In a previous commit, we introduced a sub-shell in the implementation of
`graph_git_behavior()`, in order to allow us to pass `-C "$DIR"`
directly to the git processes spawned by `graph_git_two_modes()`.
Now that its callers are always operating from the "$TRASH_DIRECTORY"
instead of one of its sub-directories, we can drop the inner sub-shell,
as it is no longer required.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The `graph_git_behavior()` helper asserts that a number of common Git
operations (such as `git log --oneline`, `git log --topo-order`, etc.)
produce identical output regardless of whether or not a commit-graph is
in use.
This helper takes as its second argument the location (relative to the
`$TRASH_DIRECTORY`) of the Git repostiory under test. In order to run
each of its commands within that repository, it first changes into that
directory, without the use of a sub-shell.
This pollutes future tests which expect to be run in the top-level
`$TRASH_DIRECTORY` as usual. We could wrap `graph_git_behavior()` in a
sub-shell, like:
graph_git_behavior() {
# ...
(
cd "$TRASH_DIRECTORY/$DIR" &&
graph_git_two_modesl
)
}
, but since we're invoking git directly, we can pass along a "-C $DIR"
when "$DIR" is non-empty.
Note, however, that until the remaining callers are cleaned up to avoid
changing working directories outside of a sub-shell, that we need to
ensure that we are operating in the top-level $TRASH_DIRECTORY. The
inner-subshell will go away in a future commit once it is no longer
necessary.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The `graph_read_expect()` function is used to ensure that the output of
the "read-graph" test helper matches certain parameters (e.g., how many
commits are in the graph, which chunks were written, etc.).
It expects the Git repository being tested to be at the current working
directory. However, a handful of t5318 tests use different repositories
stored in sub-directories. To work around this, several tests in t5318
change into the relevant repository outside of a sub-shell, altering the
context for the rest of the suite.
Prepare to remove these globally-scoped directory changes by teaching
`graph_read_expect()` to take an optional "-C dir" to specify where the
repository containing the commit-graph being tested is.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The 'read_generation_data' member of 'struct commit_graph' was
introduced by 1fdc383c5 (commit-graph: use generation v2 only if entire
chain does, 2021-01-16). The intention was to avoid using corrected
commit dates if not all layers of a commit-graph had that data stored.
The logic in validate_mixed_generation_chain() at that point incorrectly
initialized read_generation_data to 1 if and only if the tip
commit-graph contained the Corrected Commit Date chunk.
This was "fixed" in 448a39e65 (commit-graph: validate layers for
generation data, 2021-02-02) to validate that read_generation_data was
either non-zero for all layers, or it would set read_generation_data to
zero for all layers.
The problem here is that read_generation_data is not initialized to be
non-zero anywhere!
This change initializes read_generation_data immediately after the chunk
is parsed, so each layer will have its value present as soon as
possible.
The read_generation_data member is used in fill_commit_graph_info() to
determine if we should use the corrected commit date or the topological
levels stored in the Commit Data chunk. Due to this bug, all previous
versions of Git were defaulting to topological levels in all cases!
This can be measured with some performance tests. Using the Linux kernel
as a testbed, I generated a complete commit-graph containing corrected
commit dates and tested the 'new' version against the previous, 'old'
version.
First, rev-list with --topo-order demonstrates a 26% improvement using
corrected commit dates:
hyperfine \
-n "old" "$OLD_GIT rev-list --topo-order -1000 v3.6" \
-n "new" "$NEW_GIT rev-list --topo-order -1000 v3.6" \
--warmup=10
Benchmark 1: old
Time (mean ± σ): 57.1 ms ± 3.1 ms
Range (min … max): 52.9 ms … 62.0 ms 55 runs
Benchmark 2: new
Time (mean ± σ): 45.5 ms ± 3.3 ms
Range (min … max): 39.9 ms … 51.7 ms 59 runs
Summary
'new' ran
1.26 ± 0.11 times faster than 'old'
These performance improvements are due to the algorithmic improvements
given by walking fewer commits due to the higher cutoffs from corrected
commit dates.
However, this comes at a cost. The additional I/O cost of parsing the
corrected commit dates is visible in case of merge-base commands that do
not reduce the overall number of walked commits.
hyperfine \
-n "old" "$OLD_GIT merge-base v4.8 v4.9" \
-n "new" "$NEW_GIT merge-base v4.8 v4.9" \
--warmup=10
Benchmark 1: old
Time (mean ± σ): 110.4 ms ± 6.4 ms
Range (min … max): 96.0 ms … 118.3 ms 25 runs
Benchmark 2: new
Time (mean ± σ): 150.7 ms ± 1.1 ms
Range (min … max): 149.3 ms … 153.4 ms 19 runs
Summary
'old' ran
1.36 ± 0.08 times faster than 'new'
Performance issues like this are what motivated 702110aac (commit-graph:
use config to specify generation type, 2021-02-25).
In the future, we could fix this performance problem by inserting the
corrected commit date offsets into the Commit Date chunk instead of
having that data in an extra chunk.
Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The graph_git_behavior helper is useful for testing that certain Git
commands behave the same when using the commit-graph and when not using
the commit-graph. Extract it to a new lib-commit-graph.sh file for use
in new test scripts that will split out from t5318.
While doing this extraction, also extract graph_read_expect and the
logic for priming the test_oid_cache.
Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
