Both 'git for-each-ref --merged=<X>' and 'git branch --merged=<X>' use
the ref-filter machinery to select references or branches (respectively)
that are reachable from a set of commits presented by one or more
--merged arguments. This happens within reach_filter(), which uses the
revision-walk machinery to walk history in a standard way.
However, the commit-reach.c file is full of custom searches that are
more efficient, especially for reachability queries that can terminate
early when reachability is discovered. Add a new
tips_reachable_from_bases() method to commit-reach.c and call it from
within reach_filter() in ref-filter.c. This affects both 'git branch'
and 'git for-each-ref' as tested in p1500-graph-walks.sh.
For the Linux kernel repository, we take an already-fast algorithm and
make it even faster:
Test HEAD~1 HEAD
-------------------------------------------------------------------
1500.5: contains: git for-each-ref --merged 0.13 0.02 -84.6%
1500.6: contains: git branch --merged 0.14 0.02 -85.7%
1500.7: contains: git tag --merged 0.15 0.03 -80.0%
(Note that we remove the iterative 'git rev-list' test from p1500
because it no longer makes sense as a comparison to 'git for-each-ref'
and would just waste time running it for these comparisons.)
The algorithm is implemented in commit-reach.c in the method
tips_reachable_from_base(). This method takes a string_list of tips and
assigns the 'util' for each item with the value 1 if the base commit can
reach those tips.
Like other reachability queries in commit-reach.c, the fastest way to
search for "can A reach B?" is to do a depth-first search up to the
generation number of B, preferring to explore first parents before later
parents. While we must walk all reachable commits up to that generation
number when the answer is "no", the depth-first search can answer "yes"
much faster than other approaches in most cases.
This search becomes trickier when there are multiple targets for the
depth-first search. The commits with lower generation number are more
likely to be within the history of the start commit, but we don't want
to waste time searching commits of low generation number if the commit
target with lowest generation number has already been found.
The trick here is to take the input commits and sort them by generation
number in ascending order. Track the index within this order as
min_generation_index. When we find a commit, if its index in the list is
equal to min_generation_index, then we can increase the generation
number boundary of our search to the next-lowest value in the list.
With this mechanism, the number of commits to search is minimized with
respect to the depth-first search heuristic. We will walk all commits up
to the minimum generation number of a commit that is _not_ reachable
from the start, but we will walk only the necessary portion of the
depth-first search for the reachable commits of lower generation.
Add extra tests for this behavior in t6600-test-reach.sh as the
interesting data shape of that repository can sometimes demonstrate
corner case bugs.
Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Fully implement the commit-counting logic required to determine
ahead/behind counts for a batch of commit pairs. This is a new library
method within commit-reach.h. This method will be linked to the
for-each-ref builtin in the next change.
The interface for ahead_behind() uses two arrays. The first array of
commits contains the list of all starting points for the walk. This
includes all tip commits _and_ base commits. The second array specifies
base/tip pairs by pointing to commits within the first array, by index.
The second array also stores the resulting ahead/behind counts for each
of these pairs.
This implementation of ahead_behind() allows multiple bases, if desired.
Even with multiple bases, there is only one commit walk used for
counting the ahead/behind values, saving time when the base/tip ranges
overlap significantly.
This interface for ahead_behind() also makes it very easy to call
ensure_generations_valid() on the entire array of bases and tips. This
call is necessary because it is critical that the walk that counts
ahead/behind values never walks a commit more than once. Without
generation numbers on every commit, there is a possibility that a
commit date skew could cause the walk to revisit a commit and then
double-count it. For this reason, it is strongly recommended that 'git
ahead-behind' is only run in a repository with a commit-graph file that
covers most of the reachable commits, storing precomputed generation
numbers. If no commit-graph exists, this walk will be much slower as it
must walk all reachable commits in ensure_generations_valid() before
performing the counting logic.
It is possible to detect if generation numbers are available at run time
and redirect the implementation to another algorithm that does not
require this property. However, that implementation requires a commit
walk per base/tip pair _and_ can be slower due to the commit date
heuristics required. Such an implementation could be considered in the
future if there is a reason to include it, but most Git hosts should
already be generating a commit-graph file as part of repository
maintenance. Most Git clients should also be generating commit-graph
files as part of background maintenance or automatic GCs.
Now, let's discuss the ahead/behind counting algorithm.
The first array of commits are considered the starting commits. The
index within that array will play a critical role.
We create a new commit slab that maps commits to a bitmap. For a given
commit (anywhere in the history), its bitmap stores information relative
to which of the input commits can reach that commit. The ith bit will be
on if the ith commit from the starting list can reach that commit. It is
important to notice that these bitmaps are not the typical "reachability
bitmaps" that are stored in .bitmap files. Instead of signalling which
objects are reachable from the current commit, they instead signal
"which starting commits can reach me?" It is also important to know that
the bitmap is not necessarily "complete" until we walk that commit. We
will perform a commit walk by generation number in such a way that we
can guarantee the bitmap is correct when we visit that commit.
At the beginning of the ahead_behind() method, we initialize the bitmaps
for each of the starting commits. By enabling the ith bit for the ith
starting commit, we signal "the ith commit can reach itself."
We walk commits by popping the commit with maximum generation number out
of the queue, guaranteeing that we will never walk a child of that
commit in any future steps.
As we walk, we load the bitmap for the current commit and perform two
main steps. The _second_ step examines each parent of the current commit
and adds the current commit's bitmap bits to each parent's bitmap. (We
create a new bitmap for the parent if this is our first time seeing that
parent.) After adding the bits to the parent's bitmap, the parent is
added to the walk queue. Due to this passing of bits to parents, the
current commit has a guarantee that the ith bit is enabled on its bitmap
if and only if the ith commit can reach the current commit.
The first step of the walk is to examine the bitmask on the current
commit and decide which ranges the commit is in or not. Due to the "bit
pushing" in the second step, we have a guarantee that the ith bit of the
current commit's bitmap is on if and only if the ith starting commit can
reach it. For each ahead_behind_count struct, check the base_index and
tip_index to see if those bits are enabled on the current bitmap. If
exactly one bit is enabled, then increment the corresponding 'ahead' or
'behind' count. This increment is the reason we _absolutely need_ to
walk commits at most once.
The only subtle thing to do with this walk is to check to see if a
parent has all bits on in its bitmap, in which case it becomes "stale"
and is marked with the STALE bit. This allows queue_has_nonstale() to be
the terminating condition of the walk, which greatly reduces the number
of commits walked if all of the commits are nearby in history. It avoids
walking a large number of common commits when there is a deep history.
We also use the helper method insert_no_dup() to add commits to the
priority queue without adding them multiple times. This uses the PARENT2
flag. Thus, we must clear both the STALE and PARENT2 bits of all
commits, in case ahead_behind() is called multiple times in the same
process.
Co-authored-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Derrick Stolee <derrickstolee@github.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>
d91d6fbf26 (commit-reach: create repo_is_descendant_of(), 2020-06-17)
adds a repository aware version of is_descendant_of() and a backward
compatibility shim that is barely used.
Update all callers to directly use the new repo_is_descendant_of()
function instead; making the codebase simpler and pushing more
the_repository references higher up the stack.
Helped-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The in-core repository instances are passed through more codepaths.
* sb/more-repo-in-api: (23 commits)
t/helper/test-repository: celebrate independence from the_repository
path.h: make REPO_GIT_PATH_FUNC repository agnostic
commit: prepare free_commit_buffer and release_commit_memory for any repo
commit-graph: convert remaining functions to handle any repo
submodule: don't add submodule as odb for push
submodule: use submodule repos for object lookup
pretty: prepare format_commit_message to handle arbitrary repositories
commit: prepare logmsg_reencode to handle arbitrary repositories
commit: prepare repo_unuse_commit_buffer to handle any repo
commit: prepare get_commit_buffer to handle any repo
commit-reach: prepare in_merge_bases[_many] to handle any repo
commit-reach: prepare get_merge_bases to handle any repo
commit-reach.c: allow get_merge_bases_many_0 to handle any repo
commit-reach.c: allow remove_redundant to handle any repo
commit-reach.c: allow merge_bases_many to handle any repo
commit-reach.c: allow paint_down_to_common to handle any repo
commit: allow parse_commit* to handle any repo
object: parse_object to honor its repository argument
object-store: prepare has_{sha1, object}_file to handle any repo
object-store: prepare read_object_file to deal with any repo
...
Similarly to previous patches, the get_merge_base functions are used
often in the code base, which makes migrating them hard.
Implement the new functions, prefixed with 'repo_' and hide the old
functions behind a wrapper macro.
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The history traversal used to implement the tag-following has been
optimized by introducing a new helper.
* ds/add-missing-tags:
remote: make add_missing_tags() linear
test-reach: test get_reachable_subset
commit-reach: implement get_reachable_subset
The existing reachability algorithms in commit-reach.c focus on
finding merge-bases or determining if all commits in a set X can
reach at least one commit in a set Y. However, for two commits sets
X and Y, we may also care about which commits in Y are reachable
from at least one commit in X.
Implement get_reachable_subset() which answers this question. Given
two arrays of commits, 'from' and 'to', return a commit_list with
every commit from the 'to' array that is reachable from at least
one commit in the 'from' array.
The algorithm is a simple walk starting at the 'from' commits, using
the PARENT2 flag to indicate "this commit has already been added to
the walk queue". By marking the 'to' commits with the PARENT1 flag,
we can determine when we see a commit from the 'to' array. We remove
the PARENT1 flag as we add that commit to the result list to avoid
duplicates.
The order of the resulting list is a reverse of the order that the
commits are discovered in the walk.
There are a couple shortcuts to avoid walking more than we need:
1. We determine the minimum generation number of commits in the
'to' array. We do not walk commits with generation number
below this minimum.
2. We count how many distinct commits are in the 'to' array, and
decrement this count when we discover a 'to' commit during the
walk. If this number reaches zero, then we can terminate the
walk.
Tests will be added using the 'test-tool reach' helper in a
subsequent commit.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Add the necessary #includes and forward declarations to allow the header
file to pass the 'hdr-check' target.
Note that, since this header includes the commit-slab implementation
header file (indirectly via commit-slab.h), some of the commit-slab
inline functions (e.g contains_cache_at_peek()) will not compile without
the complete type of 'struct commit'. Hence, we replace the forward
declaration of 'struct commit' with the an #include of the 'commit.h'
header file.
It is possible, using the 'commit-slab-{decl,impl}.h' files, to avoid
this inclusion of the 'commit.h' header. Commit a9f1f1f9f8 ("commit-slab.h:
code split", 2018-05-19) separated the commit-slab interface from its
implementation, to allow for the definition of a public commit-slab data
structure. This enabled us to avoid including the commit-slab implementation
in a header file, which could result in the replication of the commit-slab
functions in each compilation unit in which it was included.
Indeed, if you compile with optimizations disabled, then run this script:
$ cat -n dup-static.sh
1 #!/bin/sh
2
3 nm $1 | grep ' t ' | cut -d' ' -f3 | sort | uniq -c |
4 sort -rn | grep -v ' 1'
$
$ ./dup-static.sh git | grep contains
24 init_contains_cache_with_stride
24 init_contains_cache
24 contains_cache_peek
24 contains_cache_at_peek
24 contains_cache_at
24 clear_contains_cache
$
you will find 24 copies of the commit-slab routines for the contains_cache.
Of course, when you enable optimizations again, these duplicate static
functions (mostly) disappear. Compiling with gcc at -O2, leaves two static
functions, thus:
$ nm commit-reach.o | grep contains_cache
0000000000000870 t contains_cache_at_peek.isra.1.constprop.6
$ nm ref-filter.o | grep contains_cache
00000000000002b0 t clear_contains_cache.isra.14
$
However, using a shared 'contains_cache' would result in all six of the
above functions as external public functions in the git binary. At present,
only three of these functions are actually called, so the trade-off
seems to favour letting the compiler inline the commit-slab functions.
Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The can_all_from_reach_with_flags() algorithm is currently quadratic in
the worst case, because it calls the reachable() method for every 'from'
without tracking which commits have already been walked or which can
already reach a commit in 'to'.
Rewrite the algorithm to walk each commit a constant number of times.
We also add some optimizations that should work for the main consumer of
this method: fetch negotitation (haves/wants).
The first step includes using a depth-first-search (DFS) from each
'from' commit, sorted by ascending generation number. We do not walk
beyond the minimum generation number or the minimum commit date. This
DFS is likely to be faster than the existing reachable() method because
we expect previous ref values to be along the first-parent history.
If we find a target commit, then we mark everything in the DFS stack as
a RESULT. This expands the set of targets for the other 'from' commits.
We also mark the visited commits using 'assign_flag' to prevent re-
walking the same commits.
We still need to clear our flags at the end, which is why we will have a
total of three visits to each commit.
Performance was measured on the Linux repository using
'test-tool reach can_all_from_reach'. The input included rows seeded by
tag values. The "small" case included X-rows as v4.[0-9]* and Y-rows as
v3.[0-9]*. This mimics a (very large) fetch that says "I have all major
v3 releases and want all major v4 releases." The "large" case included
X-rows as "v4.*" and Y-rows as "v3.*". This adds all release-candidate
tags to the set, which does not greatly increase the number of objects
that are considered, but does increase the number of 'from' commits,
demonstrating the quadratic nature of the previous code.
Small Case:
Before: 1.52 s
After: 0.26 s
Large Case:
Before: 3.50 s
After: 0.27 s
Note how the time increases between the two cases in the two versions.
The new code increases relative to the number of commits that need to be
walked, but not directly relative to the number of 'from' commits.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
The can_all_from_reach_with_flags method is used by ok_to_give_up in
upload-pack.c to see if we have done enough negotiation during a fetch.
This method is intentionally created to preserve state between calls to
assist with stateful negotiation, such as over SSH.
To make this method testable, add a new can_all_from_reach method that
does the initial setup and final tear-down. We will later use this
method in production code. Call the method from 'test-tool reach' for
now.
Since this is a many-to-many reachability query, add a new type of input
to the 'test-tool reach' input format. Lines "Y:<committish>" create a
list of commits to be the reachability targets from the commits in the
'X' list. In the context of fetch negotiation, the 'X' commits are the
'want' commits and the 'Y' commits are the 'have' commits.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
There are several commit walks in the codebase. Group them together into
a new commit-reach.c file and corresponding header. After we group these
walks into one place, we can reduce duplicate logic by calling
equivalent methods.
The can_all_from_reach_with_flags method is used in a stateful way by
upload-pack.c. The parameters are very flexible, so we will be able to
use its commit walking logic for many other callers.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
There are several commit walks in the codebase. Group them together into
a new commit-reach.c file and corresponding header. After we group these
walks into one place, we can reduce duplicate logic by calling
equivalent methods.
All methods are direct moves, except we also make the commit_contains()
method public so its consumers in ref-filter.c can still call it. We can
also test this method in a test-tool in a later commit.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
There are several commit walks in the codebase. Group them together into
a new commit-reach.c file and corresponding header. After we group these
walks into one place, we can reduce duplicate logic by calling
equivalent methods.
The ref_newer() method is used by 'git push -f' to check if a force-push
is necessary. By making the method public, we make it possible to test
the method directly without setting up an envieronment where a 'git
push' call makes sense.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
There are several commit walks in the codebase. Group them together into
a new commit-reach.c file and corresponding header. After we group these
walks into one place, we can reduce duplicate logic by calling
equivalent methods.
The method declarations in commit.h are not touched by this commit and
will be moved in a following commit. Many consumers need to point to
commit-reach.h and that would bloat this commit.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
