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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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RE: 2x4
By Omega215D on 1/19/2009 12:08:08 AM , Rating: 2
On my Opteron 170 (Dual core) can play all those games listed at 1680x1050 with no problem, the graphics cards still play a major part in those games. The only game I ran into a problem with was Grand Theft Auto IV in which I had to overclock my processor to 2.7GHz to get it to run smoothly.

I then jumped on board to the Phenom 9600 and GTA IV runs much smoother without having to overclock. I'm sure this will be the case with future games like Alan Wake, Deus EX 3 etc.

Dual cores for a majority of today's games will be good enough considering I was still on a socket 939 platform where the Intel Core 2 Duo shows a major performance increase in all games.

RE: 2x4
By BeastieBoy on 1/20/2009 10:02:59 AM , Rating: 2
This argument makes it clear why consoles have become so popular. Let the game developers worry about making the games playable on my system, rather than trying to make my system play the games.

RE: 2x4
By jrb531 on 1/20/2009 11:25:05 AM , Rating: 2
GTA4 is a poorly written "port" that uses as excess of CPU cycles because it is not properly programed!

The Xbox 360 (which is the version that was ported) has an ATI/AMD 1950XT in it and as such the port is not designed to take advantage of the faster video cards and as such is forced to use your CPU to compensate.

The PC version also runs at a much higher resolution than the 360. This is not an example to use.

Sure using "brute force" to try and do a quick and dirty console port may work but it's the worst example you can find for saying that the CPU is most important for games.

“So far we have not seen a single Android device that does not infringe on our patents." -- Microsoft General Counsel Brad Smith
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