SMP vs. Hyperthreading?

[Brent Wiese]

> Hi,
> 
> I've got a machine with a P4 that supports HTT (Hyperthreading) so I
> thought about setting up a SMP-kernel.

SMP <> HT. I'm not an expert on this, but when I went to an Intel conference
a couple years ago which discussed hyperthreading, it was made pretty clear
it wasn't the same as SMP. But, like I said, I'm not an expert so I don't
know what exactly its doing at the very low levels.

> Under 5.2.1 it's a different story though: /var/log/messages shows
> that there are 2 CPUs but the message indicating the second CPU has
> been launched is missing ("/kernel: SMP: AP CPU #1 Launched!"). When I
> run a "cpuburn"-Test under 5.2.1 CPU load indicated by "top" shows
> 100% so I assume SMP/HTT is not active.

That is the message for the second CPU. The first is CPU #0 and is always
active. Every dual-cpu machine I've installed SMP kernel on only shows the
second CPU launching.

> So my primary question is - how do I get SMP running under 5.2.1 with
> a hyperthreading-capable P4??

I haven't used 5.x, but in 4.9, there is a specific line in the kernel
config for hyperthreading support (to which I've read mixed reviews on this
list).

Good luck
Brent