Re: Git is not scalable with too many refs/*

From: Christian Couder <christian.couder@gmail.com>
Date: Sep 26, 2011, 12:41 UTC
Message-ID: <CAP8UFD3TWQHU0wLPuxMDnc3bRSz90Yd+yDMBe03kofeo-nr7yA@mail.gmail.com>
In-Reply-To: <201109251443.28243.mfick@codeaurora.org>

On Sun, Sep 25, 2011 at 10:43 PM, Martin Fick <mfick@codeaurora.org> wrote:

Show 6 quoted lines

> A coworker of mine pointed out to me that a simple
>
>  git checkout
>
> can also take rather long periods of time > 3 mins when run
> on a repo with ~100K refs.

Are all these refs packed?

Show 13 quoted lines

> While this is not massive like the other problem I reported,
> it still seems like it is more than one would expect.  So, I
> tried an older version of git, and to my surprise/delight,
> it was much faster (.2s).  So, I bisected this issue also,
> and it seems that the "offending" commit is
> 680955702990c1d4bfb3c6feed6ae9c6cb5c3c07:
>
> commit 680955702990c1d4bfb3c6feed6ae9c6cb5c3c07
> Author: Christian Couder <chriscool@tuxfamily.org>
> Date:   Fri Jan 23 10:06:53 2009 +0100
>
>    replace_object: add mechanism to replace objects found
> in "refs/replace/"

[...]

> Now, I suspect this commit is desirable, but I was hoping
> that perhaps a look at it might inspire someone to find an
> obvious problem with it.

I don't think there is an obvious problem with it, but it would be nice if you could dig a bit deeper.

The first thing that could take a lot of time is the call to for_each_replace_ref() in this function:

+static void prepare_replace_object(void)
+{
+       static int replace_object_prepared;
+
+       if (replace_object_prepared)
+               return;
+
+       for_each_replace_ref(register_replace_ref, NULL);
+       replace_object_prepared = 1;
+}

Another thing is calling replace_object_pos() repeatedly in lookup_replace_object().

Thanks, Christian.