[geeks] New small Intel Board

[Alois Hammer]

On Fri, 6 Jun 2008 16:15:44 -0400, "Joshua Boyd" <jdboyd at jdboyd.net>
said:
> On Fri, Jun 06, 2008 at 04:03:17PM -0400, Alois Hammer wrote:
> > On Fri, 6 Jun 2008 12:51:50 -0400 (EDT), nate at portents.com said:
> > > 
> > > > Hyper-threading
> > > 
> > > Atom does have hyperthreading (just as Nehalem will).
> > 
> > I'm a bit puzzled.  HyperThreading was invented because the Pentium 4
> > architecture -- Willamette through Cedar Mill -- had some unused
> > execution unit time resulting from the poor design, and presenting an
> > extra logical CPU got a little of the lost performance back.  Why is HT
> > suddenly making a return?
> > 
> > ...Intel hasn't suddenly added *real* threading a la SPARC T1/T2, have
> > they?
> 
> Err, hyperthreading was/is real threading.  I believe the T1/T2s just
> double the number of contexts per core, although there could be other
> improvements that I am not aware of.

Okay, "real" maybe wasn't the right word.  What I meant is that the
hardware behind the extra thread was just a little unused execution unit
time; hence an absolute maximum of ~15% real performance gain on
intensive SMP database tests vs. turning HT off.  (And pre-Northwood HT
was sufficiently buggered that it was either turned off by default in
firmware, or /unalterably/ turned off in firmware, esp. in desktop
machines.  That was even true for earlier Northwood steppings, IIRC.)  I
was under the impression that T1/T2 hardware had significant extra
execution units behind the extra threads, making them a little more...
useful.