From: Duy Nguyen <pclouds@gmail.com>
Date: Sun, 15 Mar 2015 00:17:07 GMT
Subject: Re: [PATCH v2 01/10] Define a structure for object IDs.
Message-ID: <CACsJy8CK=0QpsMpZKN6Js99xFRD6mpJW1id2BBQDMsV1DaHiPg@mail.gmail.com>
In-Reply-To: <xmqqbnjvw7c8.fsf@gitster.dls.corp.google.com>

On Sun, Mar 15, 2015 at 5:19 AM, Junio C Hamano <gitster@pobox.com> wrote:
> Duy Nguyen <pclouds@gmail.com> writes:
>
>>> Anyway, wouldn't this be all academic?  I do not see how you would
>>> keep the object name in the <pack, nth> format in-core, as the
>>> obj_hash[] is a hashtable keyed by <sha-1>, and even when we switch
>>> to a different hash, I cannot see how such a table to ensure the
>>> singleton-ness of in-core objects can be keyed sometimes by <hash>
>>> and by <pack, nth> in some other time.
>>
>> I'm implementing something to see how much we gain by avoiding object
>> lookup. The current approach is having "struct object ** obj" in
>> "struct packed_git", indexed by "nth". So when you have <pack, nth>
>> and pack->obj[nth] is valid, you'll get to "struct object *" without
>> hashing.
>
> But do you realize that the hashtable serves two purposes?  Grab the
> object from its name is one thing, and the other one I am not seeing
> how you will make it work with "sometimes <sha-1> sometimes <pack,nth>"
> is to ensure that we will have only one in-core copy for the same object.
> We even walk the hashtable when we want to drop the flag bits from
> all in-core objects, so even if you instanciated an in-core object
> without going through the object name layer, the hashtable needs to
> have a pointer to such a pointer, no?

Notice that the first time pack->obj[] is filled using
lookup_object(). So yes, the hash table has all the pointers that
pack->obj[] has. If somebody wants to remove an object out of hash
table, then all pack->obj[] are invalidated, but I don't think we ever
delete objects from hash table.
-- 
Duy