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

[程洋 <chengyang@xiaomi.com>]

>
> >     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.
>
> I'd be surprised if bitmaps make a big difference either way here, since blobs
> are very quick in the "counting" phase of pack-objects. They can't link to
> anything else, so we should not be opening the object contents at all! We
> just need to find them on disk, and then in many cases we can send them
> over the wire without even decompressing (the exception is if they are
> stored as deltas against an object the client doesn't have).
>
> I didn't generate a test case, but I'm pretty sure that is how git.git's pack-
> objects should behave. But you mentioned that the server is jgit; it's possible
> that it isn't as optimized in that area.

At first I also think it's some implementation bugs by jgit. However I can also reproduce it on cgit. Here is the steps, I'm not sure if you can reproduce too.

1. Clone a repository from AOSP to local machine:  `git clone "https://android.googlesource.com/platform/prebuilts/gradle-plugin"`
2. try to clone from localhost using cgit server.   `GIT_TRACE_PACKET=1 git clone --filter=blob:none -b master user@localhost:/home/user/repositories/gradle-plugin `
3. During checkout phase, it also takes 15 seconds before actual downloading.

It's really easy to reproduce, as long as the repository has so many blobs to checkout

>
> -Peff
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