Re: [PATCH v4 04/10] commit-graph: persist existence of changed-paths

From: Taylor Blau <me@ttaylorr.com>
Date: Oct 16, 2020, 03:18 UTC
Message-ID: <20201016031844.GA2181783@nand.local>
In-Reply-To: <103404f0-a164-2550-4730-6b8773d3ca1f@gmail.com>

On Thu, Oct 15, 2020 at 10:18:47PM -0400, Derrick Stolee wrote:

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> On 10/15/2020 5:41 PM, Taylor Blau wrote:
> > So, we need to be more aggressively checking the Bloom filter settings
> > in any layer we want to reuse a Bloom filter out of before reusing it
> > verbatim in the current layer. The patch below the scissors line is
> > sufficient to do that, and it causes the third test to start passing.
>
> I think there are three things we should keep in mind:
>
> 1. Incompatible Bloom filter settings between layers should be seen
>    as _inconsistent data_ as Git should not be writing incremental
>    commit-graph files with inconsistent Bloom filter settings. Thus,
>    when reading the commit-graph chain we should prevent incompatible
>    filters from being used. One way to do this is to notice different
>    settings and completely disable Bloom filters. The other way would
>    be to take the settings from the first layer with filters and then
>    clear the chunk_bloom_indexes and chunk_bloom_data fields for the
>    layers that don't agree. This fits with an expectation that lower
>    layers are larger, so more filters can be used in that situation.

Sure; I'd be fine with only allowing filters computed with the settings present in the lowest or largest layer in the event that multiple layers exist with incompatible settings.

I'm trying to point us towards a direction of not optimizing too far along a direction that we're unlikely to take, while also trying to do something relatively non-invasive to make it possible for a version of Git to change the default Bloom settings. That is, if a user is writing split commit-graphs, and we change the default Bloom settings, they shouldn't have to recompute or merge down all of their Bloom filters.

If that's something that we never think is going to happen, I'm fine with not thinking too hard about it. But, I also don't want to paint ourselves into a corner, so I think something like the patch I wrote in the email that you're replying to actually may be worth pursuing further. I dunno. Definitely after 2.29, though.

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> 2. We should be sure that Git will not agree to write incompatible
>    settings between layers of a commit-graph chain. Even more, it
>    should definitely not re-use filters when merging layers with
>    incompatible filter values. The strategy above to ignore Bloom
>    filters in incompatible upper layers is enough of a guard against
>    the "merge layers" situation.

Yeah, this would be fine with me.

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> 3. Allowing users (or future Git developers) to adjust the default
>    Bloom filter settings is one that is good to do for future-proofing,
>    but not one that I expect to be widely used (any gains here are
>    minuscule compared to the results already achieved with the defaults).
>    On top of that, including incompatible settings across layers is even
>    less likely to be a real use case. We should not be straining to make
>    the code even worse for this imaginary scenario.

Right, I think we're pretty much in agreement here. Doing something easy to make sure that we don't run into a wall seems to make sense, but I think modifying the revision walk machinery to keep track of multiple Bloom keys computed with different settings corresponding to the set of Bloom filter settings in commit-graph layers is probably too far in that direction.

For what it's worth, I was mainly talking about it to say that it would be more effort than it's probably worth to do. There's also nothing that we're currently discussing that would prevent us from taking that same direction up in six months from now.

Thanks, Taylor