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From: Jakub Narebski <jnareb@gmail.com>
To: Jeff King <peff@peff.net>
Cc: git@vger.kernel.org, "Junio C Hamano" <gitster@pobox.com>,
	"Ted Ts\'o" <tytso@mit.edu>,
	"Jonathan Nieder" <jrnieder@gmail.com>,
	"Ævar Arnfjörð Bjarmason" <avarab@gmail.com>,
	"Clemens Buchacher" <drizzd@aon.at>,
	"Shawn O. Pearce" <spearce@spearce.org>
Subject: Re: Generation numbers and replacement objects
Date: Sat, 16 Jul 2011 23:10:25 +0200
Message-ID: <201107162310.26808.jnareb@gmail.com> (raw)
In-Reply-To: <20110715211033.GA1943@sigill.intra.peff.net>

On Fri, Jul 15, 2011, Jeff King wrote:
> On Fri, Jul 15, 2011 at 02:01:36PM -0700, Jakub Narebski wrote:
> 
> > Peff, as Junio said somewhere else either in this thread, or the one
> > started by Linus, we would want generation numbers both without taking
> > into account replacement objects (e.g. for object traversal during
> > push / fetch), and with taking it into account (e.g. when showing log
> > or blame for end user).
> > 
> > So we would need two generation number caches: one with and one
> > without replaces.
> 
> Right. And I already outlined a solution for that by indexing the caches
> by the validity token (I haven't written the patches yet, but it's a
> pretty trivial change).

Actually we wouldn't probably want a separate cache for each validity
token, but two caches: one with and one without... well, perhaps one per
namespace.  But certainly not one per replacement.
 
> > Nb. generation header stored in commit object can give only the one
> > without replaces, i.e. speed up object enumeration (what happened to
> > caching GSoC project code?) but not git-log.
> 
> Yes. It is a weakness of putting the generation number in the header. I
> think Linus has already said he doesn't care about grafting. You are
> welcome to argue with him about that.

I tried, but he isn't responding to questions about replacement objects.

I can agree that grafts are terrible hack, and for me turning off using
generation numbers if there are grafts is reasonable solution.  Not so
with replace objects.

> > Also if replacement object has the same generation as the commit it
> > replaces, and I think also if it has lower generation number, current
> > generation numbers would still work (ne need to invalidate cache).
> 
> Yes, that is why I said elsewhere "you could be more clever about seeing
> how the cache's validity constraints changed". But ultimately, it is not
> that expensive to regenerate the cache under the new conditions, grafts
> don't change very often, and the code to figure out exactly which parts
> of the cache could be saved would be complex.

True.  Well, at least taking hash of only replacements of commit objects
that change generation number could be a reasonable thing... but probably
too complicated anyway.

-- 
Jakub Narebski
Poland

  reply index

Thread overview: 56+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2011-07-13  6:47 [RFC/PATCHv2 0/6] generation numbers for faster traversals Jeff King
2011-07-13  6:57 ` [RFC/PATCHv2 1/6] decorate: allow storing values instead of pointers Jeff King
2011-07-13 17:52   ` Jonathan Nieder
2011-07-13 20:08     ` Jeff King
2011-07-14 17:34       ` Jeff King
2011-07-14 17:51         ` [PATCH 1/3] implement generic key/value map Jeff King
2011-07-14 18:52           ` Bert Wesarg
2011-07-14 18:54             ` Bert Wesarg
2011-07-14 18:55               ` Jeff King
2011-07-14 19:07                 ` Bert Wesarg
2011-07-14 19:14                   ` Jeff King
2011-07-14 19:18                     ` Bert Wesarg
2011-07-14 17:52         ` [PATCH 2/3] fast-export: use object to uint32 map instead of "decorate" Jeff King
2011-07-15  9:40           ` Sverre Rabbelier
2011-07-15 20:00             ` Jeff King
2011-07-14 17:53         ` [PATCH 3/3] decorate: use "map" for the underlying implementation Jeff King
2011-07-14 21:06         ` [RFC/PATCHv2 1/6] decorate: allow storing values instead of pointers Junio C Hamano
2011-08-04 22:43           ` [RFC/PATCH 0/5] macro-based key/value maps Jeff King
2011-08-04 22:45             ` [PATCH 1/5] implement generic key/value map Jeff King
2011-08-04 22:46             ` [PATCH 2/5] fast-export: use object to uint32 map instead of "decorate" Jeff King
2011-08-04 22:46             ` [PATCH 3/5] decorate: use "map" for the underlying implementation Jeff King
2011-08-04 22:46             ` [PATCH 4/5] map: implement persistent maps Jeff King
2011-08-04 22:46             ` [PATCH 5/5] implement metadata cache subsystem Jeff King
2011-08-05 11:03             ` [RFC/PATCH 0/5] macro-based key/value maps Jeff King
2011-08-05 15:31               ` René Scharfe
2011-08-06  6:30                 ` Jeff King
2011-07-13  7:04 ` [RFC/PATCHv2 2/6] add metadata-cache infrastructure Jeff King
2011-07-13  8:18   ` Bert Wesarg
2011-07-13  8:31     ` Jeff King
2011-07-13  8:45       ` Bert Wesarg
2011-07-13 19:18         ` Jeff King
2011-07-13 19:40       ` Junio C Hamano
2011-07-13 19:33   ` Junio C Hamano
2011-07-13 20:25     ` Jeff King
2011-07-13  7:05 ` [RFC/PATCHv2 3/6] commit: add commit_generation function Jeff King
2011-07-13 14:26   ` Eric Sunshine
2011-07-13  7:05 ` [RFC/PATCHv2 4/6] pretty: support %G to show the generation number of a commit Jeff King
2011-07-13  7:06 ` [RFC/PATCHv2 5/6] check commit generation cache validity against grafts Jeff King
2011-07-13 14:26   ` Eric Sunshine
2011-07-13 19:35     ` Jeff King
2011-07-13  7:06 ` [RFC/PATCHv2 6/6] limit "contains" traversals based on commit generation Jeff King
2011-07-13  7:23   ` Jeff King
2011-07-13 20:33     ` Junio C Hamano
2011-07-13 20:58       ` Jeff King
2011-07-13 21:12         ` Junio C Hamano
2011-07-13 21:18           ` Jeff King
2011-07-15 18:22   ` Junio C Hamano
2011-07-15 20:40     ` Jeff King
2011-07-15 21:04       ` Junio C Hamano
2011-07-15 21:14         ` Jeff King
2011-07-15 21:01 ` Generation numbers and replacement objects Jakub Narebski
2011-07-15 21:10   ` Jeff King
2011-07-16 21:10     ` Jakub Narebski [this message]
2011-08-04 22:48 [RFC/PATCH 0/2] patch-id caching Jeff King
2011-08-04 22:49 ` [PATCH 1/2] cherry: read default config Jeff King
2011-08-04 22:49 ` [PATCH 2/2] cache patch ids on disk Jeff King
2011-08-04 22:52   ` Jeff King

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