HPCwire: The Impact of Multi-core Processors on Application Performance

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March 21, 2008

The Impact of Multi-core Processors on Application Performance

by Dave Field

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The era of single processor systems is over; the multi- and many-core systems world is here. If you're not ready for this change, there's an IT train wreck in your future. We're entering a phase where taking full advantage of the power of multi-core processors is critical for customers to continue to accelerate innovation and to improve their business success.

Single core and life was good

For many years, the microprocessor community has translated Moore's Law of transistor density into a direct doubling of single-threaded performance every 18 months. Applications ran faster on each new processor version, and new versions were released frequently. Performance tuning of applications required minor experimentation with compilers and tuning flags.

This was a period of high productivity for application developers, since they could concentrate on product functionality and performance and minimize the time to create, tune, test, and support computer-model-unique versions. It was fun, but it could not last forever.

Today, the era of single processor systems is over. The multi- and many- core systems world is here. If you're not ready for this change, there's an IT train wreck in your future. We're entering a phase where taking full advantage of the power of multi-core processors is critical for customers to continue to accelerate innovation and to improve their business success. Dual-core technology is now pervasive in the industry; quad-core processors are here and about to become the new standard for server nodes, and roadmaps pointing to octal-core processors are not far off.

For applications not designed to take advantage of the increased raw compute power that comes with the availability of the added cores, applications may run slower. This likelihood increases as the core count increases. So, even though that bright shiny new server you just bought has more raw available compute capability, your applications may run more slowly.

Dual- and quad-core processors take the heat

The real question is how did we get here? Did we really think that Moore's law could go on indefinitely? It used to be that with each new system, the clock speeds increased and the systems ran faster. However, as the chip manufacturers have learned, there are repercussions. More speed requires significantly more power. More power means more heat. More heat means more cooling. Today, we're at the point where, if we continue on this power, performance, heat and cooling curve, even your laptop will soon require water cooling followed by liquid nitrogen for cooling.

Dual-core processors, and then quad-core processors, often get the blame for making it more difficult and complex to program applications. However, they also get the credit for keeping power and cooling requirements at reasonable levels. The real issue is balancing system power, cooling, I/O, memory and cache. To meet new system balance requirements for power and cooling, clock speeds have declined: the more cores per processor chip, the lower the clock frequency. Moore's Law continues, but the additional transistors are used to implement more cores and larger caches.

As a result of this, the problem has shifted from the hardware challenge of making things run faster with faster clock cycles, to the software challenge of how to use all the additional cores (raw compute power) now available on the chip to improve performance. Unfortunately, this has created coding problems for application developers. A multi-core processor can do more work than a single-core processor, so the total amount of work, in compute jobs per month, increases on multi-core-based servers. But without taking into consideration the multi-core nature of today's systems, the performance of an individual application will not increase; it is likely to run more slowly as the number of cores increases, due to the combination of lower clock rates and competition for memory bandwidth and cache.

And it's not just the applications. Sending all your communications interrupts, for example, to one core could overwhelm that core -- possibly slowing down the rest of the system. It's that system balance thing again.

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