HPCwire: More Cores Can Mean Slower Supercomputing

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January 14, 2009

More Cores Can Mean Slower Supercomputing

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16 multicores perform barely as well as two for complex applications

ALBUQUERQUE, N.M., Jan. 13 -- The worldwide attempt to increase the speed of supercomputers merely by increasing the number of processor cores on individual chips unexpectedly worsens performance for many complex applications, Sandia simulations have found.

A Sandia team simulated key algorithms for deriving knowledge from large data sets. The simulations show a significant increase in speed going from two to four multicores, but an insignificant increase from four to eight multicores. Exceeding eight multicores causes a decrease in speed. Sixteen multicores perform barely as well as two, and after that, a steep decline is registered as more cores are added.

The problem is the lack of memory bandwidth as well as contention between processors over the memory bus available to each processor. (The memory bus is the set of wires used to carry memory addresses and data to and from the system RAM.)

A supermarket analogy

To use a supermarket analogy, if two clerks at the same checkout counter are processing your food instead of one, the checkout process should go faster. Or, you could be served by four clerks.

Or eight clerks. Or sixteen. And so on.

The problem is, if each clerk doesn't have access to the groceries, he or she doesn't necessarily help the process. Worse, the clerks may get in each other's way.

Similarly, it seems a no-brainer that if one core is fast, two would be faster, four still faster, and so on.

But the lack of immediate access to individualized memory caches -- the "food" of each processor -- slows the process down instead of speeding it up once the number of cores exceeds eight, according to a simulation of high-performance computers by Sandia's Richard Murphy, Arun Rodrigues and former student Megan Vance.

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