Re: Git commit generation numbers

From: Phil Hord <hordp@cisco.com>
Date: Jul 21, 2011, 00:39 UTC
Message-ID: <4E277540.7080408@cisco.com>
In-Reply-To: <alpine.DEB.2.02.1107201714140.6412@asgard.lang.hm>

On 07/20/2011 08:18 PM, david@lang.hm wrote:

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> On Wed, 20 Jul 2011, Phil Hord wrote:
>
>> On 07/20/2011 07:36 PM, Nicolas Pitre wrote:
>>> On Wed, 20 Jul 2011, david@lang.hm wrote:
>>>
>>>> If the generation number is part of the repository then it's going to
>>>> be the same for everyone.
>>> The actual generation number will be, and has to be, the same for
>>> everyone with the same repository content, regardless of the cache 
>>> used.
>>> It is a well defined number with no room to interpretation.
>>
>> Nonsense.
>>
>> Even if the generation number is well-defined and shared by all 
>> clients, the only quasi-essential definition is "for each A in 
>> ancestors_of(B), gen(A) < gen(B)".
>>
>> In practice, the actual generation number *will be the same* for 
>> everyone with the same repository content, unless and until someone 
>> develops a different calculation method.  But there is no reason to 
>> require that the number *has to be* the same for everyone unless you 
>> expect (or require) everyone to share their gen-caches.
>
> and I think this is why Linus is not happy with a cache. He is seeing 
> this as something that has significantly more value if it is going to 
> be consistant in a distributed manner than if it's just something 
> calculated locally that can be different from other systems.

It will only be used locally, so it needn't be consistent with anyone else's.

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>
> if it's just locally generated, then I could easily see generation 
> numbers being different on different people's ssstems, dependin on the 
> order that they see commits (either locally generated or pulled from 
> others)
>
> If it's part of the commit, then as that commit gets propogated the 
> generation number gets propogated as well, and every repository will 
> agree on what the generation number is for any commit that's shared.
>
> I agree that this consistancy guarantee seems to be valuable.

I can't see why.

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>> Surely there will be a competent and efficient gen-cache API.  But 
>> most code can just ask if B --contains A or even just use rev-list 
>> and benefit from the increased speed of the answer.  Because most 
>> code doesn't really care about the gen numbers themselves, but only 
>> the speed of determining ancestry.
>
> in that case, why bother with generation numbers at all? the improved 
> data based heristic seems to solve that problem.

Does it? Surely the ruckus would've died down in that case. But I haven't been reading pu.

It seems to me that the main drawback to a gen-cache is that it slows down the first operation after even a local clone (with just hardlinks).

On the other hand, I see too many nails in the distributed-gen-numbers coffin: legacy commits can't catch up (and therefore suffer), and legacy clients can trash or corrupt even "new-style" commits.

Phil