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Re: Git and GCC
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?