mirrors/git
1
0
Fork 0
mirror of https://github.com/git/git.git synced 2024-10-31 06:17:56 +01:00
Code Activity

commit-graph.txt: update design document

Browse source 
We now calculate generation numbers in the commit-graph file and use
them in paint_down_to_common().

Expand the section on generation numbers to discuss how the three
special generation numbers GENERATION_NUMBER_INFINITY, _ZERO, and
_MAX interact with other generation numbers.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Derrick Stolee 2018-05-01 12:47:25 +00:00 committed by Junio C Hamano
parent 7adf526670
commit 1472978ec6
1 changed files with 24 additions and 5 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

29
Documentation/technical/commit-graph.txt
View file

@ -77,6 +77,29 @@ in the commit graph. We can treat these commits as having "infinite"
generation number and walk until reaching commits with known generation
number.
We use the macro GENERATION_NUMBER_INFINITY = 0xFFFFFFFF to mark commits not
in the commit-graph file. If a commit-graph file was written by a version
of Git that did not compute generation numbers, then those commits will
have generation number represented by the macro GENERATION_NUMBER_ZERO = 0.
Since the commit-graph file is closed under reachability, we can guarantee
the following weaker condition on all commits:
If A and B are commits with generation numbers N amd M, respectively,
and N < M, then A cannot reach B.
Note how the strict inequality differs from the inequality when we have
fully-computed generation numbers. Using strict inequality may result in
walking a few extra commits, but the simplicity in dealing with commits
with generation number *_INFINITY or *_ZERO is valuable.
We use the macro GENERATION_NUMBER_MAX = 0x3FFFFFFF to for commits whose
generation numbers are computed to be at least this value. We limit at
this value since it is the largest value that can be stored in the
commit-graph file using the 30 bits available to generation numbers. This
presents another case where a commit can have generation number equal to
that of a parent.
Design Details
--------------
@ -98,18 +121,14 @@ Future Work
- The 'commit-graph' subcommand does not have a "verify" mode that is
necessary for integration with fsck.
- The file format includes room for precomputed generation numbers. These
are not currently computed, so all generation numbers will be marked as
0 (or "uncomputed"). A later patch will include this calculation.
- 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'
- 'tag --merged'
- Currently, parse_commit_gently() requires filling in the root tree
object for a commit. This passes through lookup_tree() and consequently