Re: [PATCH] check_everything_connected: assume alternate ref tips are valid

> On Fri, Jun 28, 2019 at 09:22:56AM -0700, Junio C Hamano wrote:
> 
>>>  	argv_array_push(&rev_list.args, "--quiet");
>>> +	argv_array_push(&rev_list.args, "--alternate-refs");
>>>  	if (opt->progress)
>>>  		argv_array_pushf(&rev_list.args, "--progress=%s",
>>>  				 _("Checking connectivity"));
>>
>> Quite honestly, I am very surprised that we did not do this.  The
>> idea of alternate object store, as well as reducing transfer cost by
>> advertising their tips as '.have' phony refs, is almost as old as
>> the pack protocol itself.
> 
> Yeah, as you note we are already telling the other side of the push
> "hey, we already have these objects". So we are almost always just
> walking over our own local objects in the connectivity check, which is
> silly.
> 
> I only did "clone --reference" in the perf test because it was the
> simplest, but a push to a server with alternates should be similarly
> improved. E.g., doing this in a clone of linux.git:
> 
>   git init --bare dst.git
>   echo '../../.git/objects' >dst.git/objects/info/alternates
>   time git push dst.git HEAD
> 
> goes from 40+ seconds to 100ms or so. Again, obviously that's the best
> case, but it should also improve the normal case of somebody pulling
> down "torvalds/linux.git" and pushing it back up to their own
> "peff/linux.git", too.
> 
> I don't have real-world numbers yet from GitHub, because we're not
> actually advertising .haves on push right now. All of the Git pieces are
> now in places to do so, but we still have to make some tweaks at our
> replication layer. But soon. :)