Re: [PATCH] Put sha1dc on a diet

From: Johannes Schindelin <johannes.schindelin@gmx.de>
Date: Mar 2, 2017, 14:45 UTC
Message-ID: <alpine.DEB.2.20.1703021539330.3767@virtualbox>
In-Reply-To: <CACsJy8D3h1KAaKi_Esc98za3LqXaB=YeW0Yu+VAV9UnX5vmttg@mail.gmail.com>

Hi Duy,

On Thu, 2 Mar 2017, Duy Nguyen wrote:

Show 10 quoted lines

> On Thu, Mar 2, 2017 at 6:19 AM, Jeff King <peff@peff.net> wrote:
> > You have to remember that some of the Git for Windows users are doing
> > horrific things like using repositories with 450MB .git/index files,
> > and the speed to compute the sha1 during an update is noticeable
> > there.
> 
> We probably should separate this use case from the object hashing
> anyway. Here we need a better, more reliable crc32 basically, to detect
> bit flips. Even if we move to SHA-something, we can keep staying with
> SHA-1 here (and with the fastest implementation)

I guess it was convenient to use the same hash algorithm for all hashing purposes in the beginning. The downside, of course, was that we kept talking about SHA-1s instead of commit hashes and the index checksum (i.e. using labels based on implementation details rather than semantically meaningful names).

In the meantime, we use different hash algorithms where appropriate, of course, and we typically encapsulate the exact hash algorithm so that it is easy to switch when/if necessary (think the hash functions for strings in our hashtables, and the hash functions in xdiff).

It would probably make sense to switch the index integrity check away from SHA-1 because we really only care about detecting bit flips there, and we have no need for the computational overhead of using a full-blown cryptographic hash for that purpose.

Ciao, Johannes