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

[David Chisnall via llvm-dev]

On 27 Jul 2016, at 19:03, Bruce Hoult via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> What do you believe is this increased burden?
> 
> The entire commit history of all llvm projects in a mono-repository is a 449 MB .git directory. It can be downloaded in about two minutes on a typical domestic internet connection (50 Mbps).
> 
> If you download only a snapshot of the current HEAD commit then the .git repository is 88 MB and takes under a minute. Any other individual commit should be similar.
> 
> This doesn't seem like a big burden to me.
> 
> The checked out llvm source directory -- which you say is all that many people want -- is 202 MB. That's without even building it.
> 
> Why is this burden unacceptable? It seems rather small to me.
> 
> For comparison, using svn to checkout llvm using ...
> 
> svn co http://llvm.org/svn/llvm-project/llvm/trunk llvm
>  
> ... took me 1 minute 28 seconds and gives a 222 MB .svn directory, 428 MB total (so 206 MB for the source files checked out).

The burden, as he pointed out, is for people who contribute to llvm-umbrella projects that are not llvm.

I think that this is a very good reason for not including libc++ in the monorepo.  Currently, it takes me about 15 seconds to do an checkout of libc++ and the result is about 100MB.  A clone from the git mirror takes just under 6 seconds, downloads 17.15MB, and produces a 72MB directory.  The build directory is about 7MB.

There is no dependency from libc++ to anything else in LLVM.  In FreeBSD, we use libc++ with both clang and gcc and people hacking on libc++ do not typically need to build a new toolchain to do so.  Pulling in all of LLVM, clang, lldb, lld, and so on just to hack on libc++ seems a bit excessive.

libunwind is similar.  It exposes a public interface that’s ABI compatible with two other libraries and is tiny.  We currently have people working on it in the process of replacing the libunwind in FreeBSD with the LLVM version.  Some of them also hack on other bits of LLVM, but getting the entire LLVM repo just to hack on a library that’s 1.6MB of source (1.5 seconds to do an svn checkout currently) seems a bit excessive.

I don’t think it’s a problem for the larger projects (clang, lldb, lld) to be in a combined repo, because the size of any of them is fairly small in comparison to the size of LLVM and tiny in comparison to the build directory.

David