Re: Dual Xeon + HW RAID question

Mindaugas Riauba wrote:
>
> > > I missed your orig. post, but AFAIK multiprocessing kernels will handle
> HT
> > > CPUs as 2 CPUs each. Thus, our dual Xeon 2.4 is recognized as 4 Xeon 2.4
> > > CPUs.
> > >
> > > This way, I don't think HT would improve any single query (afaik no
> postgres
> > > process uses more than one cpu), but overall multi-query performance has
> to
> > > improve.
> >
> > When you use hyperthreading, each virtual cpu runs at 70% of a full CPU,
> > so hyperthreading could be slower than non-hyperthreading.  On a fully
> > loaded dual cpu system, you are looking at 2.8 cpu's (0.70 * 4), while
> > if it isn't loaded, you are looking at slowing down if you are only
> > using 1 or 2 cpu's.
>
>   Virtual cpus are not running at 70% of real cpus :). Slowdown will happen
> if
> scheduler will run 2 processes on the same real cpu. And I read that there
> are
> patches for Linux kernel to fix that. Sooner rather than later they will
> appear
> in Linus kernel.

Right, I simplified it.  The big deal is whether the OS favors the
second real CPU over one of the virtual CPU's on the same die --- by
default, it doesn't.  Ever if it did work perfectly, you are talking
about going from 1 to 1.4 or 2 to 2.8, which doesn't seem like much.

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