Re: [PATCH 2/6] commit: add generation number to struct commmit

From: Stefan Beller <sbeller@google.com>
Date: Apr 3, 2018, 18:44 UTC
Message-ID: <CAGZ79kYeKmWidi0SqV_R6NhWrH1rT-rTXetRfZ_YBpycieFxgw@mail.gmail.com>
In-Reply-To: <20180403182800.GA8377@sigill.intra.peff.net>

On Tue, Apr 3, 2018 at 11:28 AM, Jeff King <peff@peff.net> wrote:

Show 16 quoted lines

> On Tue, Apr 03, 2018 at 11:05:36AM -0700, Brandon Williams wrote:
>
>> On 04/03, Derrick Stolee wrote:
>> > The generation number of a commit is defined recursively as follows:
>> >
>> > * If a commit A has no parents, then the generation number of A is one.
>> > * If a commit A has parents, then the generation number of A is one
>> >   more than the maximum generation number among the parents of A.
>> >
>> > Add a uint32_t generation field to struct commit so we can pass this
>>
>> Is there any reason to believe this would be too small of a value in the
>> future?  Or is a 32 bit unsigned good enough?
>
> The linux kernel took ~10 years to produce 500k commits. Even assuming
> those were all linear (and they're not),

... which you meant in terms of DAG, where a linear history is the worst case for generation numbers.

I first read it the other way round, as the best case w.r.t. timing

~/linux$ git log --oneline |wc -l 721223 $ git log --oneline --since 2012 |wc -l 421853 $ git log --oneline --since 2011 |wc -l 477155

The number of commits is growing exponentially, though the exponential part is very small and the YoY growth can be estimated using linear interpolation.

In linux, the release is a natural synchronization point IIUC as well as on a regular schedule. So an interesting question to ask there would be whether the delta in generation number goes up over time, or if the DAG just gets wider (=more parallel)

> that gives us ~80,000 years of
> leeway. So even if the pace of development speeds up or we have a
> quicker project, it still seems we have a pretty reasonable safety
> margin.

Thanks for the estimate. Stefan