Re: [PATCH 2/2] lookup_commit_reference_gently: do not read non-{tag,commit}

From: Jeff King <peff@peff.net>
Date: May 30, 2013, 21:22 UTC
Message-ID: <20130530212223.GA2135@sigill.intra.peff.net>
In-Reply-To: <5cc40825d5b4fb3382e4c054c49adf5e6b6fe110.1369943791.git.trast@inf.ethz.ch>

On Thu, May 30, 2013 at 10:00:23PM +0200, Thomas Rast wrote:

Show 7 quoted lines

> lookup_commit_reference_gently unconditionally parses the object given
> to it.  This slows down git-describe a lot if you have a repository
> with large tagged blobs in it: parse_object() will read the entire
> blob and verify that its sha1 matches, only to then throw it away.
> 
> Speed it up by checking the type with sha1_object_info() prior to
> unpacking.

This would speed up the case where we do not end up looking at the object at all, but it will slow down the (presumably common) case where we will in fact find a commit and end up parsing the object anyway.

Have you measured the impact of this on normal operations? During a traversal, we spend a measurable amount of time looking up commits in packfiles, and this would presumably double it.

This is not the first time I have seen this tradeoff in git. It would be nice if our object access was structured to do incremental examination of the objects (i.e., store the packfile index lookup or partial unpack of a loose object header, and then use that to complete the next step of actually getting the contents).

-Peff