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

On Sun, Sep 25, 2011 at 10:43 PM, Martin Fick [off-list ref] wrote:

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?

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 [off-list ref]
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.