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commit-graph: introduce commit_graph_data_slab 

The struct commit is used in many contexts. However, members
`generation` and `graph_pos` are only used for commit-graph related
operations and otherwise waste memory.

This wastage would have been more pronounced as we transition to
generation number v2, which uses 64-bit generation number instead of
current 32-bits.

As they are often accessed together, let's introduce struct
commit_graph_data and move them to a commit_graph_data slab.

While the overall test suite runs just as fast as master,
(series: 26m48s, master: 27m34s, faster by 2.87%), certain commands
like `git merge-base --is-ancestor` were slowed by 40% as discovered
by Szeder Gábor [1]. After minimizing commit-slab access, the slow down
persists but is closer to 20%.

Derrick Stolee believes the slow down is attributable to the underlying
algorithm rather than the slowness of commit-slab access [2] and we will
follow-up in a later series.

[1]: https://lore.kernel.org/git/20200607195347.GA8232@szeder.dev/
[2]: https://lore.kernel.org/git/13db757a-9412-7f1e-805c-8a028c4ab2b1@gmail.com/

Signed-off-by: Abhishek Kumar <abhishekkumar8222@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Abhishek Kumar 2020-06-17 14:44:09 +05:30 committed by Junio C Hamano
parent 6da43d937c
commit 4844812b9e
2 changed files with 78 additions and 10 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

78
commit-graph.c
View File

@ -87,6 +87,58 @@ static int commit_pos_cmp(const void *va, const void *vb)
commit_pos_at(&commit_pos, b);
}
define_commit_slab(commit_graph_data_slab, struct commit_graph_data);
static struct commit_graph_data_slab commit_graph_data_slab =
COMMIT_SLAB_INIT(1, commit_graph_data_slab);
uint32_t commit_graph_position(const struct commit *c)
{
struct commit_graph_data *data =
commit_graph_data_slab_peek(&commit_graph_data_slab, c);
return data ? data->graph_pos : COMMIT_NOT_FROM_GRAPH;
}
uint32_t commit_graph_generation(const struct commit *c)
{
struct commit_graph_data *data =
commit_graph_data_slab_peek(&commit_graph_data_slab, c);
if (!data)
return GENERATION_NUMBER_INFINITY;
else if (data->graph_pos == COMMIT_NOT_FROM_GRAPH)
return GENERATION_NUMBER_INFINITY;
return data->generation;
}
static struct commit_graph_data *commit_graph_data_at(const struct commit *c)
{
unsigned int i, nth_slab;
struct commit_graph_data *data =
commit_graph_data_slab_peek(&commit_graph_data_slab, c);
if (data)
return data;
nth_slab = c->index / commit_graph_data_slab.slab_size;
data = commit_graph_data_slab_at(&commit_graph_data_slab, c);
/*
* commit-slab initializes elements with zero, overwrite this with
* COMMIT_NOT_FROM_GRAPH for graph_pos.
*
* We avoid initializing generation with checking if graph position
* is not COMMIT_NOT_FROM_GRAPH.
*/
for (i = 0; i < commit_graph_data_slab.slab_size; i++) {
commit_graph_data_slab.slab[nth_slab][i].graph_pos =
COMMIT_NOT_FROM_GRAPH;
}
return data;
}
static int commit_gen_cmp(const void *va, const void *vb)
{
const struct commit *a = *(const struct commit **)va;
@ -1020,7 +1072,7 @@ static void write_graph_chunk_data(struct hashfile *f, int hash_len,
else
packedDate[0] = 0;
packedDate[0] |= htonl((*list)->generation << 2);
packedDate[0] |= htonl(commit_graph_data_at(*list)->generation << 2);
packedDate[1] = htonl((*list)->date);
hashwrite(f, packedDate, 8);
@ -1251,9 +1303,11 @@ static void compute_generation_numbers(struct write_commit_graph_context *ctx)
_("Computing commit graph generation numbers"),
ctx->commits.nr);
for (i = 0; i < ctx->commits.nr; i++) {
uint32_t generation = commit_graph_data_at(ctx->commits.list[i])->generation;
display_progress(ctx->progress, i + 1);
if (ctx->commits.list[i]->generation != GENERATION_NUMBER_INFINITY &&
ctx->commits.list[i]->generation != GENERATION_NUMBER_ZERO)
if (generation != GENERATION_NUMBER_INFINITY &&
generation != GENERATION_NUMBER_ZERO)
continue;
commit_list_insert(ctx->commits.list[i], &list);
@ -1264,22 +1318,26 @@ static void compute_generation_numbers(struct write_commit_graph_context *ctx)
uint32_t max_generation = 0;
for (parent = current->parents; parent; parent = parent->next) {
if (parent->item->generation == GENERATION_NUMBER_INFINITY ||
parent->item->generation == GENERATION_NUMBER_ZERO) {
generation = commit_graph_data_at(parent->item)->generation;
if (generation == GENERATION_NUMBER_INFINITY ||
generation == GENERATION_NUMBER_ZERO) {
all_parents_computed = 0;
commit_list_insert(parent->item, &list);
break;
} else if (parent->item->generation > max_generation) {
max_generation = parent->item->generation;
} else if (generation > max_generation) {
max_generation = generation;
}
}
if (all_parents_computed) {
current->generation = max_generation + 1;
struct commit_graph_data *data = commit_graph_data_at(current);
data->generation = max_generation + 1;
pop_commit(&list);
if (current->generation > GENERATION_NUMBER_MAX)
current->generation = GENERATION_NUMBER_MAX;
if (data->generation > GENERATION_NUMBER_MAX)
data->generation = GENERATION_NUMBER_MAX;
}
}
}

10
commit-graph.h
View File

@ -135,4 +135,14 @@ void free_commit_graph(struct commit_graph *);
*/
void disable_commit_graph(struct repository *r);
struct commit_graph_data {
uint32_t graph_pos;
uint32_t generation;
};
/*
* Commits should be parsed before accessing generation, graph positions.
*/
uint32_t commit_graph_generation(const struct commit *);
uint32_t commit_graph_position(const struct commit *);
#endif