[Pw_forum] multi core Vs Single core

[Marcel Mohr]

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.

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> ________________________________________________________________________________
> AS> 5, 50, 500, 5000 - Store N number of mails in your inbox. Click here.
> 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.