I came across an interesting shallow-fetch case, and I suspect git is
doing something very sub-optimal.
You can reproduce with:
git clone --bare git://github.com/CocoaPods/Specs
cd Specs.git
time git pack-objects --revs --thin --stdout \
--delta-base-offset --include-tag <<\EOF | wc -c
d7a6d9295d718c6015be496880f1a293bdd89185
--not
067f265bb512c95b22b83ccd121b9facbddcf6b1
EOF
time git pack-objects --revs --thin --stdout --shallow \
--delta-base-offset --include-tag <<\EOF | wc -c
--shallow ecd7ea6033fe8a05d5c21f3a54355fded6942659
d7a6d9295d718c6015be496880f1a293bdd89185
--not
067f265bb512c95b22b83ccd121b9facbddcf6b1
EOF
The first is a non-shallow clone; it takes a few hundred milliseconds to
generate the pack, and the result is a few hundred kilobytes. The second
is a shallow clone (logged from a real-world request); it sends 200
times as many objects, totaling 270MB, and takes almost a minute of CPU.
I'm trying to figure out why that is, and whether git can do better.
I think what's happening here is that the history looks like this:
F ... ecd7ea6 ... 067f265 ... M ... d7a6d929
\ /
X ....................... Y
That is, we are asking for 067f265 to d7a6d929, but that includes some
merge M which _crosses_ our grafted shallow-point ecd7ea6. That pulls in
essentially all of the history for the entire (missing only the commits
from the fork point F up to ecd7ea6).
So I _think_ that pack-objects is doing the best it can with the
information it was given. But presumably in a shallow repo the user
would prefer to have a segmented history rather than pull in all of
those old commits.
I don't know how the client invoked git, but we can guess what happened
and simulate with:
git tag shallow ecd7ea6033fe8a05d5c21f3a54355fded6942659
git tag old 067f265bb512c95b22b83ccd121b9facbddcf6b1
git tag new d7a6d9295d718c6015be496880f1a293bdd89185
git clone --no-local --bare --branch=shallow --depth=1 . clone.git
cd clone.git
git fetch origin old:refs/tags/old
git fetch origin new:refs/tags/new
Of the two follow-up fetches in the clone, the first is reasonably fast
(it just grabs a few new commits on top of the shallow base), but the
second is expensive (it grabs the merge which pulls in the whole
history). If we add "--depth=1" to each of those fetches, everything
remains fast.
Is this user error to call "git fetch" without "--depth" in the
subsequent cases? Or should git remember that we are in a shallow repo,
and presume that the user by default wants to keep things shallow?
-Peff
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