Re: [External Mail]Re: why git is so slow for a tiny git push?

From: Jeff King <peff@peff.net>
Date: Nov 24, 2021, 18:15 UTC
Message-ID: <YZ6BJpAY4ZAWaqkr@coredump.intra.peff.net>
In-Reply-To: <56994f02ed4542b89cc6f9586d703e05@xiaomi.com>

On Tue, Nov 23, 2021 at 06:42:12AM +0000, 程洋 wrote:

> I got another problem here.
> When I tries to clone from remote server. It took me 25 seconds to enumerating objects. And then 1 second to `couting objects` by bitmap.
> I don't understand, why a fresh clone need `enumerating objects` ? Is `couting objects` enough for the server to determine what to send?

In older versions of Git, the "counting objects" progress meter used to be the actual object graph traversal. That changed in v2.18 (via 5af050437a), but you may still seem some reference to "counting objects is expensive".

These days that is called "enumerating objects", and "counting objects" is just doing a quick-ish pass over that list to do some light analysis (e.g., if we can reuse an on-disk delta). I'd expect "enumerating" to be expensive in general, and "counting" to be quick in general.

The "enumerating" phase is where we determine what to send whether it's for a clone or a fetch, and may involve opening up a bunch of trees to walk the graph. It's what reachability bitmaps are supposed to make faster. But if you have 300k refs, as you've mentioned, you almost certainly don't have complete coverage of all of the ref tips, so we'll have to fallback to doing at least a partial graph traversal.

Taylor (cc'd) has been looking at some tricks for speeding up cases like this with a lot of refs. But I don't think there's anything to show publicly yet.

-Peff