From: Ramsay Jones <ramsay@ramsay1•demon.co.uk>
To: Jeff King <peff@peff•net>
Cc: Junio C Hamano <gitster@pobox•com>,
GIT Mailing-list <git@vger•kernel.org>
Subject: Re: [PATCH v3 2/2] alloc.c: remove the redundant commit_count variable
Date: Fri, 11 Jul 2014 10:41:32 +0100 [thread overview]
Message-ID: <53BFB14C.10000@ramsay1.demon.co.uk> (raw)
In-Reply-To: <20140711083220.GA5407@sigill.intra.peff.net>
On 11/07/14 09:32, Jeff King wrote:
> On Fri, Jul 11, 2014 at 01:59:53AM +0100, Ramsay Jones wrote:
>
>>> The code you're touching here was trying to make sure that each commit
>>> gets a unique index, under the assumption that commits only get
>>> allocated via alloc_commit_node. But I think that assumption is wrong.
>>> We can also get commit objects by allocating an OBJ_NONE (e.g., via
>>> lookup_unknown_object) and then converting it into an OBJ_COMMIT when we
>>> find out what it is.
>>
>> Hmm, I don't know how the object is converted, but the object allocator
>> is actually allocating an 'union any_object', so it's allocating more
>> space than for a struct object anyway.
>
> Right, we would generally want to avoid lookup_unknown_object where we
> can for that reason.
>
>> If you add an 'index' field to struct object, (and remove it from
>> struct commit) it could be set in alloc_object_node(). ie _all_ node
>> types get an index field.
>
> That was something I considered when we did the original commit-slab
> work, as it would let you do similar tricks for any set of objects, not
> just commits. The reasons against it are:
>
> 1. It would bloat the size of blob and tree structs by at least 4
> bytes (probably 8 for alignment). In most repos, commits make up
> only 10-20% of the total objects (so for linux.git, we're talking
> about 25MB extra in the working set).
>
> 2. It makes single types sparse in the index space. In cases where you
> do just want to keep data on commits (and that is the main use),
> you end up allocating a slab entry per object, rather than per
> commit. That wastes memory (much worse than 25MB if your slab items
> are large), and reduces cache locality.
>
Ahem, yeah I keep telling myself not to post at 2am ... ;-)
Although I haven't given this too much extra thought, I'm beginning
to think that your best course would be to revert commit 969eba63
and add an convert_object_to_commit() function to commit.c which
would set c->index. Then track down each place an OBJ_NONE gets
converted to an OBJ_COMMIT and insert a call to
convert_object_to_commit(). (which may do more than just set the
index field ...)
Hmm, I've just noticed a new series in my in-box. I guess I will
discover what you decided to do shortly ... ;-P
ATB,
Ramsay Jones
next prev parent reply other threads:[~2014-07-11 9:41 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-07-10 23:59 [PATCH v3 2/2] alloc.c: remove the redundant commit_count variable Ramsay Jones
2014-07-11 0:30 ` Jeff King
2014-07-11 0:59 ` Ramsay Jones
2014-07-11 8:32 ` Jeff King
2014-07-11 9:41 ` Ramsay Jones [this message]
2014-07-11 8:41 ` [PATCH 0/7] ensure index is set for all OBJ_COMMIT objects variable Jeff King
2014-07-11 8:42 ` [PATCH 1/7] alloc.c: remove the alloc_raw_commit_node() function Jeff King
2014-07-11 8:46 ` [PATCH 2/7] move setting of object->type to alloc_* functions Jeff King
2014-07-12 14:44 ` Ramsay Jones
2014-07-12 18:05 ` Jeff King
2014-07-13 6:41 ` Jeff King
2014-07-13 6:41 ` [PATCH v2 1/8] alloc.c: remove the alloc_raw_commit_node() function Jeff King
2014-07-15 20:06 ` Junio C Hamano
2014-07-13 6:41 ` [PATCH v2 2/8] alloc: write out allocator definitions Jeff King
2014-07-15 20:11 ` Junio C Hamano
2014-07-13 6:41 ` [PATCH v2 3/8] move setting of object->type to alloc_* functions Jeff King
2014-07-15 20:12 ` Junio C Hamano
2014-07-13 6:42 ` [PATCH v2 4/8] parse_object_buffer: do not set object type Jeff King
2014-07-13 6:42 ` [PATCH v2 5/8] add object_as_type helper for casting objects Jeff King
2014-07-13 6:42 ` [PATCH v2 6/8] alloc: factor out commit index Jeff King
2014-07-13 6:42 ` [PATCH v2 7/8] object_as_type: set " Jeff King
2014-07-13 6:42 ` [PATCH v2 8/8] diff-tree: avoid lookup_unknown_object Jeff King
2014-07-13 19:27 ` [PATCH 2/7] move setting of object->type to alloc_* functions Ramsay Jones
2014-07-14 5:57 ` Jeff King
2014-07-14 11:03 ` Ramsay Jones
2014-07-12 14:55 ` Ramsay Jones
2014-07-12 18:07 ` Jeff King
2014-07-11 8:46 ` [PATCH 3/7] parse_object_buffer: do not set object type Jeff King
2014-07-11 8:48 ` [PATCH 4/7] add object_as_type helper for casting objects Jeff King
2014-07-11 10:45 ` Ramsay Jones
2014-07-11 16:59 ` Jeff King
2014-07-11 8:48 ` [PATCH 5/7] alloc: factor out commit index Jeff King
2014-07-11 8:49 ` [PATCH 6/7] object_as_type: set " Jeff King
2014-07-11 8:50 ` [PATCH 7/7] diff-tree: avoid lookup_unknown_object Jeff King
2014-07-11 10:31 ` [PATCH 0/7] ensure index is set for all OBJ_COMMIT objects variable Ramsay Jones
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