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Re: Git and GCC


On 12/6/07, Nicolas Pitre <nico@cam.org> wrote:
> On Thu, 6 Dec 2007, Jon Smirl wrote:
>
> > On 12/6/07, Nicolas Pitre <nico@cam.org> wrote:
> > > > When I lasted looked at the code, the problem was in evenly dividing
> > > > the work. I was using a four core machine and most of the time one
> > > > core would end up with 3-5x the work of the lightest loaded core.
> > > > Setting pack.threads up to 20 fixed the problem. With a high number of
> > > > threads I was able to get a 4hr pack to finished in something like
> > > > 1:15.
> > >
> > > But as far as I know you didn't try my latest incarnation which has been
> > > available in Git's master branch for a few months already.
> >
> > I've deleted all my giant packs. Using the kernel pack:
> > 4GB Q6600
> >
> > Using the current thread pack code I get these results.
> >
> > The interesting case is the last one. I set it to 15 threads and
> > monitored with 'top'.
> > For 0-60% compression I was at 300% CPU, 60-74% was 200% CPU and
> > 74-100% was 100% CPU. It never used all for cores. The only other
> > things running were top and my desktop. This is the same load
> > balancing problem I observed earlier.
>
> Well, that's possible with a window 25 times larger than the default.
>
> The load balancing is solved with a master thread serving relatively
> small object list segments to any work thread that finished with its
> previous segment.  But the size for those segments is currently fixed to
> window * 1000 which is way too large when window == 250.
>
> I have to find a way to auto-tune that segment size somehow.

That would be nice. Threading is most important on the giant
pack/window combinations. The normal case is fast enough that I don't
real notice it. These giant pack/window combos can run 8-10 hours.

>
> But with the default window size there should not be any such noticeable
> load balancing problem.

I only spend 30 seconds in the compression phase without making the
window larger. It's not long enough to really see what is going on.

>
> Note that threading only happens in the compression phase.  The count
> and write phase are hardly paralleled.
>
>
> Nicolas
>


-- 
Jon Smirl
jonsmirl@gmail.com


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