From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on dcvr.yhbt.net X-Spam-Level: X-Spam-ASN: AS31976 209.132.180.0/23 X-Spam-Status: No, score=-5.4 required=3.0 tests=AWL,BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,RCVD_IN_DNSWL_HI,RP_MATCHES_RCVD shortcircuit=no autolearn=ham autolearn_force=no version=3.4.0 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by dcvr.yhbt.net (Postfix) with ESMTP id E809A20986 for ; Thu, 29 Sep 2016 09:26:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932591AbcI2J0B (ORCPT ); Thu, 29 Sep 2016 05:26:01 -0400 Received: from cloud.peff.net ([104.130.231.41]:49823 "EHLO cloud.peff.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932451AbcI2J0A (ORCPT ); Thu, 29 Sep 2016 05:26:00 -0400 Received: (qmail 13407 invoked by uid 109); 29 Sep 2016 09:25:58 -0000 Received: from Unknown (HELO peff.net) (10.0.1.2) by cloud.peff.net (qpsmtpd/0.84) with SMTP; Thu, 29 Sep 2016 09:25:58 +0000 Received: (qmail 32387 invoked by uid 111); 29 Sep 2016 09:26:14 -0000 Received: from sigill.intra.peff.net (HELO sigill.intra.peff.net) (10.0.0.7) by peff.net (qpsmtpd/0.84) with SMTP; Thu, 29 Sep 2016 05:26:14 -0400 Received: by sigill.intra.peff.net (sSMTP sendmail emulation); Thu, 29 Sep 2016 05:25:56 -0400 Date: Thu, 29 Sep 2016 05:25:56 -0400 From: Jeff King To: Junio C Hamano Cc: git@vger.kernel.org, torvalds@linux-foundation.org Subject: Re: [PATCH 4/4] core.abbrev: raise the default abbreviation to 12 hexdigits Message-ID: <20160929092556.3kkmdd6uprss76zx@sigill.intra.peff.net> References: <20160928233047.14313-1-gitster@pobox.com> <20160928233047.14313-5-gitster@pobox.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20160928233047.14313-5-gitster@pobox.com> Sender: git-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org On Wed, Sep 28, 2016 at 04:30:47PM -0700, Junio C Hamano wrote: > As Peff said, responding in a thread started by Linus's suggestion > to raise the default abbreviation to 12 hexdigits: > > I actually think "12" might be sane for a long time. That's 48 bits of > sha1, so we'd expect a 50% change of a _single_ collision at 2^24, or 16 > million. The biggest repository I know about (in number of objects) is > the one holding all of the objects for all of the forks of > torvalds/linux on GitHub. It's at about 15 million objects. > > Which _seems_ close, but remember that's the size where we expect to see > a single collision. They don't become common until much later (I didn't > compute an exact number, but Linus's 16x sounds about right). I know > that the growth of the kernel isn't really linear, but I think the need > to bump to "13" might not just be decades, but possibly a century or > more. > > So 12 seems reasonable, and the only downside for it (or for "13", for > that matter) is a few extra bytes. I dunno, maybe people will really > hate that, but I have a feeling these are mostly cut-and-pasted anyway. I am not sure my quote is a good rationale for this bump. It was meant to be a rationale that "12" is big enough, but the "I dunno" at the end kind of glosses over the downsides. -Peff