Hyperthreading

[David Schultz]

On Fri, Jun 27, 2003, Brooks Davis wrote:
> On Fri, Jun 27, 2003 at 06:39:12PM -0500, Glenn Johnson wrote:
> > Thanks.  I had read the smp manual page.  I know _how_ to enable HTT; I
> > was wondering whether I _should_ enable it.  It seems the answer is that
> > it is not beneficial in its current state because the scheduler does not
> > yet differentiate between physical and logical processors.
> 
> It's more complicated then that.  For many users, it's true that HTT is
> not useful due to the scheduling issues, but for some applications where
> you keep all the CPUs busy, it does help.  Somewhat suprisingly,
> SETI at Home performs better with HTT enabled then without.  The individual
> workunits take longer to process, but the overall throughput is better
> (4 workunits every 6hrs instead of 2 workunits every 4hrs).

Hyperthreading will generally give you better thoughput because
you get better utilization of the hardware; when one functional
unit would normally be idle due to a pipeline bubble, the other
logical CPU may be able to provide work for it.  On the other
hand, as you observe, latency is worse.  In particular, if you're
running a web browser on one processor, it's competing for
resources with your SETI at Home client on the other processor, even
though the SETI at Home client is niced.