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Sandia simulations reveal memory is the bottleneck for some multi-core processors

Years ago, the hallmark of processor performance was clock speed. As chipmakers hit the wall on how far they could push clock speeds processor designs started to go to multiple cores to increase performance. However, as many users can tell you performance doesn't always increase the more cores you add to a system.

Benchmarkers know that a quad core processor often offers less performance than a similarly clocked dual-core processor for some uses. The reason for this phenomenon according to Sandia is one of memory availability. Supercomputers have tried to increase performance by moving to multiple core processors, just as the world of consumer processors has done.

The Sandia team has found that simply increasing the number of cores in a processor doesn't always improve performance, and at a point the performance actually decreases. Sandia simulations have shown that moving from dual core to four core processors offers a significant increase in performance. However, the team has found that moving from four cores to eight cores offers an insignificant performance gain. When you move from eight cores to 16 cores, the performance actually drops.

Sandia team members used simulations with algorithms for deriving knowledge form large data sets for their tests. The team found that when you moved to 16 cores the performance of the system was barely as good as the performance seen with dual-cores.

The problem according to the team is the lack of memory bandwidth along with fighting between the cores over the available memory bus of each processor. The team uses a supermarket analogy to better explain the problem. If two clerks check out your purchases, the process goes faster, add four clerks and things are even quicker.

However, if you add eight clerks or 16 clerks it becomes a problem to not only get your items to each clerk, but the clerks can get in each other's way leading to slower performance than using less clerks provides. Team member Arun Rodrigues said in a statement, "To some extent, it is pointing out the obvious — many of our applications have been memory-bandwidth-limited even on a single core. However, it is not an issue to which industry has a known solution, and the problem is often ignored."

James Peery, director of Sandia's Computations, Computers, Information, and Mathematics Center said, "The difficulty is contention among modules. The cores are all asking for memory through the same pipe. It's like having one, two, four, or eight people all talking to you at the same time, saying, 'I want this information.' Then they have to wait until the answer to their request comes back. This causes delays."

The researchers say that today there are memory systems available that offer dramatically improved memory performance over what was available a year ago, but the underlying fundamental memory problem remains.

Sandia and the ORNL are working together on a project that is intended to pave the way for exaflop supercomputing. The ORNL currently has the fastest supercomputer in the world, called the Jaguar, which was the first supercomputer to break the sustained petaflop barrier.



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Why?
By jrb531 on 1/20/2009 11:42:51 AM , Rating: 2
I find it humorous (or sad) that people with differing "opinions" take this stuff so personal.

If you bought a 4x core CPU why are you "offended" if someone has the "opinion" that a 2x core CPU that runs faster "might" have been a better choice?

In some games a 4x core 2500mhz CPU runs slower (yet costs a ton more) than a 2x core running at 3000mhz.

This is FACT and not something made up. In other games (not many) the 4x core will be faster and I'm sure future games will take better advantage of extra cores.

Why does this "FACT" somehow upset people?

If you bought a 4x core (at a much greater expense) because you did your homework and some of the apps you run can take advantage of the extra cores then so be it...

but if you bought into the "hype" of more cores are "always" better without doing your homework then shame on you and getting into some kind of "my cores are greater than your cores" debate does nothing but make you look silly.

Fo me I'll stick with my lowly AMD 7500 for $75 and enjoy my Nvidia 260 until the prices of the 4x cores come down. Maybe by then more games (which is why I built this computer) will do something with the extra cores but right now I fail to see why I should pay an extra $200 for a Phenom II.

I respect that others may feel differently but as long as my games running at max resolution and setting obtain a "minimum" of 30fps (IE the framerate never drops below 30fps but is often much muhc higher) then I fail to see the difference in playing a game at a "minimum" of 30fps and one that gets 20000000000000fps




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