Re: [RFC/PATCH 0/3] protocol v2

From: Junio C Hamano <gitster@pobox.com>
Date: Mar 2, 2015, 03:10 UTC
Message-ID: <xmqqioekf629.fsf@gitster.dls.corp.google.com>
In-Reply-To: <alpine.DEB.2.02.1503011707040.12474@nftneq.ynat.uz>

David Lang <david@lang.hm> writes:

Show 5 quoted lines

> how would these approaches be affected by a client that is pulling
> from different remotes into one local repository? For example, pulling
> from the main kernel repo and from the -stable repo.
>
> David Lang

As I said in $gmane/264000, which the above came from:

    Note that the above would work if and only if we accept that it
    is OK to send objects between the remote tracking branches the
    fetcher has (i.e. the objects it last fetched from the server)
    and the current tips of branches the server has, without
    optimizing by taking into account that some commits in that set
    may have already been obtained by the fetcher from a
    third-party.

The scheme tries go gain by reducing the ref advertisement cost by sacrificing the optimization opportunity when you fetch from updated Linus's tree after having fetched from a recent next tree, the latter of which may have contained a lot of objects that went to the Linus's tree since you fetched from Linus's the last time. The current protocol, by negotiating what you have (including the objects you obtained from sideways via 'next') with the Linus's tree, allows the server to compute a minimum packfile customized just for you. By trading that off with "everybody that follow this repository will get the same set of packfiles in sequence trickled into his repository" model, it would instead allow the server to prepare these packfiles thousands of clients that follow Linus's tree will want only once.

The client-server pair may want to have a negociation mechanism (e.g. "I may have many objects I fetched from sideways, give me minimum pack that is customized for me by spending cycles---I am willing to wait until you finish computing it" vs "I am just following along and not doing anything fancy, just give me the same thing as everybody else") to select what optimization they want to use.