[Beowulf] Re: vectors vs. loops

[Philippe Blaise]

Robert G. Brown wrote:

>....
>
>Still, the marketplace speaks for itself.  It doesn't argue, and isn't
>legendary, it just is.
>....
>  
>


But, does the hpc marketplace have a direction ?

Few years ago, some people had a "fantastic vision" to replace the 
vector machines market :
use big clusters of SMPs with the help of the new paradigm of hybrid 
mpi/openmp programming.
Then the main vendors (usa), except Cray, were very happy to sell giant 
clusters of smp machines.

Nevertheless, the japanese guys built the "earth simulator" ; which is 
still the most powerful machine in the world
(don't trust this stupid top500 list).

Then Cray came back ... with vector machines...

Don't underestimate the power of vector machines.
Yes Fujitsu or NEC vector machines are still very efficient, even with 
non contiguous memory access (!!).

One year ago, the only cpus that sometimes were able to equal vectorial 
cpus were alpha (ev7) and itanum2 with
big caches and / or fast memory access. Remember that alpha is dead. 
Have a look to the itanium2 market shares.

The marketplace is not a good argument at all.

Vectorization and parallelization are compatible
Hybrid mpi/openmp programming  is a harder task than mpi/vector programming.
If you have enough money and if your program is vectorizable, buy a 
vector machine of course.

Cluster of SMPs ? they will remain an efficient and low cost solution, 
(and quite easy to be sold
by a mass vendor).
And thanks to cluster of SMPs with the help of linux, the HPC market is 
now "democratic".

Of course, it would be nice to have a true vector unit on a P4 or Opteron.
But the problem will be the memory access again.

Bye,

  Phil.