Re: Partial-clone cause big performance impact on server

[Derrick Stolee <derrickstolee@github.com>]

On 8/14/2022 2:48 AM, Jeff King wrote:
> On Thu, Aug 11, 2022 at 08:09:56AM +0000, 程洋 wrote:
> 
>>     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.

I just remembered that Gerrit specifically has branch-level security,
where some branches are not visible to all users. For that reason, blobs
cannot be served without first determining if they are reachable from a
branch visible to the current user.

I'm not sure if that's the problem in this particular case, but it could
be.

Thanks,
-Stolee