[Wien] Intel EMT64 OT

[Gerhard H Fecher]

This is nothing surprising if you have a CPU with Hyperthreading (HT), but may 
strongly depent on the size of memory you need and have. 
Roughly speaking, the HT works in some cases like 2 CPUs as it may deal with 
two processes in parallel. If you check what your CPU is doing, you will find 
that it is not very busy during writes to the hard disc, so that a second 
calculation may use the CPU during such dead time. However, it works only 
faster, if the second process does not need data from the hard disc but can 
take it from the memory.
We are using multi-CPU Kernels on our computers with single HT-CPUs and have 
rather good experience if running two, not too big, Wien calculations at the 
same time. However, if you understimate the size of memory that a calculation 
needs, the computer may become very slow because it is finally only swapping 
data between memory and hard disc, so be careful.
You may also gain speed if you use the optimum (this you have to check for 
your system and may not be maximal otimization) compiler switches when 
compiling Wien.
Indeed, very old P4 did not come with HT, and/or (at least for Suse systems) 
the Linux setup may have choosen already a multi processor Kernel in case of 
your newer computer.  

Stefaan, you are right, that is just the same effect as reported before.

Good luck,
Ciao
Gerhard

Am Mittwoch, 7. September 2005 12:52 schrieb Stefaan Cottenier:
> 
> > Besides this, we found that surprisingly 2 WIEN jobs (on one cpu) can
> > "gain" some time; i.e. while one job runs eg. 1h, 2 jobs in (k-)parallel
> > do not need 2h, but some 10-20 % less !!??
> > 
> > (PS: we do not see this on our "old" 2.6 GHz P4)
> 
> Is that the same effect as mentioned in 
> 
> http://zeus.theochem.tuwien.ac.at/pipermail/wien/2005-March/004901.html  ?
> 
> That was observed on a normal P4 (2.2 or 2.4 GHz).
> 
> Stefaan
> 
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