Question about HT

[Bruno]

Hi,

Is it possible for some applications to be slower on a 2.CPU w/HT than
on the same CPU w/o HT?

Background:
Basically, I see that a one threaded application seems to only use 50%
of the CPU in task manager in Windows XP. I guess a SMP-aware
application would spread its threads amongs the two logical CPUs, but
a single threaded application would stick to a single logical CPU.
So, if I've understood things correctly so far, the real questions
behind would be :
- Does HT make two logical CPU half the speed of the physical CPU, or
does HT emulate two logical CPUs of the same speed?
- Would an application stuck to 50% in the task manager be running
slower than it would be on a CPU w/o HT?

TIA,
Bruno

 

[Peter Cavan]

Hi
I don't know exactly how programs choose how many CPUs to use, but here are
some things. 1) Normal programs, games etc. definitely run faster W/O HT
than with it. I presume that this will change when HT programs go more
mainstream, but not at the moment. 2) If you have 2 fairly intensive
programs running they will probably use both CPUs, but I'm not 100%. You can
set which CPU for a program to use under Task Manager\Processes. I have a HT
CPU, and I usually leave HT on, but I turn it off for benchmarks, new games
etc.
Hope this helps a bit
PC

 

[Bruno]

Hi
I don't know exactly how programs choose how many CPUs to use, but here are
some things. 1) Normal programs, games etc. definitely run faster W/O HT
than with it. I presume that this will change when HT programs go more
mainstream, but not at the moment. 2) If you have 2 fairly intensive
programs running they will probably use both CPUs, but I'm not 100%. You can
set which CPU for a program to use under Task Manager\Processes. I have a HT
CPU, and I usually leave HT on, but I turn it off for benchmarks, new games
etc.
Hope this helps a bit
PC

Yes, that's what I think : If you usually use your machine for single
purposes and since many applications are not SMP aware and run ony
one, you may end up using half your processor for this single
application if HT is on...

Does anyone know if it is possible to toggle HT from the OS? I know
this can be done from the bios, but I have my computer doing some
calculations, which I cannot stop or I'll lose what has already be
done so far. I'd like to switch HT off to speed things up, without
losing the 30% already done. Am I dreaming, right? ;-)