On Thu, Sep 17, 2020 at 10:20:03AM +0200, Rasmus Villemoes wrote:
> Hi,
>
> While poking around the code, I noticed that it seems
> ->approximate_object_count_valid is never set to 1, and it never has
> been, not even back when it was a global variable. So perhaps it can
> just be removed and the logic depending on it simplified? Or am I
> missing some preprocessor trickery.
>
> Nobody seems to have noticed the lack of caching - and actually setting
> it to 1 after the count has been computed might be a little dangerous
> unless one takes care to invalidate the cache anywhere that might be
> relevant.
We should be able to construct a test where it matters. The main cost
that the flag is overcoming is the iteration through the packs. So we'd
want a lot of packs. And the primary place the function would get called
a lot is when abbreviating commits. So doing:
for i in $(seq 1000); do
echo blob
echo 'data <<EOF'
echo $i
echo EOF
echo checkpoint
done | git -c transfer.unpacklimit=0 fast-import
will get us a lot of packs. I tried that in linux.git. And then this
should get us a baseline for how much it costs to traverse and print out
object names:
$ time git rev-list --format=%H HEAD >/dev/null
real 0m6.636s
user 0m6.492s
sys 0m0.144s
And now let's see how long it takes with abbreviation:
$ time git rev-list --format=%h HEAD >/dev/null
real 0m34.518s
user 0m34.253s
sys 0m0.264s
Yow. That's a lot. But part of the cost is that we have to look up each
abbreviated hash in each pack to see if it's present there, so we'd
expect it to be a lot more expensive. But let's try it with the caching
flag:
diff --git a/packfile.c b/packfile.c
index 9ef27508f2..e69012e7f2 100644
--- a/packfile.c
+++ b/packfile.c
@@ -923,6 +923,7 @@ unsigned long repo_approximate_object_count(struct repository *r)
count += p->num_objects;
}
r->objects->approximate_object_count = count;
+ r->objects->approximate_object_count_valid = 1;
}
return r->objects->approximate_object_count;
}
$ time git rev-list --format=%h HEAD >/dev/null
real 0m29.411s
user 0m29.150s
sys 0m0.260s
Still not great, but caching the count did save us 15%. That seems worth
it to me (1000 packs is more than we'd hope for, but not uncommon in a
poorly maintained repo). The failure to set the flag is just a bug;
looks like mine from 8e3f52d778 (find_unique_abbrev: move logic out of
get_short_sha1(), 2016-10-03).
You're right that caching runs the risk of the cache being invalidated.
But in this case I think we're covered. We'd generally modify packed_git
only via reprepare_packed_git(), prepare_packed_git(), and we do reset
the flag there. Plus count is only meant to be approximate, so even if
it ended up stale within a single process, I don't think it would be
that big a deal.
-Peff
