From: Patrick Steinhardt <ps@pks.im>
To: Taylor Blau <me@ttaylorr.com>
Cc: Derrick Stolee <stolee@gmail.com>, git@vger.kernel.org
Subject: Re: [PATCH 1/2] fetch-pack: use commit-graph when computing cutoff
Date: Wed, 2 Feb 2022 13:46:34 +0100 [thread overview]
Message-ID: <Yfp9KsvvQa5yWuLE@ncase> (raw)
In-Reply-To: <YfhoWk1LFnzbFaQf@nand.local>
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On Mon, Jan 31, 2022 at 05:53:14PM -0500, Taylor Blau wrote:
> (+cc Stolee, in case anything I'm saying here is wrong)
>
> On Fri, Jan 28, 2022 at 11:17:03AM +0100, Patrick Steinhardt wrote:
> > One thing to keep in mind though is that the commit-graph corrects
> > committer dates:
> >
> > * A commit with at least one parent has corrected committer date
> > equal to the maximum of its commiter date and one more than the
> > largest corrected committer date among its parents.
>
> This snippet refers to how correct committer dates are computed, not how
> the commit dates themselves are stored.
>
> Indeed, the corrected committer date is used to compute the corrected
> commit date offset, which is the "v2" generation number scheme (as
> opposed to topological levels, which make up "v1").
>
> But that is entirely separate from the committer dates stored by the
> commit-graph file, which are faithful representations of the exact
> committer date attached to each commit.
>
> Looking at the very last few lines of the main loop in
> write_graph_chunk_data() (where the committer dates are stored):
>
> if (sizeof((*list)->date) > 4)
> packedDate[0] = htonl(((*list)->date >> 32) & 0x3);
> else
> packedDate[0] = 0;
>
> packedDate[0] |= htonl(*topo_level_slab_at(ctx->topo_levels, *list) << 2);
> packedDate[1] = htonl((*list)->date);
> hashwrite(f, packedDate, 8);
>
> the low-order 34 bits are used to store the commit's `->date` field, and
> the remaining high-order 30 bits are used to store the generation
> number. (You can look in `fill_commit_graph_info()` to see that we only
> use those 34 bits to write back the date field).
>
> So I think this paragraph (and the ones related to it) about this being
> an approximation and that being OK since this is a heuristic can all go
> away.
>
> Thanks,
> Taylor
Aha, that makes a lot of sense. Thanks a lot for correcting my
misconception! I'll send a v2 of this patch series with the corrected
commit message.
Patrick
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next prev parent reply other threads:[~2022-02-02 12:46 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-01-28 10:15 [PATCH 0/2] fetch: speed up mirror-fetches with many refs Patrick Steinhardt
2022-01-28 10:17 ` [PATCH 1/2] fetch-pack: use commit-graph when computing cutoff Patrick Steinhardt
2022-01-31 22:53 ` Taylor Blau
2022-02-02 12:46 ` Patrick Steinhardt [this message]
2022-01-28 10:19 ` [PATCH 2/2] fetch: skip computing output width when not printing anything Patrick Steinhardt
2022-02-02 12:51 ` [PATCH v2 0/2] fetch: speed up mirror-fetches with many refs Patrick Steinhardt
2022-02-02 12:51 ` [PATCH v2 1/2] fetch-pack: use commit-graph when computing cutoff Patrick Steinhardt
2022-02-09 18:01 ` Christian Couder
2022-02-10 11:43 ` Patrick Steinhardt
2022-02-02 12:51 ` [PATCH v2 2/2] fetch: skip computing output width when not printing anything Patrick Steinhardt
2022-02-09 18:10 ` Christian Couder
2022-02-10 12:23 ` Patrick Steinhardt
2022-02-10 12:28 ` [PATCH v3 0/2] fetch: speed up mirror-fetches with many refs Patrick Steinhardt
2022-02-10 12:28 ` [PATCH v3 1/2] fetch-pack: use commit-graph when computing cutoff Patrick Steinhardt
2022-02-10 12:28 ` [PATCH v3 2/2] fetch: skip computing output width when not printing anything Patrick Steinhardt
2022-02-10 18:04 ` [PATCH v3 0/2] fetch: speed up mirror-fetches with many refs Junio C Hamano
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