[Beowulf] Larrabee - Mark Hahn's personal attack

Mark Hahn hahn at mcmaster.ca
Fri Jan 27 07:37:43 PST 2012

>>> Larrabee indeed resembles itanium to some extend, but not quite.
>> wow, that has to be your most loosely-tethered-to-reality statement
>> yet!
>> it's true that Larrabee and Itanium are very close
>> in the number of letters in their name.
> Your personal attack seems to indicate you disagree with my
> qualification of the entire Larrabee line
> having any reality sense in the long run.

not surprisingly, no: I disagree that Larrabee and Itanium resemble
each other in any but really silly ways.

Itanium is a custom, VLIW architecture; Larrabee is an on-chip
cluster of non-VLIW, commodity x86_64 cores.

none of the distinctive features of Itanium (multi-instruction bundles,
dependency on compile-time scheduling, intended market, implementation,
success limited to predictable, high-bandwidth situations, directory-based
inter-node cache coherency) are anything close to the features of Larrabee
(standard x86_64 ISA, no special compiler needed, on-chip message-passing
network, suitable for complex/dynamic/unpredictable loads, possibly not even
cache-coherent across one chip.)

my guess is that you were thinking about how ia64 chips tended to run 
at low clock rates, and thinking about how gpus (probably including
larrabee) also tend to be low-clocked.

> Instead of throwing mudd, mind to explain why a Larrabee,
> an architecture far away from mainstream, makes any chance of
> competing in HPC
> with the existing architectural concepts in the long run?

as far as I know, larrabee will be a mesh of conventional x86_64 cores
that will run today's x86_64 code.  I don't know whether Intel has stated
(or even decided) whether the cores will have full or partial cache
coherency, or whether they'll really be an MPI-like shared-nothing cluster.

if you want to compare Larrabee to Fermi or AMD GCN, that might be 
interesting.  or to mainstream multicore - like bulldozer, with 
32c per package vs larrabee with ">=50".

but not ia64.  it's best we all just forget about it.

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