[PATCH v4 0/7] Use generation numbers for --topo-order
- From
Derrick Stolee via GitGitGadget <gitgitgadget@gmail.com>
- Date
- Oct 16, 2018, 22:36 UTC
- Message-ID
- <pull.25.v4.git.gitgitgadget@gmail.com>
- In-Reply-To
- <pull.25.v3.git.gitgitgadget@gmail.com>
This patch series performs a decently-sized refactoring of the revision-walk machinery. Well, "refactoring" is probably the wrong word, as I don't actually remove the old code. Instead, when we see certain options in the 'rev_info' struct, we redirect the commit-walk logic to a new set of methods that distribute the workload differently. By using generation numbers in the commit-graph, we can significantly improve 'git log --graph' commands (and the underlying 'git rev-list --topo-order').
On the Linux repository, I got the following performance results when comparing to the previous version with or without a commit-graph:
Test: git rev-list --topo-order -100 HEAD HEAD~1, no commit-graph: 6.80 s HEAD~1, w/ commit-graph: 0.77 s HEAD, w/ commit-graph: 0.02 s
Test: git rev-list --topo-order -100 HEAD -- tools HEAD~1, no commit-graph: 9.63 s HEAD~1, w/ commit-graph: 6.06 s HEAD, w/ commit-graph: 0.06 s
If you want to read this series but are unfamiliar with the commit-graph and generation numbers, then I recommend reading Documentation/technical/commit-graph.txt or a blob post [1] I wrote on the subject. In particular, the three-part walk described in "revision.c: refactor basic topo-order logic" is present (but underexplained) as an animated PNG [2].
Since revision.c is an incredibly important (and old) portion of the codebase -- and because there are so many orthogonal options in 'struct rev_info' -- I consider this submission to be "RFC quality". That is, I am not confident that I am not missing anything, or that my solution is the best it can be. I did merge this branch with ds/commit-graph-with-grafts and the "DO-NOT-MERGE: write and read commit-graph always" commit that computes a commit-graph with every 'git commit' command. The test suite passed with that change, available on GitHub [3]. To ensure that I cover at least the case I think are interesting, I added tests to t6600-test-reach.sh to verify the walks report the correct results for the three cases there (no commit-graph, full commit-graph, and a partial commit-graph so the walk starts at GENERATION_NUMBER_INFINITY).
One notable case that is not included in this series is the case of a history comparison such as 'git rev-list --topo-order A..B'. The existing code in limit_list() has ways to cut the walk short when all pending commits are UNINTERESTING. Since this code depends on commit_list instead of the prio_queue we are using here, I chose to leave it untouched for now. We can revisit it in a separate series later. Since handle_commit() turns on revs->limited when a commit is UNINTERESTING, we do not hit the new code in this case. Removing this 'revs->limited = 1;' line yields correct results, but the performance is worse.
This series was based on ds/reachable, but is now based on 'master' to not conflict with 182070 "commit: use timestamp_t for author_date_slab". There is a small conflict with md/filter-trees, because it renamed a flag in revisions.h in the line before I add new flags. Hopefully this conflict is not too difficult to resolve.
Changes in V3: I added a new patch that updates the tab-alignment for flags in revision.h before adding new ones (Thanks, Ævar!). Also, I squashed the recommended changes to run_three_modes and test_three_modes from Szeder and Junio. Thanks!
Changes in V4: I'm sending a V4 to respond to the feedback so far. Still looking forward to more on the really big commit!
* Removed the whitespace changes to the flags in revision.c that caused merge pain. * The prio-queue peek function is now covered by tests when in "stack" mode. * The "add_parents_to_list()" function is now renamed to "process_parents()" * Added a new commit that expands test coverage with alternate orders and file history (use GIT_TEST_COMMIT_GRAPH to have t6012-rev-list-simplify.sh cover the new logic). These tests found a problem with author dates (I forgot to record them during the explore walk). * Commit message edits.
Thanks, -Stolee
[1] https://blogs.msdn.microsoft.com/devops/2018/07/09/supercharging-the-git-commit-graph-iii-generations/ Supercharging the Git Commit Graph III: Generations and Graph Algorithms
[2] https://msdnshared.blob.core.windows.net/media/2018/06/commit-graph-topo-order-b-a.png Animation showing three-part walk
[3] https://github.com/derrickstolee/git/tree/topo-order/testA branch containing this series along with commits to compute commit-graph in entire test suite.
Cc: avarab@gmail.comCc: szeder.dev@gmail.com
Derrick Stolee (7): prio-queue: add 'peek' operation test-reach: add run_three_modes method test-reach: add rev-list tests revision.c: begin refactoring --topo-order logic commit/revisions: bookkeeping before refactoring revision.c: generation-based topo-order algorithm t6012: make rev-list tests more interesting
commit.c | 11 +- commit.h | 8 ++ object.h | 4 +- prio-queue.c | 9 ++ prio-queue.h | 6 + revision.c | 245 +++++++++++++++++++++++++++++++++-- revision.h | 6 + t/helper/test-prio-queue.c | 26 ++-- t/t0009-prio-queue.sh | 14 ++ t/t6012-rev-list-simplify.sh | 45 +++++-- t/t6600-test-reach.sh | 96 +++++++++++++- 11 files changed, 430 insertions(+), 40 deletions(-)
base-commit: 2d3b1c576c85b7f5db1f418907af00ab88e0c303 Published-As: https://github.com/gitgitgadget/git/releases/tags/pr-25%2Fderrickstolee%2Ftopo-order%2Fprogress-v4 Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-25/derrickstolee/topo-order/progress-v4 Pull-Request: https://github.com/gitgitgadget/git/pull/25
Range-diff vs v3:
1: cc1ec4c270 ! 1: 2358cfd5ed prio-queue: add 'peek' operation
@@ -8,7 +8,9 @@
add it as prio_queue_peek().
Add a reference-level comparison in t/helper/test-prio-queue.c
- so this method is exercised by t0009-prio-queue.sh.
+ so this method is exercised by t0009-prio-queue.sh. Further, add
+ a test that checks the behavior when the compare function is NULL
+ (i.e. the queue becomes a stack).
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
@@ -56,6 +58,11 @@
- if (!strcmp(*argv, "get"))
- show(prio_queue_get(&pq));
- else if (!strcmp(*argv, "dump")) {
+- int *v;
+- while ((v = prio_queue_get(&pq)))
+- show(v);
+- }
+- else {
+ if (!strcmp(*argv, "get")) {
+ void *peek = prio_queue_peek(&pq);
+ void *get = prio_queue_get(&pq);
@@ -63,6 +70,40 @@
+ BUG("peek and get results do not match");
+ show(get);
+ } else if (!strcmp(*argv, "dump")) {
- int *v;
- while ((v = prio_queue_get(&pq)))
- show(v);
++ void *peek;
++ void *get;
++ while ((peek = prio_queue_peek(&pq))) {
++ get = prio_queue_get(&pq);
++ if (peek != get)
++ BUG("peek and get results do not match");
++ show(get);
++ }
++ } else if (!strcmp(*argv, "stack")) {
++ pq.compare = NULL;
++ } else {
+ int *v = malloc(sizeof(*v));
+ *v = atoi(*argv);
+ prio_queue_put(&pq, v);
+
+diff --git a/t/t0009-prio-queue.sh b/t/t0009-prio-queue.sh
+--- a/t/t0009-prio-queue.sh
++++ b/t/t0009-prio-queue.sh
+@@
+ test_cmp expect actual
+ '
+
++cat >expect <<'EOF'
++3
++2
++6
++4
++5
++1
++8
++EOF
++test_expect_success 'stack order' '
++ test-tool prio-queue stack 8 1 5 4 6 2 3 dump >actual &&
++ test_cmp expect actual
++'
++
+ test_done
2: b2a1ade148 = 2: 3a4b68e479 test-reach: add run_three_modes method
3: b0ceb96076 = 3: 12a3f6d367 test-reach: add rev-list tests
4: fd1a0ab7cd = 4: cd9eef9688 revision.c: begin refactoring --topo-order logic
5: e86f304082 ! 5: f3e291665d commit/revisions: bookkeeping before refactoring
@@ -16,7 +16,11 @@
around the UNINTERESTING flag and other special cases depending
on the struct rev_info. Allow this method to ignore a NULL 'list'
parameter, as we will not be populating the list for our walk.
+ Also rename the method to the slightly more generic name
+ process_parents() to make clear that this method does more than
+ add to a list (and no list is required anymore).
+ Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
diff --git a/commit.c b/commit.c
@@ -28,7 +32,8 @@
-/* record author-date for each commit object */
-define_commit_slab(author_date_slab, timestamp_t);
--
++implement_shared_commit_slab(author_date_slab, timestamp_t);
+
-static void record_author_date(struct author_date_slab *author_date,
- struct commit *commit)
+void record_author_date(struct author_date_slab *author_date,
@@ -64,7 +69,7 @@
extern int check_commit_signature(const struct commit *commit, struct signature_check *sigc);
+/* record author-date for each commit object */
-+define_commit_slab(author_date_slab, timestamp_t);
++define_shared_commit_slab(author_date_slab, timestamp_t);
+
+void record_author_date(struct author_date_slab *author_date,
+ struct commit *commit);
@@ -77,6 +82,17 @@
diff --git a/revision.c b/revision.c
--- a/revision.c
+++ b/revision.c
+@@
+ *cache = new_entry;
+ }
+
+-static int add_parents_to_list(struct rev_info *revs, struct commit *commit,
+- struct commit_list **list, struct commit_list **cache_ptr)
++static int process_parents(struct rev_info *revs, struct commit *commit,
++ struct commit_list **list, struct commit_list **cache_ptr)
+ {
+ struct commit_list *parent = commit->parents;
+ unsigned left_flag;
@@
if (p->object.flags & SEEN)
continue;
@@ -97,3 +113,39 @@
}
if (revs->first_parent_only)
break;
+@@
+
+ if (revs->max_age != -1 && (commit->date < revs->max_age))
+ obj->flags |= UNINTERESTING;
+- if (add_parents_to_list(revs, commit, &list, NULL) < 0)
++ if (process_parents(revs, commit, &list, NULL) < 0)
+ return -1;
+ if (obj->flags & UNINTERESTING) {
+ mark_parents_uninteresting(commit);
+@@
+
+ static void expand_topo_walk(struct rev_info *revs, struct commit *commit)
+ {
+- if (add_parents_to_list(revs, commit, &revs->commits, NULL) < 0) {
++ if (process_parents(revs, commit, &revs->commits, NULL) < 0) {
+ if (!revs->ignore_missing_links)
+ die("Failed to traverse parents of commit %s",
+ oid_to_hex(&commit->object.oid));
+@@
+ for (;;) {
+ struct commit *p = *pp;
+ if (!revs->limited)
+- if (add_parents_to_list(revs, p, &revs->commits, &cache) < 0)
++ if (process_parents(revs, p, &revs->commits, &cache) < 0)
+ return rewrite_one_error;
+ if (p->object.flags & UNINTERESTING)
+ return rewrite_one_ok;
+@@
+ try_to_simplify_commit(revs, commit);
+ else if (revs->topo_walk_info)
+ expand_topo_walk(revs, commit);
+- else if (add_parents_to_list(revs, commit, &revs->commits, NULL) < 0) {
++ else if (process_parents(revs, commit, &revs->commits, NULL) < 0) {
+ if (!revs->ignore_missing_links)
+ die("Failed to traverse parents of commit %s",
+ oid_to_hex(&commit->object.oid));
6: fa6d5ef152 < -: ---------- revision.h: add whitespace in flag definitions
7: 020b2f50c5 ! 6: aa0bb2221d revision.c: refactor basic topo-order logic
@@ -1,6 +1,15 @@
Author: Derrick Stolee <dstolee@microsoft.com>
- revision.c: refactor basic topo-order logic
+ revision.c: generation-based topo-order algorithm
+
+ The current --topo-order algorithm requires walking all
+ reachable commits up front, topo-sorting them, all before
+ outputting the first value. This patch introduces a new
+ algorithm which uses stored generation numbers to
+ incrementally walk in topo-order, outputting commits as
+ we go. This can dramatically reduce the computation time
+ to write a fixed number of commits, such as when limiting
+ with "-n <N>" or filling the first page of a pager.
When running a command like 'git rev-list --topo-order HEAD',
Git performed the following steps:
@@ -139,11 +148,12 @@
frequently, including by merge commits. A less-frequently-changed
path (such as 'README') has similar end-to-end time since we need
to walk the same number of commits (before determining we do not
- have 100 hits). However, get get the benefit that the output is
+ have 100 hits). However, get the benefit that the output is
presented to the user as it is discovered, much the same as a
normal 'git log' command (no '--topo-order'). This is an improved
user experience, even if the command has the same runtime.
+ Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
diff --git a/object.h b/object.h
@@ -216,10 +226,13 @@
+ if (parse_commit_gently(c, 1) < 0)
+ return;
+
++ if (revs->sort_order == REV_SORT_BY_AUTHOR_DATE)
++ record_author_date(&info->author_date, c);
++
+ if (revs->max_age != -1 && (c->date < revs->max_age))
+ c->object.flags |= UNINTERESTING;
+
-+ if (add_parents_to_list(revs, c, NULL, NULL) < 0)
++ if (process_parents(revs, c, NULL, NULL) < 0)
+ return;
+
+ if (c->object.flags & UNINTERESTING)
@@ -366,10 +379,10 @@
static void expand_topo_walk(struct rev_info *revs, struct commit *commit)
{
-- if (add_parents_to_list(revs, commit, &revs->commits, NULL) < 0) {
+- if (process_parents(revs, commit, &revs->commits, NULL) < 0) {
+ struct commit_list *p;
+ struct topo_walk_info *info = revs->topo_walk_info;
-+ if (add_parents_to_list(revs, commit, NULL, NULL) < 0) {
++ if (process_parents(revs, commit, NULL, NULL) < 0) {
if (!revs->ignore_missing_links)
die("Failed to traverse parents of commit %s",
- oid_to_hex(&commit->object.oid));
@@ -404,9 +417,9 @@
--- a/revision.h
+++ b/revision.h
@@
- #define USER_GIVEN (1u<<25) /* given directly by the user */
- #define TRACK_LINEAR (1u<<26)
- #define ALL_REV_FLAGS (((1u<<11)-1) | USER_GIVEN | TRACK_LINEAR)
+ #define USER_GIVEN (1u<<25) /* given directly by the user */
+ #define TRACK_LINEAR (1u<<26)
+ #define ALL_REV_FLAGS (((1u<<11)-1) | USER_GIVEN | TRACK_LINEAR)
+#define TOPO_WALK_EXPLORED (1u<<27)
+#define TOPO_WALK_INDEGREE (1u<<28)
-: ---------- > 7: a21febe112 t6012: make rev-list tests more interesting-- gitgitgadget