Re: upload-pack is slow with lots of refs

[Jeff King <peff@peff.net>]

On Wed, Oct 03, 2012 at 02:36:00PM +0200, Ævar Arnfjörð Bjarmason wrote:

> I'm creating a system where a lot of remotes constantly fetch from a
> central repository for deployment purposes, but I've noticed that even
> with a remote.$name.fetch configuration to only get certain refs a
> "git fetch" will still call git-upload pack which will provide a list
> of all references.
> 
> This is being done against a repository with tens of thousands of refs
> (it has a tag for each deployment), so it ends up burning a lot of CPU
> time on the uploader/receiver side.

Where is the CPU being burned? Are your refs packed (that's a huge
savings)? What are the refs like? Are they .have refs from an alternates
repository, or real refs? Are they pointing to commits or tag objects?

What version of git are you using?  In the past year or so, I've made
several tweaks to speed up large numbers of refs, including:

  - cff38a5 (receive-pack: eliminate duplicate .have refs, v1.7.6); note
    that this only helps if they are being pulled in by an alternates
    repo. And even then, it only helps if they are mostly duplicates;
    distinct ones are still O(n^2).

  - 7db8d53 (fetch-pack: avoid quadratic behavior in remove_duplicates)
    a0de288 (fetch-pack: avoid quadratic loop in filter_refs)
    Both in v1.7.11. I think there is still a potential quadratic loop
    in mark_complete()

  - 90108a2 (upload-pack: avoid parsing tag destinations)
    926f1dd (upload-pack: avoid parsing objects during ref advertisement)
    Both in v1.7.10. Note that tag objects are more expensive to
    advertise than commits, because we have to load and peel them.

Even with those patches, though, I found that it was something like ~2s
to advertise 100,000 refs.

> Has there been any work on extending the protocol so that the client
> tells the server what refs it's interested in?

I don't think so. It would be hard to do in a backwards-compatible way,
because the advertisement is the first thing the server says, before it
has negotiated any capabilities with the client at all.

-Peff