What does 24GFLOPs GPU speed mean?

Per Wikipedia a GFLOP is a Giga(Billion) Floating-point Operations Per Second. That can be calculated by

http://en.wikipedia.org/wiki/FLOPS

All info provided by Boardcom about the BCM2935 which uses the VideoCore 4 GPU.

http://www.broadcom.com/products/BCM2835

I the link below states that the default GPU is set at 250MHz

http://elinux.org/RPiconfig

That is about all I was able to dig up. I will move this thread over to the Raspberry Pi Group to see if anyone there can provide furnther input.

Thanks,

Jason G.

Hi Jason,

   I really did intend this as a question for Element14 to answer via Feedback and Support,

asking what does Element14 mean when they claim 24GFLOPS performance,

rather than a question for discussion between RPi users as to what we think

Element14 is claiming.

   I don't see anything at the Broadcom page you mentioned that claims 24GFLOPS.

   User shabaz cites a definition requiring 64-bit precision, which I agree is standard,

but I don't think I've seen any claims that the GPU can even do 64-bit floating point.

   I presume the claim is that a single RPi does 24GFLOPS, so I'm not sure why you

selected Wikipedia's long formula that includes sockets/node and nodes/chassis.

   There is a claim here:

http://www.element14.com/community/groups/raspberry-pi/blog/2012/02/29/raspberry-pi-gpu-performance-dominates-the-iphone-4s-and-tegra-2-based-smartphones

that the RPi VideoCore IV is "dual core", but I think that's a mistake.

Hi Jason,

   I really did intend this as a question for Element14 to answer via Feedback and Support,

asking what does Element14 mean when they claim 24GFLOPS performance,

rather than a question for discussion between RPi users as to what we think

Element14 is claiming.

   I don't see anything at the Broadcom page you mentioned that claims 24GFLOPS.

   User shabaz cites a definition requiring 64-bit precision, which I agree is standard,

but I don't think I've seen any claims that the GPU can even do 64-bit floating point.

   I presume the claim is that a single RPi does 24GFLOPS, so I'm not sure why you

selected Wikipedia's long formula that includes sockets/node and nodes/chassis.

   There is a claim here:

http://www.element14.com/community/groups/raspberry-pi/blog/2012/02/29/raspberry-pi-gpu-performance-dominates-the-iphone-4s-and-tegra-2-based-smartphones

that the RPi VideoCore IV is "dual core", but I think that's a mistake.

It's been a long time since I was active in supercomputing, but here's my understanding.  AFAIK, there's no requirement that FLOPS have to be double-precision.  There's no reason for a GPU to do double precision, so I can't imagine why Broadcom or anyone else would do so.

Generally, GFLOPS is a marketing number so the specs are as optimistic as possible.  It's generally a "peak" number, and according to one wag it's "a guarantee from the manufacturer that you won't go faster than this".  So they generally just multiply FLOPS times the clock rate, and say "sure, you could go this fast if you could get the operands to the arithmetic hardware".  That's usually very hard to do in general, so it's only with highly structured computations like inner products that you can actually get close to the peak.  If the problem is embarassingly parallel enough and you do some hand-tuning, you can get pretty close.

Another spec game: the main processing element of a floating-point engine is usually multiply-accumulate, since it's easy to do both at the same time.  These are counted as two FLOPs, so the GFLOPS rating immediately doubles.

As far as benchmarks are concerned, I note that chez RasPi a certain moderator often claims that VideoCore 4 is higher performance than its competitors, but I've never seen him quote an independent study.  If anyone has seen any, let us know.