Derrick Stolee <dsto...@microsoft.com> writes: > While preparing commits to be written into a commit-graph file, compute > the generation numbers using a depth-first strategy.
Sidenote: for generation numbers it does not matter if we use depth-first or breadth-first strategy, but it is more natural to use depth-first search because generation numbers need post-order processing (parents before child). > > The only commits that are walked in this depth-first search are those > without a precomputed generation number. Thus, computation time will be > relative to the number of new commits to the commit-graph file. A question: what happens if the existing commit graph is from older version of git and has _ZERO for generation numbers? Answer: I see that we treat both _INFINITY (not in commit-graph) and _ZERO (in commit graph but not computed) as not computed generation numbers. All right. > > If a computed generation number would exceed GENERATION_NUMBER_MAX, then > use GENERATION_NUMBER_MAX instead. All right, though I guess this would remain theoretical for a long while. We don't have any way of testing this, at least not without recompiling Git with lower value of GENERATION_NUMBER_MAX -- which means not automatically, isn't it? > > Signed-off-by: Derrick Stolee <dsto...@microsoft.com> > --- > commit-graph.c | 45 +++++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 45 insertions(+) > > diff --git a/commit-graph.c b/commit-graph.c > index 9ad21c3ffb..047fa9fca5 100644 > --- a/commit-graph.c > +++ b/commit-graph.c > @@ -439,6 +439,9 @@ static void write_graph_chunk_data(struct hashfile *f, > int hash_len, > else > packedDate[0] = 0; > > + if ((*list)->generation != GENERATION_NUMBER_INFINITY) > + packedDate[0] |= htonl((*list)->generation << 2); > + If we stumble upon commit marked as "not in commit-graph" while writing commit graph, it is a BUG(), isn't it? (Problem noticed by Junio.) It is a bit strange to me that the code uses get_be32 for reading, but htonl for writing. Is Git tested on non little-endian machines, like big-endian ppc64 or s390x, or on mixed-endian machines (or selectable-endian machines with data endianness set to non little-endian, like ia64)? If not, could we use for example openSUSE Build Service (https://build.opensuse.org/) for this? > packedDate[1] = htonl((*list)->date); > hashwrite(f, packedDate, 8); > > @@ -571,6 +574,46 @@ static void close_reachable(struct packed_oid_list *oids) > } > } > > +static void compute_generation_numbers(struct commit** commits, > + int nr_commits) > +{ > + int i; > + struct commit_list *list = NULL; All right, commit_list will work as stack. > + > + for (i = 0; i < nr_commits; i++) { > + if (commits[i]->generation != GENERATION_NUMBER_INFINITY && > + commits[i]->generation != GENERATION_NUMBER_ZERO) > + continue; All right, we consider _INFINITY and _SERO as not computed. If generation number is computed (by 'recursion' or from commit graph), we (re)use it. This means that generation number calculation is incremental, as intended -- good. > + > + commit_list_insert(commits[i], &list); Start depth-first walks from commits given. > + while (list) { > + struct commit *current = list->item; > + struct commit_list *parent; > + int all_parents_computed = 1; Here all_parents_computed is a boolean flag. I see that it is easier to start with assumption that all parents will have computed generation numbers. > + uint32_t max_generation = 0; The generation number value of 0 functions as sentinel; generation numbers start from 1. Not that it matters much, as lowest possible generation number is 1, and we could have started from that value. > + > + for (parent = current->parents; parent; parent = > parent->next) { > + if (parent->item->generation == > GENERATION_NUMBER_INFINITY || > + parent->item->generation == > GENERATION_NUMBER_ZERO) { > + all_parents_computed = 0; > + commit_list_insert(parent->item, &list); > + break; If some parent doesn't have generation number calculated, we add it to stack (and break out of loop because it is depth-first walk), and mark this situation. All right. > + } else if (parent->item->generation > > max_generation) { > + max_generation = > parent->item->generation; Otherwise, update max_generation. All right. > + } > + } > + > + if (all_parents_computed) { > + current->generation = max_generation + 1; > + pop_commit(&list); > + } > + > + if (current->generation > GENERATION_NUMBER_MAX) > + current->generation = GENERATION_NUMBER_MAX; This conditional should be inside all_parents_computed test, for example like this: + if (all_parents_computed) { + current->generation = max_generation + 1; + if (current->generation > GENERATION_NUMBER_MAX) + current->generation = GENERATION_NUMBER_MAX; + + pop_commit(&list); + } (Noticed by Junio.) Sidenote: when we revisit the commit, returning from depth-first walk of one of its parents, we calculate max_generation from scratch again. This does not matter for performance, as it's just data access and calculating maximum - any workaround to not restart those calculations would take more time and memory. And it's simple. > + } > + } > +} > + > void write_commit_graph(const char *obj_dir, > const char **pack_indexes, > int nr_packs, > @@ -694,6 +737,8 @@ void write_commit_graph(const char *obj_dir, > if (commits.nr >= GRAPH_PARENT_MISSING) > die(_("too many commits to write graph")); > > + compute_generation_numbers(commits.list, commits.nr); > + Nice and simple. All right. I guess that we do not pass "struct packed_commit_list commits" as argument to compute_generation_numbers instead of "struct commit** commits.list" and "int commits.nr" to compute_generation_numbers() to keep the latter nice and generic? > graph_name = get_commit_graph_filename(obj_dir); > fd = hold_lock_file_for_update(&lk, graph_name, 0); Best, -- Jakub Narębski
