Re: hyperthreaded cpu still an issue in 8.4?

On 7/28/09 1:28 PM, "Greg Smith" <gsmith(at)gregsmith(dot)com> wrote:

> On Tue, 28 Jul 2009, Matthew Wakeling wrote:
>
>> Unlikely. Different threads on the same CPU core share their resources, so
>> they don't need an explicit communication channel at all (I'm simplifying
>> massively here). A real interconnect is only needed between CPUs and between
>> different cores on a CPU, and of course to the outside world.
>
> The question was "why are the new CPUs benchmarking so much faster than
> the old ones", and I believe that's mainly because the interconnection
> both between CPUs and between CPUs and memory are dramatically faster.

I believe he was answering the question "What makes SMT work well with
Postgres for these CPUs when it had problems on old Xeons?" -- and that
doesn't have a lot to do with the interconnect or bandwidth. It may also be
a more advanced compiler / OS toolchain. Postgres 8.0 compiled on an older
system and OS might not work so well with the new HT.

As for the question as to what is so good about the Nehalems -- the on-die
memory controller and point-to-point interprocessor interconnect is the
biggest performance change. Turbo and SMT are pretty good icing on the cake
though.