Re: Compressing packed-refs

From: Jeff King <peff@peff.net>
Date: Jul 17, 2020, 06:27 UTC
Message-ID: <20200717062723.GA1179001@coredump.intra.peff.net>
In-Reply-To: <20200716225429.i7pb6xorkwdsf5fn@chatter.i7.local>

On Thu, Jul 16, 2020 at 06:54:29PM -0400, Konstantin Ryabitsev wrote:

Show 11 quoted lines

> Let me offer a more special-case (but not crazy) example from 
> git.kernel.org. The newer version of grokmirror that I'm working on is 
> built to take advantage of the pack-islands feature that was added a 
> while back. We fetch all linux forks into a single "object storage" 
> repo, with each fork going into its own 
> refs/virtual/[uniquename]/(heads|tags) place. This means there's lots of 
> duplicates in packed-refs, as all the tags from torvalds/linux.git will 
> end up duplicated in almost every fork.
> 
> So, after running git pack-refs --all, the packed-refs file is 50-ish MB 
> in size, with a lot of same stuff like:

We do the same thing at GitHub. 50MB is on the easy side. We have cases in the gigabytes.

Show 9 quoted lines

> etc, duplicated 600 times with each fork. It compresses decently well 
> with gzip -9, and *amazingly* well with xz -9:
> 
> $ ls -ahl packed-refs
> -rw-r--r--. 1 mirror mirror 46M Jul 16 22:37 packed-refs
> $ ls -ahl packed-refs.gz
> -rw-r--r--. 1 mirror mirror 19M Jul 16 22:47 packed-refs.gz
> $ ls -ahl packed-refs.xz
> -rw-r--r--. 1 mirror mirror 2.3M Jul 16 22:47 packed-refs.xz

Yes, it does compress well. Just gzipping like that would have worked once upon a time, when accessing it meant reading the whole thing linearly. These days, though, we mmap() the file and binary-search it. That lets us examine a subset of the refs quickly (this is from our torvalds/linux.git fork network):

  $ wc -c packed-refs 
  2394623761 packed-refs

  $ time git for-each-ref | wc -l
  19552978
  
  real	1m12.297s
  user	1m2.441s
  sys	0m10.235s

  $ time git for-each-ref refs/remotes/2325298 | wc -l
  2590
  
  real	0m0.077s
  user	0m0.025s
  sys	0m0.055s

Show 6 quoted lines

> Which really just indicates how much duplicated data is in that file. If 
> reftables will eventually replace refs entirely, then we probably 
> shouldn't expend too much effort super-optimizing it, especially if I'm 
> one of the very few people who would benefit from it. However, I'm 
> curious if a different sorting strategy would help remove most of the 
> duplication without requiring too much engineering time.

You definitely could store it in a more efficient way. Reftables will have most of the things you'd want: prefix compression, binary oids, etc. I wouldn't be opposed to a tweak to packed-refs in the meantime if it was simple to implement. But definitely we'd want to retain the ability to find a subset of refs in sub-linear time. That might get tricky and push it from "simple" to "let's just invest in reftable".

You might also consider whether you need all of those refs at all in the object storage repo. The main uses are:

  - determining reachability during repacks; but you could generate this
    on the fly from the refs in the individual forks (de-duplicating as
    you go). We don't do this at GitHub, because the information in the
    duplicates is useful to our delta-islands config.

  - getting new objects into the object store. It sounds like you might
    do this with "git fetch", which does need up-to-date refs. We used
    to do that, too, but it can be quite slow. These days we migrate the
    objects directly via hardlinks, and then use "update-ref --stdin" to
    sync the refs into the shared storage repo.

  - advertising alternate ref tips in receive-pack (i.e., saying "we
    already know about object X" if it's in somebody else's fork, which
    means people pulling from Linus and then pushing to their fork don't
    have to send the objects again). You probably don't want to
    advertise all of them (just sifting the duplicates is too
    expensive). We use core.alternateRefsCommand to pick out just the
    ones from the parent fork. We _do_ still use the copy of the refs in
    our shared storage, not the ones in the actual fork. But that's
    because we migrate objects to shared storage asynchronously (so it's
    possible for one fork to have refs pointing to objects that aren't
    yet available to the other forks).

So it's definitely not a no-brainer, but possibly something to explore.

-Peff