[ale] Hyperthreading

[Steven A. DuChene]

I have been reading the responses in this thread so far but i have not as of
yet actually heard anything about checking to see if a particular P4 processor
is really Hyperthread capable. Not all P4 processors are created equal and
some are not Hyperthread capable. Typically other than actually research the
particular processor specs on the Intel web site the best way to tell is to
see if there is an option in the motherboard BIOS to turn Hyperthreading on
or off. Once it is turned on if you then run a SMP kernel you will actually
see more than one processor being reported in /proc/cpuinfo

If you run an SMP kernel and still only see one processor in /proc/cpuinfo
then either the Hyperthreading support is not turned on OR your processor
is not Hyperthread capable.

Now, as to whether it is a good thing or a bad thing to turn on, it is all
a question of the particular types of work loads the system sees. There are
cases where Hyperthreading is not actually a help and in some cases it may
actually be a performance degradation.

On Wed, Apr 14, 2004 at 10:40:40AM -0400, Dow Hurst wrote:
> Yes, I run two minimizations simultaneously on my single P4 and until the 
> second one starts the machine is very responsive.  It is truly like having a 
> dual CPU from my user perspective.  Run times are doubled on the minimizations.
> 
> 25m37.821s   Single run
> 
> 49m54.143s
> 44m10.795s   Dual runs (The second is the shorter due to different start 
> times, so this is general datapoints)
> 
> 
> These are GPCR receptor minimizations with delta 9 THC bound inside the 
> receptor.  About ~2700 atoms in vacuum using molecular mechanics and a lot of 
> torsional restraints applied.  Not a difficult calculation but is pure FPU on 
> C and Fortran code.  But, my point is the machine was acting like a dual CPU 
> machine in a very efficient manner.
> Dow
> 
> 
> Christopher Fowler wrote:
> > How is the performance? Is there a real gain?
> > 
> > On Wed, 2004-04-14 at 10:03, Jonathan Glass wrote:
> > 
> >>On Wed, 2004-04-14 at 10:02, Mike Millson wrote:
> >>
> >>>I have a box with a P4 processor running Slackware 9.1. What do I need
> >>>to do to enable hyperthreading support? Do I just have to compile a
> >>>kernel with SMP support? Are there any other kernel options or config
> >>>changes I need to make? Does anyone know of any good articles or howtos
> >>>on configuring linux for hyperthreading?
> >>>
> >>>Thank you,
> >>>Mike
> >>>

-- 
Steven A. DuChene     linux-clusters at mindspring.com
                      sduchene at mindspring.com