Re: Git commit generation numbers

On Wed, 20 Jul 2011, Phil Hord wrote:

On 07/20/2011 07:36 PM, Nicolas Pitre wrote:

quoted

On Wed, 20 Jul 2011, david@lang.hm wrote:

quoted

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.

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.

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.

David Lang