[Beowulf] getting a phi, dammit.

Mark Hahn hahn at mcmaster.ca
Fri Mar 8 10:56:55 PST 2013

> How about turning this on its head and installing a Phi directly on
> the mainboard, with lots of memory attached (unless it has some
> in-built limits, haven't checked...)

Phi is implemented similar to a GPU: it has a very wide interface 
to soldered-on gddr5 ram.  gddr5 channels are 32b wide, and it has 16x, 
which for around 300 GB/s works out to a gddr5 performance of 5.860 Gt/s.
AFAIK, gddr5 can't tolerate multiple banks (so that's 16 chips,
each obviously 4Gb, which is the largest in production.)

> only for I/O - for which it should be fast enough. To avoid changing
> the architecture too much, attach the whole thing to an Atom CPU - low
> power, but fast enough to handle the I/O and to control the
> computation on the Phi.

no, the atom would be fairly pointless, since the Phi already has plenty
of cores.  as far as I know, you could plug the Phi into an otherwise 
hostless PCI backplane - it would mainly just need a BIOS like a normal
PC, and it would probably have to do some emulation for that, since I
think I remember reading that Phi doesn't implement the same set of modes
as a "normal" x86 (good old real mode, for instance.)

in short, I think it's a question of pin counts and when higher-density
gddr5 chips will arrive.

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