RE: [External Mail]Re: why git is so slow for a tiny git push?

[程洋 <chengyang@xiaomi.com>]

Oh my god.
Jesus. It works for me. After disable writebitmap, time cost decrease from 33 seconds to 0.9 seconds.

But now it turns out that, remote side takes 13 seconds to receive the pack,  since git receive-pack is triggered automatically from remote side, is there anyway to enable GIT_TRACE2_PERF on server side?

-----Original Message-----
From: Jeff King <peff@peff.net>
Sent: Tuesday, October 12, 2021 4:40 PM
To: 程洋 <chengyang@xiaomi.com>
Cc: git@vger.kernel.org
Subject: Re: [External Mail]Re: why git is so slow for a tiny git push?

*This message originated from outside of XIAOMI. Please treat this email with caution*


On Tue, Oct 12, 2021 at 08:04:44AM +0000, 程洋 wrote:

> I have bitmap indeed because my master server also serves as download server.
> However I'm using git 2.17.0, and I didn't set repack.writeBitmaps

On that version and without the config, then perhaps you (or somebody) passed "-b" to git-repack.

> But why bitmaps can cause push to be slow? Do you mean that if
> writeBitmaps is true, every push will regenerate bitmap file? If
> that's what you mean, what I see is the only bitmap file in my repo
> didn't change across time (the modify time is one month ago, long
> before I run the experiment)

No, it is not regenerating the on-disk bitmaps. But when deciding the set of objects to send, pack-objects will generate an internal bitmap which is the set difference of objects reachable from the pushed refs, minus objects reachable from the refs the other the other side told us they had.

It uses the on-disk bitmaps as much as possible, but there may be commits not covered by bitmaps (either because they were pushed since the last repack which built bitmaps, or simply because it's too expensive to put a bitmap on every commit, so we sprinkle them throughout the commit history). In those cases we have to traverse parts of the object graph by walking commits and opening up trees. This can be expensive, and is where your time is going.

Reachability bitmaps _usually_ make things faster, but they have some cases where they make things worse (especially if you have a ton of refs, or haven't repacked recently).

If bitmaps are causing a problem for your push, they are likely to be causing problems for fetches, too. But if you want to keep them to serve fetches, but not use them for push, you should be able to do:

  git -c pack.usebitmaps=false push

-Peff
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