091f4cf (commit: don't use generation numbers if not needed,
2018-08-30) changed paint_down_to_common() to use commit dates instead
of generation numbers v1 (topological levels) as the performance
regressed on certain topologies. With generation number v2 (corrected
commit dates) implemented, we no longer have to rely on commit dates and
can use generation numbers.
For example, the command `git merge-base v4.8 v4.9` on the Linux
repository walks 167468 commits, taking 0.135s for committer date and
167496 commits, taking 0.157s for corrected committer date respectively.
While using corrected commit dates, Git walks nearly the same number of
commits as commit date, the process is slower as for each comparision we
have to access a commit-slab (for corrected committer date) instead of
accessing struct member (for committer date).
This change incidentally broke the fragile t6404-recursive-merge test.
t6404-recursive-merge sets up a unique repository where all commits have
the same committer date without a well-defined merge-base.
While running tests with GIT_TEST_COMMIT_GRAPH unset, we use committer
date as a heuristic in paint_down_to_common(). 6404.1 'combined merge
conflicts' merges commits in the order:
- Merge C with B to form an intermediate commit.
- Merge the intermediate commit with A.
With GIT_TEST_COMMIT_GRAPH=1, we write a commit-graph and subsequently
use the corrected committer date, which changes the order in which
commits are merged:
- Merge A with B to form an intermediate commit.
- Merge the intermediate commit with C.
While resulting repositories are equivalent, 6404.4 'virtual trees were
processed' fails with GIT_TEST_COMMIT_GRAPH=1 as we are selecting
different merge-bases and thus have different object ids for the
intermediate commits.
As this has already causes problems (as noted in 859fdc0 (commit-graph:
define GIT_TEST_COMMIT_GRAPH, 2018-08-29)), we disable commit graph
within t6404-recursive-merge.
Signed-off-by: Abhishek Kumar <abhishekkumar8222@gmail.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Since there are released versions of Git that understand generation
numbers in the commit-graph's CDAT chunk but do not understand the GDAT
chunk, the following scenario is possible:
1. "New" Git writes a commit-graph with the GDAT chunk.
2. "Old" Git writes a split commit-graph on top without a GDAT chunk.
If each layer of split commit-graph is treated independently, as it was
the case before this commit, with Git inspecting only the current layer
for chunk_generation_data pointer, commits in the lower layer (one with
GDAT) whould have corrected commit date as their generation number,
while commits in the upper layer would have topological levels as their
generation. Corrected commit dates usually have much larger values than
topological levels. This means that if we take two commits, one from the
upper layer, and one reachable from it in the lower layer, then the
expectation that the generation of a parent is smaller than the
generation of a child would be violated.
It is difficult to expose this issue in a test. Since we _start_ with
artificially low generation numbers, any commit walk that prioritizes
generation numbers will walk all of the commits with high generation
number before walking the commits with low generation number. In all the
cases I tried, the commit-graph layers themselves "protect" any
incorrect behavior since none of the commits in the lower layer can
reach the commits in the upper layer.
This issue would manifest itself as a performance problem in this case,
especially with something like "git log --graph" since the low
generation numbers would cause the in-degree queue to walk all of the
commits in the lower layer before allowing the topo-order queue to write
anything to output (depending on the size of the upper layer).
Therefore, When writing the new layer in split commit-graph, we write a
GDAT chunk only if the topmost layer has a GDAT chunk. This guarantees
that if a layer has GDAT chunk, all lower layers must have a GDAT chunk
as well.
Rewriting layers follows similar approach: if the topmost layer below
the set of layers being rewritten (in the split commit-graph chain)
exists, and it does not contain GDAT chunk, then the result of rewrite
does not have GDAT chunks either.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Abhishek Kumar <abhishekkumar8222@gmail.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
As discovered by Ævar, we cannot increment graph version to
distinguish between generation numbers v1 and v2 [1]. Thus, one of
pre-requistes before implementing generation number v2 was to
distinguish between graph versions in a backwards compatible manner.
We are going to introduce a new chunk called Generation DATa chunk (or
GDAT). GDAT will store corrected committer date offsets whereas CDAT
will still store topological level.
Old Git does not understand GDAT chunk and would ignore it, reading
topological levels from CDAT. New Git can parse GDAT and take advantage
of newer generation numbers, falling back to topological levels when
GDAT chunk is missing (as it would happen with a commit-graph written
by old Git).
We introduce a test environment variable 'GIT_TEST_COMMIT_GRAPH_NO_GDAT'
which forces commit-graph file to be written without generation data
chunk to emulate a commit-graph file written by old Git.
To minimize the space required to store corrrected commit date, Git
stores corrected commit date offsets into the commit-graph file, instea
of corrected commit dates. This saves us 4 bytes per commit, decreasing
the GDAT chunk size by half, but it's possible for the offset to
overflow the 4-bytes allocated for storage. As such overflows are and
should be exceedingly rare, we use the following overflow management
scheme:
We introduce a new commit-graph chunk, Generation Data OVerflow ('GDOV')
to store corrected commit dates for commits with offsets greater than
GENERATION_NUMBER_V2_OFFSET_MAX.
If the offset is greater than GENERATION_NUMBER_V2_OFFSET_MAX, we set
the MSB of the offset and the other bits store the position of corrected
commit date in GDOV chunk, similar to how Extra Edge List is maintained.
We test the overflow-related code with the following repo history:
F - N - U
/ \
U - N - U N
\ /
N - F - N
Where the commits denoted by U have committer date of zero seconds
since Unix epoch, the commits denoted by N have committer date of
1112354055 (default committer date for the test suite) seconds since
Unix epoch and the commits denoted by F have committer date of
(2 ^ 31 - 2) seconds since Unix epoch.
The largest offset observed is 2 ^ 31, just large enough to overflow.
[1]: https://lore.kernel.org/git/87a7gdspo4.fsf@evledraar.gmail.com/
Signed-off-by: Abhishek Kumar <abhishekkumar8222@gmail.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
With most of preparations done, let's implement corrected commit date.
The corrected commit date for a commit is defined as:
* A commit with no parents (a root commit) has corrected commit date
equal to its committer date.
* A commit with at least one parent has corrected commit date equal to
the maximum of its commit date and one more than the largest corrected
commit date among its parents.
As a special case, a root commit with timestamp of zero (01.01.1970
00:00:00Z) has corrected commit date of one, to be able to distinguish
from GENERATION_NUMBER_ZERO (that is, an uncomputed corrected commit
date).
To minimize the space required to store corrected commit date, Git
stores corrected commit date offsets into the commit-graph file. The
corrected commit date offset for a commit is defined as the difference
between its corrected commit date and actual commit date.
Storing corrected commit date requires sizeof(timestamp_t) bytes, which
in most cases is 64 bits (uintmax_t). However, corrected commit date
offsets can be safely stored using only 32-bits. This halves the size
of GDAT chunk, which is a reduction of around 6% in the size of
commit-graph file.
However, using offsets be problematic if a commit is malformed but valid
and has committer date of 0 Unix time, as the offset would be the same
as corrected commit date and thus require 64-bits to be stored properly.
While Git does not write out offsets at this stage, Git stores the
corrected commit dates in member generation of struct commit_graph_data.
It will begin writing commit date offsets with the introduction of
generation data chunk.
Signed-off-by: Abhishek Kumar <abhishekkumar8222@gmail.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
In a preparatory step for introducing corrected commit dates, let's
return timestamp_t values from commit_graph_generation(), use
timestamp_t for local variables and define GENERATION_NUMBER_INFINITY
as (2 ^ 63 - 1) instead.
We rename GENERATION_NUMBER_MAX to GENERATION_NUMBER_V1_MAX to
represent the largest topological level we can store in the commit data
chunk.
With corrected commit dates implemented, we will have two such *_MAX
variables to denote the largest offset and largest topological level
that can be stored.
Signed-off-by: Abhishek Kumar <abhishekkumar8222@gmail.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
In a later commit we will introduce corrected commit date as the
generation number v2. Corrected commit dates will be stored in the new
seperate Generation Data chunk. However, to ensure backwards
compatibility with "Old" Git we need to continue to write generation
number v1 (topological levels) to the commit data chunk. Thus, we need
to compute and store both versions of generation numbers to write the
commit-graph file.
Therefore, let's introduce a commit-slab `topo_level_slab` to store
topological levels; corrected commit date will be stored in the member
`generation` of struct commit_graph_data.
The macros `GENERATION_NUMBER_INFINITY` and `GENERATION_NUMBER_ZERO`
mark commits not in the commit-graph file and commits written by a
version of Git that did not compute generation numbers respectively.
Generation numbers are computed identically for both kinds of commits.
A "slab-miss" should return `GENERATION_NUMBER_INFINITY` as the commit
is not in the commit-graph file. However, since the slab is
zero-initialized, it returns 0 (or rather `GENERATION_NUMBER_ZERO`).
Thus, we no longer need to check if the topological level of a commit is
`GENERATION_NUMBER_INFINITY`.
We will add a pointer to the slab in `struct write_commit_graph_context`
and `struct commit_graph` to populate the slab in
`fill_commit_graph_info` if the commit has a pre-computed topological
level as in case of split commit-graphs.
Signed-off-by: Abhishek Kumar <abhishekkumar8222@gmail.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
In a preparatory step to implement generation number v2, we add tests to
ensure Git can read and parse commit-graph files without Generation Data
chunk. These files represent commit-graph files written by Old Git and
are neccesary for backward compatability.
We extend run_three_modes() and test_three_modes() to *_all_modes() with
the fourth mode being "commit-graph without generation data chunk".
Signed-off-by: Abhishek Kumar <abhishekkumar8222@gmail.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Both fill_commit_graph_info() and fill_commit_in_graph() parse
information present in commit data chunk. Let's simplify the
implementation by calling fill_commit_graph_info() within
fill_commit_in_graph().
fill_commit_graph_info() used to not load committer data from commit data
chunk. However, with the upcoming switch to using corrected committer
date as generation number v2, we will have to load committer date to
compute generation number value anyway.
e51217e15 (t5000: test tar files that overflow ustar headers,
30-06-2016) introduced a test 'generate tar with future mtime' that
creates a commit with committer date of (2^36 + 1) seconds since
EPOCH. The CDAT chunk provides 34-bits for storing committer date, thus
committer time overflows into generation number (within CDAT chunk) and
has undefined behavior.
The test used to pass as fill_commit_graph_info() would not set struct
member `date` of struct commit and load committer date from the object
database, generating a tar file with the expected mtime.
However, with corrected commit date, we will load the committer date
from CDAT chunk (truncated to lower 34-bits to populate the generation
number. Thus, Git sets date and generates tar file with the truncated
mtime.
The ustar format (the header format used by most modern tar programs)
only has room for 11 (or 12, depending on some implementations) octal
digits for the size and mtime of each file.
As the CDAT chunk is overflow by 12-octal digits but not 11-octal
digits, we split the existing tests to test both implementations
separately and add a new explicit test for 11-digit implementation.
To test the 11-octal digit implementation, we create a future commit
with committer date of 2^34 - 1, which overflows 11-octal digits without
overflowing 34-bits of the Commit Date chunks.
To test the 12-octal digit implementation, the smallest committer date
possible is 2^36 + 1, which overflows the CDAT chunk and thus
commit-graph must be disabled for the test.
Signed-off-by: Abhishek Kumar <abhishekkumar8222@gmail.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
In indegree_walk_step(), we add unvisited parents to the indegree queue.
However, parents are not guaranteed to be parsed. As the indegree queue
sorts by generation number, let's parse parents before inserting them to
ensure the correct priority order.
Signed-off-by: Abhishek Kumar <abhishekkumar8222@gmail.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Before computing Bloom filters, the commit-graph machinery uses
commit_gen_cmp to sort commits by generation order for improved diff
performance. 3d11275505 (commit-graph: examine commits by generation
number, 2020-03-30) claims that this sort can reduce the time spent to
compute Bloom filters by nearly half.
But since c49c82aa4c (commit: move members graph_pos, generation to a
slab, 2020-06-17), this optimization is broken, since asking for a
'commit_graph_generation()' directly returns GENERATION_NUMBER_INFINITY
while writing.
Not all hope is lost, though: 'commit_gen_cmp()' falls back to
comparing commits by their date when they have equal generation number,
and so since c49c82aa4c is purely a date comparison function. This
heuristic is good enough that we don't seem to loose appreciable
performance while computing Bloom filters.
Applying this patch (compared with v2.30.0) speeds up computing Bloom
filters by factors ranging from 0.40% to 5.19% on various repositories [1].
So, avoid the useless 'commit_graph_generation()' while writing by
instead accessing the slab directly. This returns the newly-computed
generation numbers, and allows us to avoid the heuristic by directly
comparing generation numbers.
[1]: https://lore.kernel.org/git/20210105094535.GN8396@szeder.dev/
Signed-off-by: Abhishek Kumar <abhishekkumar8222@gmail.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The previous change reduced time spent in strlen() while comparing
consecutive paths in verify_cache(), but we can do better. The
conditional checks the existence of a directory separator at the correct
location, but only after doing a string comparison. Swap the order to be
logically equivalent but perform fewer string comparisons.
To test the effect on performance, I used a repository with over three
million paths in the index. I then ran the following command on repeat:
git -c index.threads=1 commit --amend --allow-empty --no-edit
Here are the measurements over 10 runs after a 5-run warmup:
Benchmark #1: v2.30.0
Time (mean ± σ): 854.5 ms ± 18.2 ms
Range (min … max): 825.0 ms … 892.8 ms
Benchmark #2: Previous change
Time (mean ± σ): 833.2 ms ± 10.3 ms
Range (min … max): 815.8 ms … 849.7 ms
Benchmark #3: This change
Time (mean ± σ): 815.5 ms ± 18.1 ms
Range (min … max): 795.4 ms … 849.5 ms
This change is 2% faster than the previous change and 5% faster than
v2.30.0.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Use the name length field of cache entries instead of calculating its
value anew.
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The end of the cache tree index extension format trails off with
ellipses ever since 23fcc98 (doc: technical details about the index
file format, 2011-03-01). While an intuitive reader could gather what
this means, it could be better to use "and so on" instead.
Really, this is only justified because I also wanted to point out that
the number of subtrees in the index format is used to determine when the
recursive depth-first-search stack should be "popped." This should help
to add clarity to the format.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
I had difficulty in my efforts to learn about the cache tree extension
based on the documentation and code because I had an incorrect
assumption about how it behaved. This might be due to some ambiguity in
the documentation, so this change modifies the beginning of the cache
tree format by expanding the description of the feature.
My hope is that this documentation clarifies a few things:
1. There is an in-memory recursive tree structure that is constructed
from the extension data. This structure has a few differences, such
as where the name is stored.
2. What does it mean for an entry to be invalid?
3. When exactly are "new" trees created?
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The index has a "cache tree" extension. This corresponds to a
significant API implemented in cache-tree.[ch]. However, there are a few
places that refer to this erroneously as "cached tree". These are rare,
but notably the index-format.txt file itself makes this error.
The only other reference is in t7104-reset-hard.sh.
Reported-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Commands such as "git reset --hard" rebuild the in-memory representation
of the cache tree index extension by parsing tree objects starting at a
known root tree. The performance of this operation can vary widely
depending on the width and depth of the repository's working directory
structure. Measure the time in this operation using trace2 regions in
prime_cache_tree().
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
As we write or read the cache tree index extension, it can be good to
isolate how much of the file I/O time is spent constructing this
in-memory tree from the existing index or writing it out again to the
new index file. Use trace2 regions to indicate that we are spending time
on this operation.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Comments update.
* ab/gettext-charset-comment-fix:
gettext.c: remove/reword a mostly-useless comment
Makefile: remove a warning about old GETTEXT_POISON flag
Fix 2.29 regression where "git mergetool --tool-help" fails to list
all the available tools.
* pb/mergetool-tool-help-fix:
mergetool--lib: fix '--tool-help' to correctly show available tools
"git for-each-repo --config=<var> <cmd>" should not run <cmd> for
any repository when the configuration variable <var> is not defined
even once.
* ds/for-each-repo-noopfix:
for-each-repo: do nothing on empty config
Some tests expect that "ls -l" output has either '-' or 'x' for
group executable bit, but setgid bit can be inherited from parent
directory and make these fields 'S' or 's' instead, causing test
failures.
* mt/t4129-with-setgid-dir:
t4129: don't fail if setgid is set in the test directory
Follow-up on the "maintenance part-3" which introduced scheduled
maintenance tasks to support platforms whose native scheduling
methods are not 'cron'.
* ds/maintenance-part-4:
maintenance: use Windows scheduled tasks
maintenance: use launchctl on macOS
maintenance: include 'cron' details in docs
maintenance: extract platform-specific scheduling
Bash completion (in contrib/) update to make it easier for
end-users to add completion for their custom "git" subcommands.
* fc/completion-aliases-support:
completion: add proper public __git_complete
test: completion: add tests for __git_complete
completion: bash: improve function detection
completion: bash: add __git_have_func helper
"git stash" did not work well in a sparsely checked out working
tree.
* en/stash-apply-sparse-checkout:
stash: fix stash application in sparse-checkouts
stash: remove unnecessary process forking
t7012: add a testcase demonstrating stash apply bugs in sparse checkouts
Retire more names with "sha1" in it.
* ma/sha1-is-a-hash:
hash-lookup: rename from sha1-lookup
sha1-lookup: rename `sha1_pos()` as `hash_pos()`
object-file.c: rename from sha1-file.c
object-name.c: rename from sha1-name.c
Code clean-up.
* ma/t1300-cleanup:
t1300: don't needlessly work with `core.foo` configs
t1300: remove duplicate test for `--file no-such-file`
t1300: remove duplicate test for `--file ../foo`
"git rev-parse" can be explicitly told to give output as absolute
or relative path with the `--path-format=(absolute|relative)` option.
* bc/rev-parse-path-format:
rev-parse: add option for absolute or relative path formatting
abspath: add a function to resolve paths with missing components
The configuration variable 'core.abbrev' can be set to 'no' to
force no abbreviation regardless of the hash algorithm.
* ew/decline-core-abbrev:
core.abbrev=no disables abbreviations
While we currently have the `GIT_CONFIG_PARAMETERS` environment variable
which can be used to pass runtime configuration data to git processes,
it's an internal implementation detail and not supposed to be used by
end users.
Next to being for internal use only, this way of passing config entries
has a major downside: the config keys need to be parsed as they contain
both key and value in a single variable. As such, it is left to the user
to escape any potentially harmful characters in the value, which is
quite hard to do if values are controlled by a third party.
This commit thus adds a new way of adding config entries via the
environment which gets rid of this shortcoming. If the user passes the
`GIT_CONFIG_COUNT=$n` environment variable, Git will parse environment
variable pairs `GIT_CONFIG_KEY_$i` and `GIT_CONFIG_VALUE_$i` for each
`i` in `[0,n)`.
While the same can be achieved with `git -c <name>=<value>`, one may
wish to not do so for potentially sensitive information. E.g. if one
wants to set `http.extraHeader` to contain an authentication token,
doing so via `-c` would trivially leak those credentials via e.g. ps(1),
which typically also shows command arguments.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The `getenv_safe()` helper function helps to safely retrieve multiple
environment values without the need to depend on platform-specific
behaviour for the return value's lifetime. We'll make use of this
function in a following patch, so let's make it available by making it
non-static and adding a declaration.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The previous commit added a new format for $GIT_CONFIG_PARAMETERS which
is able to robustly handle subsections with "=" in them. Let's start
writing the new format. Unfortunately, this does much less than you'd
hope, because "git -c" itself has the same ambiguity problem! But it's
still worth doing:
- we've now pushed the problem from the inter-process communication
into the "-c" command-line parser. This would free us up to later
add an unambiguous format there (e.g., separate arguments like "git
--config key value", etc).
- for --config-env, the parser already disallows "=" in the
environment variable name. So:
git --config-env section.with=equals.key=ENVVAR
will robustly set section.with=equals.key to the contents of
$ENVVAR.
The new test shows the improvement for --config-env.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
