Advanced Micro Devices Inc (AMD)

[dacaw]

Large cache & real world problems.

I've done a lot of fluid dynamics. I've published (15 years ago now) in this field. When I started CFD 25 years ago it was really in its infancy.

You always start with a small number of steps (finite-difference based methods) or a small number of mesh points (finite element methods). The governing parameter is the speed of solution - it used to be the size of virtual memory, now that is relaxed on workstations.

You want to have really detailed modeling of the system -i.e. lots of points - but you always hit the limit of run time. Its no fun waiting for your solution all day to find out it blew up.

So there is no magic "ideal" working set. More is always better if you have a fast enough system.

The specfp tests have a significant working set but its a fixed set. Cynical manufacturers could make processors where all of the code+data fits on-chip. Of course when the real user then tries to extend the simulation, and the code+data spills into physical memory, he/she is going to be disappointed. There is another point where the spillage from physical->virtual (i.e.disk) kills you but physical is cheap and relatively easily extended. On-chip cache is neither cheap nor easily extended.

That's why you don't take these specfp results only at face value. Take a trip to somewhere like comp.arch and see what people think about the itanic there. They will say exactly the same thing as me: these specfp results are all about false benchmarking.