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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: This is not all that surprising...
By Motoman on 1/18/2009 11:09:22 AM , Rating: 2
Intel and AMD have already dealt with this in the real world.

Really? Please illucidate this topic for us.

RE: This is not all that surprising...
By Reclaimer77 on 1/18/2009 11:54:53 AM , Rating: 1
It's a non topic. You think Intel and AMD are a bunch of idiots who blindly add cores to CPU's without taking memory usage into account ?

I'm not sure what you want me to say. The article is simply stating the obvious, and it sure as hell isn't news to Intel or AMD. Why do you think we have on die memory controllers and dual and triple channel memory now ?

How do you explain that software WRITTEN for 8 threads runs faster in the i7 than quad cores ?

RE: This is not all that surprising...
By Motoman on 1/18/2009 12:08:10 PM , Rating: 2
...How do you explain that we can expect *all* applications to benefit from an 8-core processor? Or 16-core?

I think that Intel and AMD are geniuses...they ran into a wall and found a way around it. But I think people like you are either far too into specialized niches that *will* benefit from lots of cores, or too far bought into the marketing to actually think about the ramifications for the typical consumer.

Applications and games that are used by the typical consumer are simply not going to be able to spread across a whole lot of parallel cores. They just aren't. So if there is some magic that will allow purely serial processes to run across multiple parallel cores, please let me know. If there isn't, please stop apparently pretending that more cores is better for everything...because it isn't.

By retrospooty on 1/19/2009 8:49:05 AM , Rating: 2
"...How do you explain that we can expect *all* applications to benefit from an 8-core processor? Or 16-core?"

??? I dont... Because we dont. Who expects that?

What we ALL know is that only mutithreaded apps benefit from multi cores and we ALL know that most games and high end apps that need extra CPU power ARE being written for multiple threads. Apps that dont need the CPU power are generally left alone.

By Jeff7181 on 1/18/2009 1:57:12 PM , Rating: 3
Ever heard of double data rate memory? Dual memory channels? Quad memory channels? Quad pumped busses?

CPU manufacturers understood a ALONG time ago that as the processing power of CPU's increase, the demands on the external bus increase also. All those things mentioned above are designed to provide the CPU with more memory bandwidth to allow the CPU to operate to it's potential.

Do you think Intel is using three memory channels for their newest chips because they got sick of seeing either 2 or 4 memory slots on a motherboard and wanted to mix it up a little with 3 or 6? Of course not... it's because they've already identified a problem feeding their new dual and quad core processors with enough data for them to crunch so they increased the memory bandwidth by adding a third channel.

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