Re: Performance issue of 'git branch'

From: Carlos R. Mafra <crmafra2@gmail.com>
Date: Jul 23, 2009, 03:18 UTC
Message-ID: <20090723031843.GA9152@Pilar.aei.mpg.de>
In-Reply-To: <alpine.LFD.2.01.0907221921570.3352@localhost.localdomain>

First of all:
      * yes, my VAIO has a slow 4200 rpm disc :-(
      * strace -T indeed showed that lstat() was not guilty
      * GIT_DEBUG_LOOKUP=1 git branch produced ugly 2200+ lines

Now to the patch,

On Wed 22.Jul'09 at 19:23:39 -0700, Linus Torvalds wrote:

Show 10 quoted lines

> > Ooh yes. That would do it. It's going to peel and look up every single ref 
> > it finds, so it's going to look up _hundreds_ of objects (all the tags, 
> > all the commits they point to, etc etc). Even if it then only shows a 
> > couple of branches.
> > 
> > Junio, any ideas?
> 
> I had one of my own.
> 
> Does this fix it?

Yes!

[mafra@Pilar:linux-2.6]$ time git branch
  27-stable
  28-stable
  29-stable
  30-stable
  dev-private
* master
  option
  sparse
  stern
0.00user 0.01system 0:01.50elapsed 1%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (42major+757minor)pagefaults 0swaps

01.50 is not that good, but it doesn't "feel" terrible as 4 seconds. [ It is incredible how 4 secs feels really bad while 2 is acceptable... ]

So thank you very much, Linus! A 50% improvement here!

And I am happy to have finally reported it, after quietly suffering for so long thinking that "git is as fast as possible, so it is probably my fault".

PS: Out of curiosity, how many femtoseconds does it take in your 
state-of-the-art machine? :-)