Re: [PATCH v4 02/13] graph: add commit graph design document

From: Junio C Hamano <gitster@pobox.com>
Date: Feb 20, 2018, 21:42 UTC
Message-ID: <xmqq1shf2v8d.fsf@gitster-ct.c.googlers.com>
In-Reply-To: <1519066406-81663-3-git-send-email-dstolee@microsoft.com>

Derrick Stolee <stolee@gmail.com> writes:

> +2. Walking the entire graph to avoid topological order mistakes.

You have at least one more mention of "topological order mistakes" below, but we commonly refer to this issue and blame it for "clock skew". Using the word highlights that there is no "mistake" in topo order algorithm and mistakes are in the commit timestamps.

> +In practice, we expect some commits to be created recently and not stored
> +in the commit graph. We can treat these commits as having "infinite"
> +generation number and walk until reaching commits with known generation
> +number.

Hmm, "pretend infinity" is an interesting approach---I need to think about it a bit more if it is sufficient.

> +- .graph files are managed only by the 'commit-graph' builtin. These are not
> +  updated automatically during clone, fetch, repack, or creating new commits.

OK. s/builtin/subcommand/; it does not make much difference if it is a built-in or standalone command.

> +- There is no 'verify' subcommand for the 'commit-graph' builtin to verify
> +  the contents of the graph file agree with the contents in the ODB.

I am not entirely sure about the merit of going into this level of detail. Being able to use only a single file looks like a more fundamental design limitation, which deserves to be decribed in this section, and we could ship the subsystem with that limitation.

But the lack of verify that can be called from fsck is merely the matter of not the subsystem being mature enough (to be written, reviewed and tested) and not a fundamental one, and we will not be shipping the subsystem until that limitation is lifted.

So I'd guess that we prefer this bullet item to be in the commit log message, not here, that describes the current status of the development (as opposed to the state of the subsystem).

> +- Generation numbers are not computed in the current version. The file
> +  format supports storing them, along with a mechanism to upgrade from
> +  a file without generation numbers to one that uses them.

Exactly the same comment as above applies to this item.

> +- The commit graph is currently incompatible with commit grafts. This can be
> +  remedied by duplicating or refactoring the current graft logic.

Hmm. Can it be lifted without first allowing us to use more than one commit graph file (i.e. one for "traverse while honoring the grafts", the other for "traverse while ignoring the grafts")?

Show 8 quoted lines

> +- After computing and storing generation numbers, we must make graph
> +  walks aware of generation numbers to gain the performance benefits they
> +  enable. This will mostly be accomplished by swapping a commit-date-ordered
> +  priority queue with one ordered by generation number. The following
> +  operations are important candidates:
> +
> +    - paint_down_to_common()
> +    - 'log --topo-order'

Yes.

> +- The graph currently only adds commits to a previously existing graph.
> +  When writing a new graph, we could check that the ODB still contains
> +  the commits and choose to remove the commits that are deleted from the
> +  ODB. For performance reasons, this check should remain optional.

The last sentence is somehow unconvincing. It probably is not appropriate for the "Future Work" section to be making a hurried design decision before having any working verification code to run benchmark on.

Show 12 quoted lines

> +- Currently, parse_commit_gently() requires filling in the root tree
> +  object for a commit. This passes through lookup_tree() and consequently
> +  lookup_object(). Also, it calls lookup_commit() when loading the parents.
> +  These method calls check the ODB for object existence, even if the
> +  consumer does not need the content. For example, we do not need the
> +  tree contents when computing merge bases. Now that commit parsing is
> +  removed from the computation time, these lookup operations are the
> +  slowest operations keeping graph walks from being fast. Consider
> +  loading these objects without verifying their existence in the ODB and
> +  only loading them fully when consumers need them. Consider a method
> +  such as "ensure_tree_loaded(commit)" that fully loads a tree before
> +  using commit->tree.

Very good idea.

Show 5 quoted lines

> +- The current design uses the 'commit-graph' builtin to generate the graph.
> +  When this feature stabilizes enough to recommend to most users, we should
> +  add automatic graph writes to common operations that create many commits.
> +  For example, one coulde compute a graph on 'clone', 'fetch', or 'repack'
> +  commands.

s/coulde/could/.

Also do not forget "fsck" that calls "verify". That is more urgent than intergration with any other subcommand.

> +- A server could provide a commit graph file as part of the network protocol
> +  to avoid extra calculations by clients.

We need to assess the riskiness and threat models regarding this, if we really want to follow this "could" through. I would imagine that the cost for verification is comparable to the cost for regenerating, in which case it may not be worth doing this _unless_ the user opts into it saying that the other side over the wire is trusted without any reservation.