a “lean” checkout:
>>>>>
>>>>> I fork the unified repo here:
>>>>> https://github.com/joker-eph/llvm-project/commits/master
<https://github.com/joker-eph/llvm-project/commits/master> and then:  svn
co
>>>>> https://github.com/joker-eph/llvm-project/trunk/compiler-rt
<https://github.com/joker-eph/llvm-project/trunk/compiler-rt>
>>>>>
>>>>> So that’s a net “no regression” compared to the current
state :)
>>>>>
>>>>>
>>>>> Is the github SVN interface's "co" magically
as fast as a git clone?
>>>>>
>>>>>
>>>>> $ time svn co
https://github.com/joker-eph/llvm-project/trunk/compiler-rt
<https://github.com/joker-eph/llvm-project/trunk/compiler-rt>
>>>>> ….
>>>>> real 0m8.539s user 0m0.919s  sys 0m1.917s
>>>>> $ time git clone
https://github.com/joker-eph/compiler-rt.git
<https://github.com/joker-eph/compiler-rt.git>
>>>>> real 0m5.487s user 0m1.208s sys 0m0.825s
>>>>>
>>>>>
>>>>> That’s actually not terrible! Color me impressed.
>>>>>
>>>>>
>>>>>
>>>>> If not, it is a performance regression because today I use
git clone and
>>>>> git-svn on my VMs just like on my physical machines, and
either way it adds
>>>>> some crazy complexity.
>>>>>
>>>>>
>>>>> No problem, I get it, exactly same workflow as today:
>>>>>
>>>>>
>>>>> Yep. Which isn’t bad. I do however have two concerns.
>>>>>
>>>>> (1) What happens if we move to pull request-based
workflows? Do we still
>>>>> support this workflow?
>>>>> (2) If I’m stuck using git-svn I kinda feel like there is
no real point in
>>>>> changing anything. I dislike this workflow less than the
earlier proposals,
>>>>> but I see no reason to move to this instead of staying on
SVN (other than
>>>>> the hosting issues which could be solved in other ways).
>>>>>
>>>>> -Chris
>>>>>
>>>>>
>>>>> # Clone from the single read-only git repo
>>>>> $ git clone https://github.com/joker-eph/compiler-rt.git
<https://github.com/joker-eph/compiler-rt.git>
>>>>> …
>>>>> # Configure the SVN remote and initialize the svn metadata
>>>>> $ cd compiler-rt
>>>>> $ git svn init
https://github.com/joker-eph/llvm-project/trunk/compiler-rt
<https://github.com/joker-eph/llvm-project/trunk/compiler-rt>
>>>>> —username>>>>> $ git config
svn-remote.svn.fetch :refs/remotes/origin/master
>>>>> $ git svn rebase -l
>>>>> ...
>>>>> # Remove and empty file and commit with git
>>>>> $ git rm empty
>>>>> $ git commit -m "remove empty file"
>>>>> # commit/push with svn to the unified git repo
>>>>> $ git svn dcommit
>>>>> Committing to
https://github.com/joker-eph/llvm-project/trunk/compiler-rt
<https://github.com/joker-eph/llvm-project/trunk/compiler-rt>
>>>>> ...
>>>>> D empty
>>>>> Committed r354148
>>>>>
>>>>>
>>>>> Here is the commit:
>>>>>
https://github.com/joker-eph/llvm-project/commit/5f7e977c8cf3c33153d91be9b556143b49911ebe
<https://github.com/joker-eph/llvm-project/commit/5f7e977c8cf3c33153d91be9b556143b49911ebe>
>>>>>
>>>>>
>>>>> —
>>>>> Mehdi
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> While admittedly you do get a linear history with using the
mono-repository,
>>>>> that isn’t the only way to solve the problem, and I don’t
really think that
>>>>> the benefit (not needing to write some tooling) justifies
the increased
>>>>> burden applied to contributors that don’t use the full LLVM
family of
>>>>> projects.
>>>>>
>>>>>
>>>>> I think the trade-off you're considering here (cost to
developers who
>>>>> use llvm plus a version-locked subrepo vs. cost to
developers who
>>>>> don't want an llvm clone) is the right one.
>>>>>
>>>>>
>>>>> I actually think there are *a lot* more considerations we
need to be making
>>>>> for an infrastructure change like this. While it is true
that our SCM
>>>>> hosting strategy primarily impacts developers, it also
impacts our users. We
>>>>> should be conscious of the impact to downstream users in
making
>>>>> infrastructure changes like this. That is part of why the
idea of a survey
>>>>> holds appeal to me; it would give us the opportunity to get
feedback from a
>>>>> much wider audience than the current “people on llvm-dev
who haven’t been
>>>>> scared away”.
>>>>>
>>>>> But as someone who has
>>>>> extensively used git submodules and repo (a wrapper
script), I
>>>>> strongly disagree with the judgement that a monorepo would
not be a
>>>>> significant improvement.
>>>>>
>>>>> Our primary disagreement, I think, is over how much cost
there is to
>>>>> "writing some tooling".  To me, this is a
significant barrier standing
>>>>> in the way of developer productivity.  Here at Google I did
a quick
>>>>> survey, and more than half of us don't have scripts of
the sort that
>>>>> Justin Bogner described.  We are all just floundering
around rebasing
>>>>> clang and llvm until it compiles.  It *sucks*.
>>>>>
>>>>>
>>>>> I actually think we’re both talking about solutions that
require tooling,
>>>>> and while we *could* be disagreeing over how much effort
each tooling
>>>>> initiative would require (I think they’re pretty close, so
I don’t care to
>>>>> have that argument), my actual disagreement with your
proposal is that it is
>>>>> a change that impacts developers and users universally and
I don’t think
>>>>> that it is justified. Simply put, I don’t feel that the
benefits are
>>>>> substantial enough to warrant the kind of disruptive change
you’re
>>>>> proposing.
>>>>>
>>>>>
>>>>> I suggest that saying that all of these developers are
"doing it
>>>>> wrong" is not helpful.
>>>>>
>>>>>
>>>>> Maybe I’m missing something, but I don’t think I said
anyone was “doing it
>>>>> wrong”. Bisecting across multiple git repositories isn’t a
great experience.
>>>>> But neither is bisecting across a half dozen separate
folders in an SVN
>>>>> repository. Both the submodule solution and the mono-repo
solution solve
>>>>> this problem equivalently well.
>>>>>
>>>>> Not everyone has the git and python/bash chops
>>>>> to write the necessary scripts.  Not everyone has the
personality to
>>>>> obsessively script around stuff, or the desire to maintain
said
>>>>> scripts.  Not everyone works on llvm/clang so much that
it's worth
>>>>> adopting a special-snowflake workflow.  And some of us --
myself
>>>>> included -- have extensive git scripts which work with the
standard
>>>>> git workflow but would be completely broken by adding a
custom level
>>>>> of indirection around git.
>>>>>
>>>>> When put this way, maybe it's clear that it's
actually a niche set of
>>>>> people for whom "script around the brokenness" is
a good solution.
>>>>>
>>>>>
>>>>> I’m not sure what “brokenness” you’re referring to. We have
a collection of
>>>>> loosely connected projects by design. As a result of that
intentional design
>>>>> certain workflows will be impacted. I don’t think that is
brokenness. I
>>>>> think our loose coupling is a feature even if it makes some
workflows
>>>>> harder.
>>>>>
>>>>> -Chris
>>>>>
>>>>>
>>>>> As I've said a bunch of times above, we have to weigh a
cost paid by
>>>>> all of us every time we type a command that starts with
"git" --
>>>>> something we do tens or hundreds of times a day -- versus
the one-time
>>>>> cost of asking people to download 1gb of data.
>>>>>
>>>>> On Wed, Jul 27, 2016 at 9:47 AM, Chris Bieneman via
llvm-dev
>>>>> <llvm-dev at lists.llvm.org> wrote:
>>>>>
>>>>> I’m just now catching up on this massive thread after being
on vacation last
>>>>> week, and I have a few thoughts I’d like to share.
>>>>>
>>>>> First and foremost please don’t consider lack of dissent on
the thread as
>>>>> presence of consensus. The various git-related threads on
LLVM-dev lately
>>>>> have been so active and contentious that I think a lot of
people are zoning
>>>>> out on the conversations. As supporting evidence of this, I
was discussing
>>>>> this thread yesterday around the office yesterday and had
quite a few people
>>>>> responding something along the lines of “they’re proposing
what?”.
>>>>>
>>>>> I think it would be great for us to have several different
proposals for how
>>>>> the git-transition could work, and have a survey to get
people’s opinions. I
>>>>> know this has been discussed repeatedly, and I want to put
in my vote in
>>>>> favor of having a survey that takes into account multiple
different
>>>>> approaches.
>>>>>
>>>>> WRT the actual proposal in this thread, I’m strongly
opposed to a
>>>>> mono-repository. While I understand the argument that the
full clone’s cost
>>>>> on disk space is minimal compared to an LLVM object
directory, what about
>>>>> for contributors that contribute to the smaller runtimes
projects but *not*
>>>>> to LLVM or Clang. A contributor that only contributes to
libcxx or
>>>>> compiler-rt being forced to do a full clone of all the LLVM
projects in
>>>>> order to push a patch kinda sucks.
>>>>>
>>>>> I want to point out a few workflows people may not be
considering.
>>>>>
>>>>> Clang can be built against an installed LLVM. I know this
workflow is used
>>>>> by some people because I’ve broken it in the past and had
to fix it. With a
>>>>> mono-repo this workflow gets a bit more complicated because
you’d need to do
>>>>> sparse checkouts, and it probably means we should just nuke
the workflow
>>>>> entirely because there is no real value added by having it.
>>>>>
>>>>> Compiler-RT’s sanitizers are used with GCC; no LLVM
required. While for the
>>>>> common use case maintaining sparse repository mirrors would
limit impact of
>>>>> this on users, should any GCC user want to contribute to
Compiler-RT, you’re
>>>>> forcing them to clone a much larger repository than
necessary.
>>>>>
>>>>> The same problem with Compiler-RT’s sanitizers also applies
to libcxx,
>>>>> libcxxabi, libunwind, and potentially any other runtime
library projects
>>>>> that we may create in the future.
>>>>>
>>>>> Beyond all that I want to point out that the git
multi-repository story is
>>>>> basically the same thing we have today with SVN except for
the absence of a
>>>>> monotonically increasing number that corresponds across
repositories. While
>>>>> admittedly you do get a linear history with using the
mono-repository, that
>>>>> isn’t the only way to solve the problem, and I don’t really
think that the
>>>>> benefit (not needing to write some tooling) justifies the
increased burden
>>>>> applied to contributors that don’t use the full LLVM family
of projects.
>>>>>
>>>>> I think we have some pretty strong evidence in the form of
the github fork
>>>>> counts (https://github.com/llvm-mirror/) that most people
aren’t using all
>>>>> of the LLVM projects. In fact, by that evidence Clang (the
second most
>>>>> popular project) is forked less than 2/3 as many times as
LLVM.
>>>>>
>>>>> -Chris
>>>>>
>>>>>
>>>>> On Jul 26, 2016, at 11:31 AM, Renato Golin via llvm-dev
>>>>> <llvm-dev at lists.llvm.org> wrote:
>>>>>
>>>>> On 26 July 2016 at 19:28, Sanjoy Das via llvm-dev
>>>>> <llvm-dev at lists.llvm.org> wrote:
>>>>>
>>>>> Even if it were possible, I would still keep my upstream
checkout
>>>>> separate just as a safety measure, to keep from sending
private stuff
>>>>> upstream by accident.
>>>>>
>>>>>
>>>>> Just FYI, this is our (Azul's) workflow as well, and
for similar
>>>>> reasons.
>>>>>
>>>>>
>>>>> Same here.
>>>>>
>>>>> cheers,
>>>>> --renato
>>>>> _______________________________________________
>>>>> LLVM Developers mailing list
>>>>> llvm-dev at lists.llvm.org
>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> LLVM Developers mailing list
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>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>
>>>>>
>>>>> _______________________________________________
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>>>>>
>>>>>
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