Re: Roadrunner Supercomputer using 12,960 CELL Processors Hits 1

On Jun 12, 3:54 am, already5cho...@[EMAIL PROTECTED]
 wrote:
> On Jun 12, 3:43 am, Robert Myers <rbmyers...@[EMAIL PROTECTED]
> wrote:
>
>
> > BG/L will do fine on some kinds of problems.  Just not ones requiring
> > significant global communication.  That was my beef about Blue Gene
> > and Red Storm.
>
> It depends on the kind of communication.
> If the communication consist mostly of small bi-directional messages
> these machines seem to be much better than anything else in existence.
> For the large bandwidth-bound messages on BG/L the picture is less
> rosy. I didn't see numbers for Crays XTn of comparable size (not sure
> they exist); in theory they should be significantly better than BG/L.
> However, I do not see why the roadrunner should be any better for
> bandwidth-bound global communication. If anything, I expect it to do
> worse, esp. in the worst case.
>

Well, actually, it looks like you're right.  I calculated the
bisection of bandwidth of BG/L in the tens of millibytes/flop (because
of the geometry and large mesh), and it turns out that the bisection
bandwidth of Roadrunner could be no better than 10 millibytes/flop,
because of the number of flops loaded onto a single node by way of the
Cell processors (about 100 gigaflop DP per node) connected by a
infiniband interconnect (about 1 gigabyte per second) that is wimpy by
comparison. :-(

Using the fat-tree topology addresses the scalability of bisection
bandwidth, but the individual links aren't properly scaled to the
nodes.

Robert.