From: Junio C Hamano <gitster@pobox.com>
To: "Jon Smirl" <jonsmirl@gmail.com>
Cc: "Johannes Schindelin" <Johannes.Schindelin@gmx.de>,
"Shawn O. Pearce" <spearce@spearce.org>,
"Git Mailing List" <git@vger.kernel.org>
Subject: Re: Calculating tree nodes
Date: Mon, 03 Sep 2007 21:28:41 -0700 [thread overview]
Message-ID: <7vmyw3835y.fsf@gitster.siamese.dyndns.org> (raw)
In-Reply-To: <9e4733910709032054y4407ce62o6b21935502bfacdb@mail.gmail.com> (Jon Smirl's message of "Mon, 3 Sep 2007 23:54:00 -0400")
"Jon Smirl" <jonsmirl@gmail.com> writes:
>> Yes. For performance reasons, since a simple commit would kill you in any
>> reasonably sized repo.
>
> That's not an obvious conclusion. A new commit is just a series of
> edits to the previous commit. Start with the previous commit, edit it,
> delta it and store it. Storing of the file objects is the same. Why
> isn't this scheme fast than the current one?
I think you seem to be forgetting about tree comparison.
With a large project that has a reasonable directory structure
(i.e. not insanely narrow), a commit touches isolated subparts
of the whole tree. Think of an architecture specific patch to
the Linux kernel touching only include/asm-i386 and arch/i386
directories.
Being able to cull an entire subdirectory (e.g. drivers/ which
has 5700 files underneath) by only looking at the tree SHA-1 of
the containing tree is a _HUGE_ win.
And this is not just about two tree comparison. When you say:
git log v2.6.20 -- arch/i386/
what you are seeing is a simplified history that consists of
commits that touch only these paths. How would we determine if
a commit touch these paths efficiently? By comparing the "i386"
entry in tree objects for $commit^:arch and $commit:arch. You
do not have to look inside arch/i386/ trees to see if any of the
330 files in it is different. You just check a single SHA-1
pair.
next prev parent reply other threads:[~2007-09-04 4:28 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-09-04 2:13 Calculating tree nodes Jon Smirl
2007-09-04 2:51 ` Shawn O. Pearce
2007-09-04 3:26 ` Jon Smirl
2007-09-04 3:40 ` Johannes Schindelin
2007-09-04 3:54 ` Jon Smirl
2007-09-04 4:21 ` Martin Langhoff
2007-09-04 5:37 ` Jon Smirl
2007-09-04 5:51 ` Andreas Ericsson
2007-09-04 10:33 ` Johannes Schindelin
2007-09-04 14:31 ` Jon Smirl
2007-09-04 15:05 ` Johannes Schindelin
2007-09-04 15:14 ` Andreas Ericsson
2007-09-04 21:02 ` Martin Langhoff
2007-09-04 4:28 ` Junio C Hamano [this message]
2007-09-04 5:50 ` Jon Smirl
2007-09-04 4:19 ` David Tweed
2007-09-04 5:52 ` Jon Smirl
2007-09-04 5:55 ` Andreas Ericsson
2007-09-04 6:16 ` Shawn O. Pearce
2007-09-04 14:19 ` Jon Smirl
2007-09-04 14:41 ` Andreas Ericsson
2007-09-04 6:16 ` David Tweed
2007-09-04 6:26 ` Shawn O. Pearce
2007-09-04 17:39 ` Junio C Hamano
2007-09-06 3:20 ` Shawn O. Pearce
2007-09-06 5:21 ` Junio C Hamano
2007-09-04 16:20 ` Daniel Hulme
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