Re: Git and GCC

> On 12/6/07, Linus Torvalds <torvalds@linux-foundation.org> wrote:
> >
> >
> > On Thu, 6 Dec 2007, Jeff King wrote:
> > >
> > > What is really disappointing is that we saved only about 20% of the
> > > time. I didn't sit around watching the stages, but my guess is that we
> > > spent a long time in the single threaded "writing objects" stage with a
> > > thrashing delta cache.
> >
> > I don't think you spent all that much time writing the objects. That part
> > isn't very intensive, it's mostly about the IO.
> >
> > I suspect you may simply be dominated by memory-throughput issues. The
> > delta matching doesn't cache all that well, and using two or more cores
> > isn't going to help all that much if they are largely waiting for memory
> > (and quite possibly also perhaps fighting each other for a shared cache?
> > Is this a Core 2 with the shared L2?)
> 
> 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.