Re: [PATCH 3/3] commit-graph: respect 'core.useBloomFilters'

[Taylor Blau <me@ttaylorr.com>]

On Tue, Jun 30, 2020 at 03:18:34PM -0400, Jeff King wrote:
> On Tue, Jun 30, 2020 at 01:17:48PM -0400, Taylor Blau wrote:
>
> > Git uses the 'core.commitGraph' configuration value to control whether
> > or not the commit graph is used when parsing commits or performing a
> > traversal.
>
> I think this is a good thing to have, and the patch itself makes sense
> to me (this is actually my first time reviewing it, despite its intended
> use within GitHub :) ).
>
> If I may bikeshed for a moment:
>
> > Introduce 'core.useBloomFilters' to control whether or not Bloom filters
> > are read. Note that this configuration is independent from both:
> >
> >   - 'core.commitGraph', to allow flexibility in using all parts of a
> >     commit-graph _except_ for its Bloom filters.
> >
> >   - The '--changed-paths' option for 'git commit-graph write', to allow
> >     reading and writing Bloom filters to be controlled independently.
>
> Should we avoid exposing the user to the words "Bloom filter"?
>
> The command-line option for writing them was genericized to
> "changed-paths", which I think is good. The use of Bloom filters is an
> implementation detail. What the user cares about is whether we can
> optimize queries of which paths changed in a commit.
>
> When we introduced reachability bitmaps long ago, we made the mistake of
> just calling them "bitmaps". That jargon is well understood by people
> who work with that code, but it's confusing outside of that (even within
> other parts of Git) because bitmaps are just a generic data structure.
> You can have a bitmap of just about anything (and indeed we do use other
> bitmaps these days). Consistently calling them "reachability bitmaps",
> especially in the user facing bits, would have reduced confusion over
> the years.
>
> Similarly, Bloom filters are a generic structure we might use elsewhere.
> I don't really care if we use the word "Bloom" internally to refer to
> this feature, but we'll be stuck with this config option for all time. I
> think it's worth picking something more clear.

All good thoughts. I wondered about this, too, when writing the patch,
but ultimately decided to expose the name since this is the only usage
of Bloom filters within Git to date. Whether that will continue to be
true, I'm not sure, so it probably isn't a great idea to lock ourselves
into that decision within the 'core' namespace.

So, I'm certainly open to changing it, although I'm not sure that I'm as
worried about exposing the implementation detail as I am about squatting
on Bloom filters within Git in general. I don't think that this
configuration will end up getting used by folks other than server
administrators and for debugging purposes, so those populations are
already likely to be aware of changed-path Bloom filters beforehand.

But, hiding the implementation detail seems like sane advice either way.

> It might even be worth considering whether "changed paths" needs more
> context (or would if we add new features in the future). On a "git
> commit-graph write" command-line it is perfectly clear, but would
> core.commitGraphChangedPaths be worth it? It's definitely more specific,
> but it's also way more ugly. ;)

Here's a third option what about 'graph.readChangedPaths'. I think that
Stolee and I discussed a new top-level 'graph' section, since we now
have a few commit-graph-related configuration variables in 'core'.

That's a little shorter, and it adds the verb 'read', which is more
descriptive than 'use' (I touch on this in the third patch, where I say
that this configuration variable _doesn't_ affect the '--changed-path'
option when writing).

Either way, I'd love to hear your thoughts and others', too, to figure
out what we think the most agreeable configuration name is.

> > diff --git a/t/helper/test-read-graph.c b/t/helper/test-read-graph.c
> > index 6d0c962438..5f585a1725 100644
> > --- a/t/helper/test-read-graph.c
> > +++ b/t/helper/test-read-graph.c
> > @@ -12,11 +12,12 @@ int cmd__read_graph(int argc, const char **argv)
> >  	setup_git_directory();
> >  	odb = the_repository->objects->odb;
> >
> > +	prepare_repo_settings(the_repository);
> > +
> >  	graph = read_commit_graph_one(the_repository, odb);
>
> I wondered why we would need this prepare_repo_settings() now, when it
> should have been needed already to cover core.commitGraph already. I
> strongly suspect the answer is: "test-tool read-graph" never properly
> respected core.commitGraph in the first place.

Yep. Could probably be broken out into a separate patch (or mentioned as
an aside in this one), but you're right: this helper did not respect
any configuration that 'prepare_repo_settings' picks up.

> And now presumably it would. If true, I don't think any tests need
> adjusted because the only places we set it are:
>
>   - on a "git -c" command line, which wouldn't run a test-tool helper
>
>   - when we do set it, it is always to "true", which is the default
>     anyway
>
> >  	if (!graph)
> >  		return 1;
> >
> > -
> >  	printf("header: %08x %d %d %d %d\n",
> >  		ntohl(*(uint32_t*)graph->data),
> >  		*(unsigned char*)(graph->data + 4),
>
> Oh good, I happened to be looking at this code earlier today for an
> unrelated reason and was bothered by this extra newline. :)

I hoped that nobody would mine me sneaking this in ;-).

>
> -Peff
Thanks,
Taylor