[Wien] What type of CPU's for big calculations?

[Peter Blaha]

> 1) Has anyone tested big cases (e.g. matrix sizes more than 14000,
> perhaps as high as 25000) on the newer CPU's with more RAM?

There is not much size dependency and definitely TODAY Intels Dore2 Duo 
E6X00 processors are best.

However, for systems with NMAT=25000 you will hardly need any 
k-point-parallelism AND you will RUN OUT OF MEMORY (in ther complex 
version more than 13GB). So you would have to use the mpi version.

For such a case I'd consider to get cpu time on one of the big 
"computercenters", which have Infiniband-based clusters for such type of 
calculations. (Infiniband is quite expensive and you don't need it for 
"every day").

PS: Of course, soon quadcore CPUs come out ...

> 2) Does one have to have a 64bit system for big cases.
Can you still buy a 32 bit system ? An of course: Yes for big cases you 
definitely need a 64 bit system (with 32 bit you are limited to 2 GB 
addressspace).

> 3) Does threading the cores really matter with big matrices
> (OMP_NUM_THREADS=2) ?
Switch it on if you run only one job on a dual core node (you gain, but 
of course not a factor of two)
Switch it off, when running 2 k-point-parallel jobs on the same node. 
You loose a bit because of memory constrains.

> 4) What about the specifics of the memory architecture (shared versus
> independent) and the L2 cache (size and whether it is shared)?
Not particular important. All big jobs run out of cache anyway. Thus it 
is only a matter how often you need to load data in the cache, but our 
blocked algorithm will lead to almost cache-size independency. (It 
matters of course for "small" progreams, which may fit totally in the 
cache (or not).

> 5) Has anyone tried running two jobs each with two threads to a two
> CPU dual-core machine (four effective cores) -- or does the OS get
> confused about who does what

"Hyperthreading" is a big "flop" (at least for hight performance computing.
Try to avoid that and run either 1-k point + OMP=2 or 2-kpoints and OMP=1