Re: [RFC] send-pack: allow skipping delta when sending pack

From: Junio C Hamano <junkio@cox.net>
Date: May 21, 2006, 08:31 UTC
Message-ID: <7vlksvzsmd.fsf@assigned-by-dhcp.cox.net>
In-Reply-To: <20060521081435.GA4526@coredump.intra.peff.net>

Jeff King <peff@peff.net> writes:

Show 15 quoted lines

> On Sat, May 20, 2006 at 11:17:42PM -0700, Junio C Hamano wrote:
>
>> base delta for that object to skip computation".  What you want
>> here is "if the object we are going to send is not a delta in
>> the source, and there are sufficient number of other objects the
>> object could have been deltified against, then it is very likely
>> that it was not worth deltifying when it was packed; so it is
>> probably not worth deltifying it now".
>
> I think we can make a stronger statement in many cases: "if the object
> we are going to send is not a delta in the source, and there are no
> other objects it could be deltified against, then it is not worth
> deltifying." That is, in the case that we just packed we KNOW that it's
> not worth it, since we're not sending anything that isn't already
> packed.

Careful.

We do not delta an otherwise perfectly deltifiable object if its delta base happens to be at the depth edge in the original pack. So no, we do _NOT_ know if it is not worth it merely from the fact that it is not deltified in the existing pack. And the latter part of your test "there are no other objects it could be deltified against" is either expensive (you have to try first to see if that is the case to really see it) or stupid (you just assume there is no suitable delta base without looking at other objects like we currently do).