It is not unreasonable to ask cat-file for a batch-check format of simply "%(objectname)". At first glance this seems like a noop (you are generally already feeding the object names on stdin!), but it has a few uses:
1. With --batch-all-objects, you can generate a listing of
the sha1s present in the repository, without any input. 2. You do not have to feed sha1s; you can feed arbitrary
sha1 expressions and have git resolve them en masse. 3. You can even feed a raw sha1, with the result that git
will tell you whether we actually have the object or
not.In case 3, the call to sha1_object_info is useful; it tells us whether the object exists or not (technically we could swap this out for has_sha1_file, but the cost is roughly the same).
In case 2, the existence check is of debatable value. A mass-resolution might prefer performance to safety (against outputting a value for a corrupted ref, for example). However, the object lookup cost is likely not as noticeable compared to the resolution cost. And since we have provided that safety in the past, the conservative choice is to keep it.
In case 1, though, the object lookup is a definite noop; we know about the object because we found it in the object database. There is no new information gained by making the call.
This patch detects that case and optimizes out the call. Here are best-of-five timings for linux.git:
[before]
$ time git cat-file --buffer \
--batch-all-objects \
--batch-check='%(objectname)'
real 0m2.117s
user 0m2.044s
sys 0m0.072s [after]
$ time git cat-file --buffer \
--batch-all-objects \
--batch-check='%(objectname)'
real 0m1.230s
user 0m1.176s
sys 0m0.052sThere are two implementation details to note here.
One is that we detect the noop case by seeing that "struct object_info" does not request any information. But besides object existence, there is one other piece of information which sha1_object_info may fill in: whether the object is cached, loose, or packed. We don't currently provide that information in the output, but if we were to do so later, we'd need to take note and disable the optimization in that case.
And that leads to the second note. If we were to output that information, a better implementation would be to remember where we saw the object in --batch-all-objects in the first place, and avoid looking it up again by sha1.
In fact, we could probably squeeze out some extra performance for less-trivial cases, too, by remembering the pack location where we saw the object, and going directly there to find its information (like type, size, etc). That would in theory make this optimization unnecessary.
I didn't pursue that path here for two reasons:
1. It's non-trivial to implement, and has memory
implications. Because we sort and de-dup the list of
output sha1s, we'd have to record the pack information
for each object, too. 2. It doesn't save as much as you might hope. It saves the
find_pack_entry() call, but getting the size and type
for deltified objects requires walking down the delta
chain (for the real type) or reading the delta data
header (for the size). These costs tend to dominate the
non-trivial cases.By contrast, this optimization is easy and self-contained, and speeds up a real-world case I've used.
Signed-off-by: Jeff King <peff@peff.net>
---
builtin/cat-file.c | 17 ++++++++++++++++-
1 file changed, 16 insertions(+), 1 deletion(-)
Show changes to builtin/cat-file.c +16 −1
diff --git a/builtin/cat-file.c b/builtin/cat-file.c
index 54db118..5d3180e 100644
--- a/builtin/cat-file.c
+++ b/builtin/cat-file.c
@@ -154,6 +154,13 @@ struct expand_data {
* elements above, so you can retrieve the response from there.
*/
struct object_info info;
+
+ /*
+ * This flag will be true if the requested batch format and options
+ * don't require us to call sha1_object_info, which can then be
+ * optimized out.
+ */
+ unsigned skip_object_info : 1;
};
static int is_atom(const char *atom, const char *s, int slen)
@@ -258,7 +265,8 @@ static void batch_object_write(const char *obj_name, struct batch_options *opt,
{
struct strbuf buf = STRBUF_INIT;
- if (sha1_object_info_extended(data->sha1, &data->info, LOOKUP_REPLACE_OBJECT) < 0) {
+ if (!data->skip_object_info &&
+ sha1_object_info_extended(data->sha1, &data->info, LOOKUP_REPLACE_OBJECT) < 0) {
printf("%s missing\n", obj_name ? obj_name : sha1_to_hex(data->sha1));
fflush(stdout);
return;
@@ -369,6 +377,13 @@ static int batch_objects(struct batch_options *opt)
strbuf_expand(&buf, opt->format, expand_format, &data);
data.mark_query = 0;
+ if (opt->all_objects) {
+ struct object_info empty;
+ memset(&empty, 0, sizeof(empty));
+ if (!memcmp(&data.info, &empty, sizeof(empty)))
+ data.skip_object_info = 1;
+ }
+
/*
* If we are printing out the object, then always fill in the type,
* since we will want to decide whether or not to stream.
Jeff King