Re: [GSoC Patch 0/3] Move generation, graph_pos to a slab

Abhishek Kumar [off-list ref] writes:

The struct commit is used in many contexts. However, members generation
and graph_pos are only used for commit-graph related operations and
otherwise waste memory.

Very minor nitpick: this sentence would read better if the names of
`generation` and `graph_pos` fields (but especially the 'generation')
were quoted.

This wastage would have been more pronounced as transistion to
generation number v2, which uses 64-bit generation number instead of
current 32-bits.

Good.  Moving reachability index value into a commit slab was one of
prerequisites to switching to the generation number v2, see [2]

[2]: https://public-inbox.org/git/cfa2c367-5cd7-add5-0293-caa75b103f34@gmail.com/t/#u

The other prerequisite was proper handling of commit-graph format
change, either by using "metadata chunk" as more flexible replacement of
mishandled format version field in the commit-graph file header, or as
proposed in [3] (and subsequent posts), removing "CDAT" chunk and
replacing it with "CDA2" chunk.

[3]: https://public-inbox.org/git/xmqq369z7i1b.fsf@gitster.c.googlers.com/t/#u


Also, we should probably stop mishandling the format version field, that
is do not error out [4] when commit-graph version of the file does not
match version supported by git code running the command, but just simply
not use the commit-graph (like it is done for Bloom filter chunks).

[4]: https://github.com/git/git/blob/master/commit-graph.c#L253

The third patch ("commit: convert commit->graph_pos to a slab",
2020-06-04) is currently failing diff-submodule related tests (t4041,
t4059 and t4060) for gcc [1]. I am going to send a second version soon,
fixing that.

[1]: https://travis-ci.com/github/abhishekkumar2718/git/jobs/343441189

Abhishek Kumar (3):
  commit: introduce helpers for generation slab
  commit: convert commit->generation to a slab
  commit: convert commit->graph_pos to a slab

 alloc.c                             |   2 -
 blame.c                             |   2 +-
 bloom.c                             |   6 +-
 commit-graph.c                      | 116 +++++++++++++++++++++-------
 commit-graph.h                      |   8 ++
 commit-reach.c                      |  50 ++++++------
 commit.c                            |   6 +-
 commit.h                            |   6 --
 contrib/coccinelle/generation.cocci |  12 +++
 contrib/coccinelle/graph_pos.cocci  |  12 +++

It is nice to see the use of Coccinelle scripts.

 revision.c                          |  16 ++--
 11 files changed, 158 insertions(+), 78 deletions(-)
 create mode 100644 contrib/coccinelle/generation.cocci
 create mode 100644 contrib/coccinelle/graph_pos.cocci

Best,
-- 
Jakub Narębski