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pack-objects: don't traverse objects unnecessarily

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This brings back some of the performance lost in optimizing recency
order inside pack objects. We were doing extreme amounts of object
re-traversal: for the 2.14 million objects in the Linux kernel
repository, we were calling add_to_write_order() over 1.03 billion times
(a 0.2% hit rate, making 99.8% of of these calls extraneous).

Two optimizations take place here- we can start our objects array
iteration from a known point where we left off before we started trying
to find our tags, and we don't need to do the deep dives required by
add_family_to_write_order() if the object has already been marked as
filled.

These two optimizations bring some pretty spectacular results via `perf
stat`:

task-clock:   83373 ms        --> 43800 ms         (50% faster)
cycles:       221,633,461,676 --> 116,307,209,986  (47% fewer)
instructions: 149,299,179,939 --> 122,998,800,184  (18% fewer)

Helped-by: Ramsay Jones (format string fix in "die" message)
Signed-off-by: Dan McGee <dpmcgee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Dan McGee 2011-10-18 00:21:23 -05:00 committed by Junio C Hamano
parent f380872f0a
commit 38d4debb6d
1 changed files with 12 additions and 6 deletions
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18
builtin/pack-objects.c
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@ -520,7 +520,7 @@ static void add_family_to_write_order(struct object_entry **wo,
static struct object_entry **compute_write_order(void)
{
unsigned int i, wo_end;
unsigned int i, wo_end, last_untagged;
struct object_entry **wo = xmalloc(nr_objects * sizeof(*wo));
@ -551,7 +551,7 @@ static struct object_entry **compute_write_order(void)
for_each_tag_ref(mark_tagged, NULL);
/*
* Give the commits in the original recency order until
* Give the objects in the original recency order until
* we see a tagged tip.
*/
for (i = wo_end = 0; i < nr_objects; i++) {
@ -559,6 +559,7 @@ static struct object_entry **compute_write_order(void)
break;
add_to_write_order(wo, &wo_end, &objects[i]);
}
last_untagged = i;
/*
* Then fill all the tagged tips.
@ -571,7 +572,7 @@ static struct object_entry **compute_write_order(void)
/*
* And then all remaining commits and tags.
*/
for (i = 0; i < nr_objects; i++) {
for (i = last_untagged; i < nr_objects; i++) {
if (objects[i].type != OBJ_COMMIT &&
objects[i].type != OBJ_TAG)
continue;
@ -581,7 +582,7 @@ static struct object_entry **compute_write_order(void)
/*
* And then all the trees.
*/
for (i = 0; i < nr_objects; i++) {
for (i = last_untagged; i < nr_objects; i++) {
if (objects[i].type != OBJ_TREE)
continue;
add_to_write_order(wo, &wo_end, &objects[i]);
@ -590,8 +591,13 @@ static struct object_entry **compute_write_order(void)
/*
* Finally all the rest in really tight order
*/
for (i = 0; i < nr_objects; i++)
add_family_to_write_order(wo, &wo_end, &objects[i]);
for (i = last_untagged; i < nr_objects; i++) {
if (!objects[i].filled)
add_family_to_write_order(wo, &wo_end, &objects[i]);
}
if (wo_end != nr_objects)
die("ordered %u objects, expected %"PRIu32, wo_end, nr_objects);
return wo;
}