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RE: CELL processors considered interesting?
It is definitely an architecture that will need support for concurrency
allowing to overlap communication and computation.
However, do the SPE allows concurrent processes? Or are they slaves under
control of the PPC?
Secondly, process switching should be fast else one can only afford
coarse-grain concurreny and the model starts to break down, especially when
combined with channel communication. On the TI C40 and ADA Sharc they also
speak of 'channels' but it takes a few pages of tricky assembly to put these
at wokr in a general purpose way (=asynchronous).
-----Original Message-----
From: owner-occam-com@xxxxxxxxxx [mailto:owner-occam-com@xxxxxxxxxx]On
Behalf Of Jim Moores
Sent: Friday, February 18, 2005 12:06 PM
To: 'Tom Locke'
Cc: occam-com@xxxxxxxxxx
Subject: RE: CELL processors considered interesting?
> -----Original Message-----
> From: owner-occam-com@xxxxxxxxxx [mailto:owner-occam-com@xxxxxxxxxx] On
> Behalf Of Tom Locke
> Sent: 18 February 2005 05:52
> To: occam-com@xxxxxxxxxx
> Subject: Re: CELL processors considered interesting?
>
> From what I understand the Cell has one general purpose core and 8 SIMD
> cores.
> Please correct me if I'm wrong... as I understand it this makes it an
> entirely different beast to the multi-core Transputer that was once on
> the cards. Better suited to crunching large data sets than general
> purpose CSP style concurrency? (the vendors certainly seem to be
> pitching it like this - video-game graphics, 'multimedia workstation')
Hi Tom,
My understanding is that they are self contained hybrid integer/Vector
processors and not just extra vector units tacked onto the standard core.
Yes they will be best at processing large amounts of single precision
floating point data but then a Pentium 4 will also get the best throughput
processing large amounts of floating point data in its SSE2/3 units but that
isn't what it spends most of it's time doing...
Each SPE seems to be a fully functional integer machine in its own right
with their own branching, logical tests, load-stores and all the other stuff
that makes up a standard RISCy processor.
The unusual things about the architecture are that the (large) register set
is shared between vector, floating point and integer operations (from what I
can tell) and also that the 256K of private memory is just that: private,
and not a hardware controlled primary cache (although I think you can use
software to make it act a bit like a cache).
I admit it isn't 100% clear that my understanding is correct, but have a
look at this
http://www.realworldtech.com/page.cfm?ArticleID=RWT021005084318, which is
easily the most detailed article I've seen yet on the CELL and see what you
think.
It also mentions communications channel DMA/compute overlap and 'channel'
instructions, which sounds rather familiar.
Don't get me wrong, I'm not trying to say it's a Transputer, but I think it
could be a useful target for CSP style concurrency. Think of it as a T800
with a fixed width instruction set, a less rich set of
concurrency/communications primitives, a large register set and a vector
unit with 256K of on-chip memory. Doesn't sound so bad?
Jim
--
Jim Moores
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