Re: Git and GCC

[Jeff King <peff at peff dot net>]

On Thu, Dec 06, 2007 at 10:35:22AM -0800, Linus Torvalds 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.

It can get nasty with super-long deltas thrashing the cache, I think.
But in this case, I think it ended up being just a poor division of
labor caused by the chunk_size parameter using the quite large window
size (see elsewhere in the thread for discussion).

> 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?)

I think the chunk_size more or less explains it. I have had reasonable
success keeping both CPUs busy on similar tasks in the past (but with
smaller window sizes).

For reference, it was a Core 2 Duo; do they all share L2, or is there
something I can look for in /proc/cpuinfo?

-Peff