[Pw_forum] multi core Vs Single core

[Axel Kohlmeyer]

On Feb 8, 2008 12:10 PM, Marcel Mohr <marcel at physik.tu-berlin.de> wrote:
> There are some posts about this topic some time ago.
>
> Planewave codes use a lot of memory bandwith, so an Intel
> quadcore is not much faster than a dual core on the same mainboard,
> because the memory bus becomes the bottleneck of the system, and all core
> share the same bus.

for some additional numbers on multi-core. have a look at:
http://groups.google.com/group/cp2k/msg/aa1ce54b56598aab?dmode=source

FYI, CP2K/QuickStep is even more demanding in terms of memory bandwidth than QE.

however, since some vendors are offering quad-core nodes for about the
same price
as dual-core nodes, it is actually a better deal to get quad-core and
then use only
half of the cores (and have twice the cache) if you get processor
affinity set up right.

a.
>
> Cheers
> Marcel
>
>
> On Fri, 8 Feb 2008, Axel Kohlmeyer wrote:
>
> > On Fri, 8 Feb 2008, ambavale sagar wrote:
> >
> >
> > sagar,
> >
> > AS> Is multi (quad) core processor more efficient than single core for our
> >
> > please clarify what you mean by "efficient".
> > more throughput for the same money, faster execution,
> > better utilization of the cpu, less room needed for
> > a given computational capability?
> >
> > AS> abinitio calculations? How efficiently multi-threading is done while
> > AS> working with serial version of code on multicore machine?
> >
> > it is a common misconceptions that you need to use multi-threading
> > to take advantage of multicore cpus. the vendors of those cpus
> > are somewhat promoting it, because it is simpler to add multi-threading
> > to a code than to parallelize it completely, and also because
> > their new cpus don't have any advantage over the older ones without
> > paralleization. fortunately, the methods used in quantum espresso
> > are well parallelizable and the code offers multiple levels of MPI
> > parallelism which i found to be almost always more efficient than
> > using OpenMP (and thus multi-threading). even more so, QE does not
> > support OpenMP (only indirectly via threaded BLAS/LAPACK libraries).
> >
> > depending on your hardware, problem set size and compiler/library
> > combination, you can get a speedup of roughly 2.5x to 3.5x from
> > a quad core cpu over a corresponding single core. when using multiple
> > nodes via a network, however, the speed and the latency of the
> > network (and the options of parallization, i.e. if you can parallelize
> > over k-points or not) start to matter and can make it all very
> > confusing.
> >
> > in the end, apart from a few general observations, there is no other
> > way to get a definite answer than running tests with representative
> > input files.
> >
> > cheers,
> >   axel.
> >
> > p.s.: this brings up the question again, that we should compile a
> > list of such typical inputs and run them on mutually available
> > machines so people can see what to expect....
> >
> > AS>
> > AS> Thanx.
> > AS>
> > AS>
> > AS>
> > AS> Sagar K. Ambavale
> > AS>
> > AS> PhD student,
> > AS>
> > AS> The M.S. University of Baroda,
> > AS>
> > AS> India
> > AS>
> > AS>
> > AS> ________________________________________________________________________________
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> > AS>
> >
> > --
> > =======================================================================
> > Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
> >   Center for Molecular Modeling   --   University of Pennsylvania
> > Department of Chemistry, 231 S.34th Street, Philadelphia, PA 19104-6323
> > tel: 1-215-898-1582,  fax: 1-215-573-6233,  office-tel: 1-215-898-5425
> > =======================================================================
> > If you make something idiot-proof, the universe creates a better idiot.
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-- 
=======================================================================
Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
  Center for Molecular Modeling   --   University of Pennsylvania
Department of Chemistry, 231 S.34th Street, Philadelphia, PA 19104-6323
tel: 1-215-898-1582,  fax: 1-215-573-6233,  office-tel: 1-215-898-5425
=======================================================================
If you make something idiot-proof, the universe creates a better idiot.