Re: upload-pack is slow with lots of refs

On Wed, Oct 3, 2012 at 8:03 PM, Jeff King [off-list ref] wrote:

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

quoted

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.

I can't provide all the details now (not with access to that machine
now), but briefly:

 * The git client/server version is 1.7.8

 * The repository has around 50k refs, they're "real" refs, almost all
   of them (say all but 0.5k-1k) are annotated tags, the rest are
   branches.

 * >99% of them are packed, there's a weekly cronjob that packs them
   all up, there were a few newly pushed branches and tags outside of
   the

 * I tried "echo -n | git upload-pack <repo>" on both that 50k
   repository and a repository with <100 refs, the former took around
   ~1-2s to run on a 24 core box and the latter ~500ms.

 * When I ran git-upload-pack with GNU parallel I managed around 20/s
   packs on the 24 core box on the 50k ref one, 40/s on the 100 ref
   one.

 * A co-worker who was working on this today tried it on 1.7.12 and
   claimed that it had the same performance characteristics.

 * I tried to profile it under gcc -pg && echo -n | ./git-upload-pack
   <repo> but it doesn't produce a profile like that, presumably
   because the process exits unsuccessfully.

   Maybe someone here knows offhand what mock data I could feed
   git-upload-pack to make it happy to just list the refs, or better
   yet do a bit more work which it would do if it were actually doing
   the fetch (I suppose I could just do a fetch, but I wanted to do
   this from a locally compiled checkout).

quoted

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.

I suppose at least for the ssh protocol we could just do:

    ssh server "(git upload-pack <repo> --refs=* || git upload-pack <repo>)"

And something similar with HTTP headers, but that of course leaves the
git:// protocol.