On 22.06.2020 at 11:34, Abhishek Kumar wrote:
One of the remaining pre-requisites for implementing generation number
v2 was distinguishing between corrected commit dates with monotonically
increasing offsets and topological level without incrementing generation
number version.
Two approaches were proposed [1]:
1. New chunk for commit data (generation data chunk, "GDAT")
2. Metadata/versioning chunk
Actually in [1] there was also proposed another distinct approach,
namely to 'rename' the "CDAT" chunk to something else, like "CDA2"
(or proposed here "GDAT").
If I read the code correctly, with old Git if one of required chunks
is missing then Git would continue work as if commit-graph was not
present -- as opposed to current handling of unknown commit-graph
file format version number, where Git would stop working with an
error message.
Since both approaches have their advantages and disadvantages, I wrote
up a prototype [2] to investigate their performance.
[1]: https://lore.kernel.org/git/86mu87qj92.fsf@xxxxxxxxx/
[2]: https://github.com/abhishekkumar2718/git/pull/1
That's very nice. Thanks for investigating that.
TL;DR: I recommend we should use generation data chunk approach.
Generation Data Chunk
=====================
We could move the generation number v2 into a separate chunk, storing
topological levels in CDAT and the corrected commit date into a new,
"GDAT" chunk. Thus, old Git would use generation number v1, and
new Git would use corrected commit dates from GDAT.
Using generation data chunk has the advantage that we would no longer
be restricted to using 30 bits for generation number. It also works
well for commit-graph chains with a mix of v1 and v2 generation numbers.
However, it increases the time required for I/O as commit data and
generation numbers are no longer contiguous.
Note: While it also increases disk space required for storing
commit-graph files by 8 bytes per commit, I don't consider it relevant,
especially on modern systems. A repo of the size of Linux repo would be
larger by a mere 7.2 Mb.
All right.
Another advantage: we don't have to store the corrected committer date
_offset_, we can store the date (as epoch) directly. This saves some
calculation, though a minuscule amount.
Yet another advantage: we don't need backward-compatibility for
generation number v2, i.e. corrected commit date.
Another disadvantage: we need to compute both topological levels and
corrected commit date when creating a commit-graph file or a chunk of
it. This means that `git commit-graph write` could be slightly more
expensive.
Metadata / Versioning Chunk
===========================
We could also introduce an optional metadata chunk to store generation
number version and store corrected date offsets in CDAT. Since the
offsets are backward compatible, Old Git would still yield correct
results by assuming the offsets to be topological levels. New Git would
correctly use the offsets to create corrected commit dates.
This also means that we need to use backward-compatible generation
number v2, that is corrected commit date with strictly monotonic
offsets. Which may lead to more cases where 30 bits for label is not
enough, and thus worse performance (no detailed reachability
information for newest commits).
It works just as well as generation number v1 in parsing and writing
commit-graph files.
However, the generation numbers are still restricted to 30 bits in CDAT
chunk and it does not work well with commit-graph chains with a mix of
v1 and v2 generation numbers.
CDAT chunk replaced with another chunk
======================================
In this approach the "CDAT" chunk is missing from the commit-graph file.
We can craft the replacement ("CDA2") however we like, but it can also
look like the "CDAT" chunk, only with the offsets for corrected commit
date rather than topological level for generation number part (30 bits).
If we choose to follow the "CDAT" format (modified), then the file
size would not change, and neither would change the amount of I/O
needed -- there would be the same access structure as in current
version.
There should be no confusion with a mix of v1 and v2 generation numbers.
The disadvantage is of course that older version of Git would no longer
be able to make use of serialized commit-graph if the file was written
by newer version of Git.
Performance
===========
What is the repository where those benchmarks took place?
| Command | Master | Metadata | Generation Data |
|--------------------------------|--------|----------|-----------------|
| git commit-graph write | 14.45s | 14.28s | 14.63s |
| git log --topo-order -10000 | 0.211s | 0.206s | 0.208s |
| git log --topo-order -100 A..B | 0.019s | 0.015s | 0.015s |
| git merge-base A..B | 0.137s | 0.137s | 0.137s |
Nice.
How those two (or three) approaches work on gen-test [3] test cases,
both in terms of commits walked (extracted via trace2 mechanism) and
actual wall-time clock, like in the result above?
[3]: https://github.com/derrickstolee/gen-test
- Metadata and generation data chunks perform better than master on
using commit-graph files since they use corrected commit dates.
- The increased I/O time for parsing GDAT does not affect performance as
much as expected.
- Generation data commit-graph takes longer to write since more
information is written into the file.
As using the commit-graph is much more frequent than writing, we can
consider both approaches to perform equally well.
I prefer generation data chunk approach as it also removes 30-bit length
restriction on generation numbers.
Thank you for your work.
Best,
P.S. I hope I haven't sent it twice...
--
Jakub Narębski
