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Re: [RFC PATCH] We should add a "git gc --auto" after "git clone" due to commit graph

On 10/5/2018 9:05 AM, Ævar Arnfjörð Bjarmason wrote:
On Fri, Oct 05 2018, Derrick Stolee wrote:

On 10/4/2018 5:42 PM, Ævar Arnfjörð Bjarmason wrote:
I don't have time to polish this up for submission now, but here's a WIP
patch that implements this, highlights:

   * There's a gc.clone.autoDetach=false default setting which overrides
     gc.autoDetach if 'git gc --auto' is run via git-clone (we just pass a
     --cloning option to indicate this).
I'll repeat that it could make sense to do the same thing on clone
_and_ fetch. Perhaps a "--post-fetch" flag would be good here to
communicate that we just downloaded a pack from a remote.
I don't think that makes sense, but let's talk about why, because maybe
I've missed something, you're certainly more familiar with the
commit-graph than I am.

The reason to do it on clone as a special-case or when the file is
missing, is because we know the file is desired (via the GC config), and
presumably is expected to help performance, and we have 0% of it. So by
going from 0% to 100% on clone we'll get fast --contains and other
goodies the graph helps with.

But when we're doing a fetch, or really anything else that runs "git gc
--auto" we can safely assume that we have a recent enough graph, because
it will have been run whenever auto-gc kicked in.


     # Slow, if we assume background forked commit-graph generation
     # (which I'm avoiding)
     git clone x && cd x && git tag --contains
     # Fast enough, since we have an existing commit-graph
     cd x && git fetch && git tag --contains

I *do* think it might make sense to in general split off parts of "gc
--auto" that we'd like to be more aggressive about, simply because the
ratio of how long it takes to do, and how much it helps with performance
makes more sense than a full repack, which is what the current heuristic
is based on.

And maybe when we run in that mode we should run in the foreground, but
I don't see why git-fetch should be a special case there, and in this
regard, the gc.clone.autoDetach=false setting I've made doesn't make
much sence. I.e. maybe we should also skip forking to the background in
such a mode when we trigger such a "mini gc" via git-commit or whatever.

My misunderstanding was that your proposed change to gc computes the commit-graph in either of these two cases:

(1) The auto-GC threshold is met.

(2) There is no commit-graph file.

And what I hope to have instead of (2) is (3):

(3) The commit-graph file is "sufficiently behind" the tip refs.

This condition is intentionally vague at the moment. It could be that we hint that (3) holds by saying "--post-fetch" (i.e. "We just downloaded a pack, and it probably contains a lot of new commits") or we could create some more complicated condition based on counting reachable commits with infinite generation number (the number of commits not in the commit-graph file).

I like that you are moving forward to make the commit-graph be written more frequently, but I'm trying to push us in a direction of writing it even more often than your proposed strategy. We should avoid creating too many orthogonal conditions that trigger the commit-graph write, which is why I'm pushing on your design here.

Anyone else have thoughts on this direction?