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

[程洋 <chengyang@xiaomi.com>]

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 <adlternative@gmail.com>
> Sent: Monday, August 15, 2022 1:16 PM
> To: 程洋 <chengyang@xiaomi.com>
> Cc: Jonathan Tan <jonathantanmy@google.com>; git@vger.kernel.org; 何浩
> <hehao@xiaomi.com>; Xin7 Ma 马鑫 <maxin7@xiaomi.com>; 石奉兵
> <shifengbing@xiaomi.com>; 凡军辉 <fanjunhui@xiaomi.com>; 王汉基
> <wanghanji@xiaomi.com>
> Subject: Re: [External Mail]Re: Partial-clone cause big performance impact on
> server
>
> [外部邮件] 此邮件来源于小米公司外部，请谨慎处理。
>
> 程洋 <chengyang@xiaomi.com> 于2022年8月13日周六 16:00写道：
> >
> > > >     3. with GIT_TRACE_PACKET=1. We found on big repositories
> (200K+refs, 6m+ objects). Git will sends 40k want.
> > > >     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)
> > > >     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.
> > > >
> > > > 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.
> > > > Is it possible to let git only send "commit want" rather than all the
> objects SHA1 one by one?
> > >
> > > 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?
> > 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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