Re: What's the difference between `git show branch:file | diff -u - file` vs `git diff branch file`?

From: Michael J Gruber <git@drmicha.warpmail.net>
Date: Aug 23, 2011, 15:34 UTC
Message-ID: <4E53C89A.9000604@drmicha.warpmail.net>
In-Reply-To: <1314096731.15017.2.camel@n900.home.ru>

Marat Radchenko venit, vidit, dixit 23.08.2011 12:52:

Show 7 quoted lines

>> Is that a very large tree or a very slow file system?
> Tree is large (500k files), file system is irrelevant since all time is spend on CPU.
> 
>> Do we enumerate all
>> differing files and only then limit diff output by path??
> 
> Dunno, that's why I am asking why it is so slow.

Well, we have to read the full tree before diffing. But I can't reproduce the extreme difference which you observed (0.003s vs. 30s), only a factor of ten or so for a repo with 100k files:

git init seq 0 100000|while read n ; do echo a > a$n;done git add . git commit -m m echo b > a0

On a ramdisk, I get:

time git diff  > /dev/null

real 0m0.160s user 0m0.064s sys 0m0.190s

time git diff -- a0 > /dev/null

real 0m0.070s user 0m0.051s sys 0m0.021s

time git diff HEAD > /dev/null

real 0m0.266s user 0m0.145s sys 0m0.212s

time git diff HEAD -- a0 > /dev/null

real 0m0.171s user 0m0.136s sys 0m0.033s

time git show HEAD:a0  > /dev/null

real 0m0.018s user 0m0.009s sys 0m0.007s

time git show HEAD:a0 | diff -u - a0 > /dev/null

real 0m0.019s user 0m0.010s sys 0m0.008s

Stumped.

In your case, do you have a lot of differing files besides the one you are limitting to? Anyway, that does not seem to make a huge difference in my timings. (Just tried.) Still Stumped.

Michael