RE: Systems engineering

From: Michael Wilson (
Date: Fri May 20 2005 - 11:41:09 MDT

The debate about minimum compute power required for AGI in general
and brain simulation in particular is relevant and open, but something
of a dead horse for the SL4 list. However;

> 100 to 100,000 Tflops assumes simulating a parallel process on
> serial computing substrate correct? How then does the advent of
> parallel hardware such as the Cell Processor (2 Tflops each)
> factor into the calculation?

Firstly absolute compute power is a distinct measurement from the
parallelisability of a particular computation. In a wholly serial
process the dependency between calculations forms a single chain.
In a wholly parallel operation (e.g. vector multiplication) every
calculation can be performed independently. In addition to the
absolute parallelisability of a process (i.e. minimum serial
step count and minimum parallelism required to achieve this),
computational and memory locality is very important as they
determine internode and CPU-memory bandwidth requirements.

Secondly the Cell processor /does not/ have a capability of 2
teraflops. This is the claimed 'total system performance' of the
PS3, the vast majority of which is contributed by the dedicated
graphics processor. The document you linked to only claims 300
gigaflops and that is both out of date (for the PS3 at least the
Cell will have 7 SPEs not 8 and will run at 3.2 GHz not 4.6 GHz)
and wildly optimistic. Looking at the functional units, instruction
set and issue width of the Cell it has a theoretical peak capacity
of around 200 gigaflops, assuming the bulk of your computation
consists of vector operations on 32-bit floats (16-bit data will
double that). However for various reasons the IPC of the cell is
likely to be relatively low, so it's unlikely that any practial
code will exceed 100 gigaflops. That's still very impressive, but
it's 20 times lower than the impression you gave and even that
relies on AGI code being reasonably parallelisable at the vector,
instruction and thread level. I'm excited about the Cell too, but
lets not get carried away by the waves of spin and PR.

 * Michael Wilson

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