Re: The most efficient way to test if repositories share the same objects

From: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
Date: Mar 22, 2018, 21:32 UTC
Message-ID: <906555df-e906-775a-0255-fbc71f7138f6@linuxfoundation.org>
In-Reply-To: <xmqqlgejlx8e.fsf@gitster-ct.c.googlers.com>

On 03/22/18 15:35, Junio C Hamano wrote:

Show 6 quoted lines

> I am not sure how Konstantin defines "the most efficient", but if it
> is "with the smallest number of bits exchanged between the
> repositories", then the answer would probably be to find the root
> commit(s) in each repository and if they share any common root(s).
> If there isn't then there is no hope to share objects between them,
> of course.

Hmm... so, this a cool idea that I'd like to use, but there are two annoying gotchas:

1. I cannot assume that refs/heads/master is meaningful -- my problem is
actually with something like
https://source.codeaurora.org/quic/la/kernel/msm-3.18 -- you will find
that master is actually unborn and there are 7700 other heads (don't get
me started on that unless you're buying me a lot of drinks).

2. Even if there is a HEAD I know I can use, it's pretty slow on large
repos (e.g. linux.git):

$ time git rev-list --max-parents=0 HEAD a101ad945113be3d7f283a181810d76897f0a0d6 cd26f1bd6bf3c73cc5afe848677b430ab342a909 be0e5c097fc206b863ce9fe6b3cfd6974b0110f4 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2

real 0m6.311s user 0m6.153s sys 0m0.110s

If I try to do this for each of the 7700 heads, this will take roughly 12 hours.

My current strategy has been pretty much:

git -C msm-3.10.git show-ref --tags -s | sort -u > /tmp/refs1 git -C msm-3.18.git show-ref --tags -s | sort -u > /tmp/refs2

and then checking if an intersection of these matches at least half of refs in either repo:

---- #/usr/bin/env python import numpy

refs1 = numpy.array(open('/tmp/refs1').readlines()) refs2 = numpy.array(open('/tmp/refs2').readlines())

in_common = len(numpy.intersect1d(refs1, refs2))
if in_common > len(refs1)/2 or in_common > len(refs2)/2:
    print('Lots of shared refs')
else:
    print('None or too few shared refs')
----

This works well enough at least for those repos with lots of shared tags, but will miss potentially large repos where there's only heads that can be pointing at commits that aren't necessarily the same between two repos.

Thanks for your help!

Best,

-- 
Konstantin Ryabitsev
Director, IT Infrastructure Security
The Linux Foundation