Re: [RFC][GSoC] Implement Generation Number v2

Derrick Stolee [off-list ref] writes:

On 3/23/2020 9:43 AM, Jakub Narebski wrote:

quoted

Junio C Hamano [off-list ref] writes:

quoted

Abhishek Kumar [off-list ref] writes:

quoted

Jakub Narębski [off-list ref] writes:

[...]
Proposed solutions are:
 - metadata / versioning chunk,
 - flag file: `.git/info/generation-number-v2`,
 - new chunk for commit data: "CDA2".

I would like to propose yet another solution: putting generation number
v2 data in a separate chunk (and possibly keeping generation number v1
in CDAT commit data chunk).  In this case we could even use ordinary
corrected commit date as generation number v2 (storing offsets as 32-bit
unsigned values), instead of backward-compatibile corrected commit date
with monotonic offsets.

I agree that if we are creating a new (optional) chunk, then that gets
around our versioning issues and could store just the offsets to get
the "corrected commit date" option instead of the backwards-compatible one.
By including yet another version number at the beginning of that chunk,
we could present a way to update this "second reachability index chunk"
with things like your interval mechanism with very little cost.

All this may be just a transitory phase, waiting until Git versions that
fail hard on commit-graph format version change die out... then we will
be able to use version number as intended (though it has the
disadvantage of turning off commit-graph wholly for older Git versions).

quoted

Each solution has its advantages and disadvantages.


With the flag file, the problem is (as Junio noticed) that if file gets
accidentally deleted, new Git would think incorrectly that commit-graph
uses generation number v1... which while suboptimal should not be bad
thanks to backward compatibility.  But I think the flag file should have
some kind of checksum as its contents (perhaps simply a copy of
commit-graph file checksum, or one checksum per file in chain with
incremental commit-graph), so that it old Git rewrites commit-graph file
leaving flag file present, new Git would notice this.

I'm not a fan of the flag file idea. Optional chunks are a good way forward.
That _could_ mean the metadata chunk, whose length can grow in the future
if/when we add more fixed-width metadata values.

So it looks like metadata / versioning chunk would be the best solution,
isn't it?

quoted

Metadata or versioning chunk cannot be deleted by mistake; if old Git
copies unknown chunks to new updated commit-graph file instead of
skipping them we would need to add some kind of checksum (similarly to
the case for flag file).  The problem to be solved is what to do if some
files in the chain of commit-graph files have v2 (and this chunk),
and> some have v1 generation number (and do not have this chunk).

The incremental commit-graph format is newer than our previous tests
for generation number v2, which will be a big reason why that old code
cannot be immediately adapted here.

The simplest thing to do is usually right: if we try to write a
generation number version that doesn't match the current commit-graph,
then we need to flatten the entire chain into one layer and recompute
the values from scratch. While it is _technically_ possible to mix
the backwards-compatible corrected commit date with generation number
v1, it requires taking the "lowest version" when doing comparisons and
that may behave very strangely. Better to avoid that complication.

Right.

quoted

About moving commit data with generation number v2 to "CDA2" chunk: if
"CDAT" chunk is missing then (I think) old Git would simply not use
commit-graph file at all; it may crash, but I don't think so.  If "CDAT"
chunk has zero length... I don't know what would happen then, possibly
also old Git would simply not use commit-graph data at all.

CDAT is required as it contains more than just generation numbers. It
has the commit date, parent int-ids, and root tree oid. The generation
numbers _could_ be left as all zeroes, which is a special case for the
format before generation numbers were introduced, but it would be better
to have values there.

I think (but I might be wrong) that the most expensive part of
calculating generation numbers is actually walking the commits.  Because
both generation number v1 (topological level) and generation number v2
(corrected committerdate, with or without monotonic restriction on
offsets) can be computed at the same time, during the same walk,
possibly with negligible cost compared to computing single geneation
number.

But this should be perhaps benchmarked.

quoted

Putting generation number v2 into separate chunk (which might be called
"GEN2" or "OFFS"/"DOFF") has the disadvantage of increasing the on disk
size of the commit graph, and possibly also increasing memory
consumption (the latter depends on how it would be handled), but has the
advantage of being fullly backward compatibile.  Old Git would simply
use generation numbers v1 in "CDAT", new Git would use generation
numbers v2 in "OFFS" -- combining commit creation date from "CDAT" and
offset from "OFFS"), and there should be no problems with updating
commit-graph file (either rewriting, or adding new commit-graph to the
chain).

I share these concerns but also the locality of the data within the file.
As we parse commits, we need the parent and commit date information out
of the CDAT chunk anyway, so it is not difficult to grab the nearby
generation number. If we put that data further away in a separate chunk,
then it can be more expensive to flip between the CDAT chunk and the
GEN2 chunk.

Right, I forgot about this issue, that Git is lazily parsing
commit-graph data, so keeping all [possible] commit data close is a good
idea from the performance point of view.

So it looks like metadata / versioning chunk would be the best solution
to the backward-compatibility interoperability problem.

In terms of your prototyping for performance checks, it may be good to
have a number of "GEN<X>" chunks so you can compute one commit-graph
that stores all of the candidate reachability indexes, then use one
of the chunks based on a config value or environment variable. I think
that would only be appropriate for testing if you are evaluating which
to build, so if you are focusing entirely on backwards-compatible
corrected commit date, this is not worth spending time on.

Right, but it looks like there is nobody taking new labelings for GSoC
2020.

Good idea for prototyping, true.

Best,
-- 
Jakub Narębski