RE: [External Mail]Re: Partial-clone cause big performance impact on server

There is a really easy way to reproduce it

git clone --filter=blob:none -b master "https://android.googlesource.com/platform/prebuilts/gradle-plugin"

Even Google AOSP Gerrit will have this problem. You will find it hang for minutes on checkout

-----Original Message-----
From: ZheNing Hu <redacted>
Sent: Monday, August 15, 2022 1:16 PM
To: 程洋 <redacted>
Cc: Jonathan Tan <redacted>; git@vger.kernel.org; 何浩
[off-list ref]; Xin7 Ma 马鑫 [off-list ref]; 石奉兵
[off-list ref]; 凡军辉 [off-list ref]; 王汉基
[off-list ref]
Subject: Re: [External Mail]Re: Partial-clone cause big performance impact on
server

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程洋 [off-list ref] 于2022年8月13日周六 16:00写道：

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    3. with GIT_TRACE_PACKET=1. We found on big repositories

(200K+refs, 6m+ objects). Git will sends 40k want.

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    4. And we then track our server(which is gerrit with jgit). We found

the server is couting objects. Then we check those 40k objects, most of them
are blobs rather than commit. (which means they're not in bitmap)

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    5. We believe that's the root cause of our problem. Git sends too

many "want SHA1" which are not in bitmap, cause the server to count
objects frequently, which then slow down the server.

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What we want is, download the things we need to checkout to specific

commit. But if one commit contain so many objects (like us , 40k+). It takes
more time to counting than downloading.

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Is it possible to let git only send "commit want" rather than all the

objects SHA1 one by one?

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On a technical level, it may be possible - at the point in the Git
code where the batch prefetch occurs, I'm not sure if we have the
commit, but we could plumb the commit information there. (We have
the tree, but this doesn't help us here because as far as I know,
the tree won't be in the bitmap so the server would need to count
objects anyway, resulting in the same problem.)

However, sending only commits as wants would mean that we would be
fetching more blobs than needed. For example, if we were to clone
(with
checkout) and then checkout HEAD^, sending a "commit want" for the
latter checkout would result in all blobs referenced by the commit's
tree being fetched and not only the blobs that are different.

It seems your solution require changes from both server side and
client side Why not we just add another filter, allow partial-clone always

sends commit level want?

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If we checkout HEAD~1, then client can send "want HEAD~1 HEAD~2".

I am interesting about this question too, maybe I can try if we can do this.. ;-)

ZheNing Hu

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