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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 SlyNine on 1/18/2009 11:55:18 PM , Rating: 2
Mean while the 4850 is around 8x faster then your 7900GS.

In games since they only really use 2 cores right now your Q6600 is about equal to a opty at 3ghz. My friend I have run many benchmarks with the A64 x2 and Q6600.

You are dead wrong.


RE: 2x4
By Denithor on 1/19/09, Rating: 0
RE: 2x4
By SlyNine on 1/19/2009 11:43:29 PM , Rating: 2
First off core per core the E8500 is better then the Q6600, Second off I benchmarked both the Q6600 and 4200X2 at stock and overclocked speeds . At 2.4 ghz the Q6600 does not score hardly any more FPS then a 4200X2 at 3Ghz in games that do not support good SMT.

I didn't say any thing about gaming on a single core, But I can tell you assassins creed isn't even playable on a single core.

SupCom may get higher FPS and have a smoother interface w/quad, but suffers from the same simulation slow down at 2.4 ghz as a 4200X2 @ 3.0ghz. Supreme Commander does not support Quad core's with a crap unfortunately because right now it's my favorite game.

UT3 is good and I have yet to try GTA4.

With a Q6600 stock I've seen maybe a 2% increase in FPS from my old A64 at 3.0ghz. an e8500 is probably a bit better. But still not night at day in games, This isn't sisoft sandra were talking about.

Moving forward yes, the Q6600 is a better future investment then any dual core.


RE: 2x4
By mindless1 on 1/21/2009 4:57:42 PM , Rating: 2
Why would anyone use a turd like GTA4 as an example?

Bottom line - You don't need more than 3GHz from a dual core processor to play most modern games fine, providing the video card is also up to snuff.

A single core at 3GHz is also viable for over 50% of the games out there today. It may not win the benchmark graphing contest, but it will maintain a playable framerate at enough games. Will it play the extra demanding ones well? Consider the question before answering, benchmarks and reviewers deliberately try to find something to show contrast, instead of the typical games that will run fine on most of the hardware tested!

Truth is, yes someone with a single core Athlon 64 who upgraded their video a couple times has had a great value run at gaming. They may not be able to extend this much into the future, but as always we can't ever think any gaming combo will last far into the future until the future is here.

Any game that won't run properly on a $60 CPU is defective. That's what the eyecandy adjustment settings are for, but in the end those make not much difference in gameplay enjoyment compared to the eyecandy settings related to GPU performance and resolutions possible at playable framerate.

It always was and still is more about the video card than anything else. Moreso than ever today with monitors continuing to rise in resolution.


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