Re: Git and GCC

From: Jon Smirl <jonsmirl@gmail.com>
Date: Dec 7, 2007, 04:06 UTC
Message-ID: <9e4733910712062006l651571f3w7f76ce64c6650dff@mail.gmail.com>
In-Reply-To: <alpine.LFD.0.9999.0712061857060.13796@woody.linux-foundation.org>

On 12/6/07, Linus Torvalds <torvalds@linux-foundation.org> wrote:

Show 21 quoted lines

>
>
> On Thu, 6 Dec 2007, Harvey Harrison wrote:
> >
> > I've updated the public mirror repo with the very-packed version.
>
> Side note: it might be interesting to compare timings for
> history-intensive stuff with and without this kind of very-packed
> situation.
>
> The very density of a smaller pack-file might be enough to overcome the
> downsides (more CPU time to apply longer delta-chains), but regardless,
> real numbers talks, bullshit walks. So wouldn't it be nice to have real
> numbers?
>
> One easy way to get real numbers for history would be to just time some
> reasonably costly operation that uses lots of history. Ie just do a
>
>         time git blame -C gcc/regclass.c > /dev/null
>
> and see if the deeper delta chains are very expensive.

jonsmirl@terra:/video/gcc$ time git blame -C gcc/regclass.c > /dev/null

real 1m21.967s user 1m21.329s sys 0m0.640s

The Mozilla repo is at least 50% larger than the gcc one. It took me 23 minutes to repack the gcc one on my $800 Dell. The trick to this is lots of RAM and 64b. There is little disk IO during the compression phase, everything is cached.

I have a 4.8GB git process with 4GB of physical memory. Everything started slowing down a lot when the process got that big. Does git really need 4.8GB to repack? I could only keep 3.4GB resident. Luckily this happen at 95% completion. With 8GB of memory you should be able to do this repack in under 20 minutes.

jonsmirl@terra:/video/gcc$ time git repack -a -d -f --depth=250 --window=250 real 22m54.380s user 69m18.948s sys 0m23.773s

Show 14 quoted lines

> (Yeah, the above is pretty much designed to be the worst possible case for
> this kind of aggressive history packing, but I don't know if that choice
> of file to try to annotate is a good choice or not. I suspect that "git
> blame -C" with a CVS import is just horrid, because CVS commits tend to be
> pretty big and nasty and not as localized as we've tried to make things in
> the kernel, so doing the code copy detection is probably horrendously
> expensive)
>
>                         Linus
> -
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-- 
Jon Smirl
jonsmirl@gmail.com