Multicore Doesn't Mean Equal Core

Via GCN -

As anyone who has worked on a group project knows all too well, not all team members contribute equally to the success of a project. And now Virginia Tech researchers have found the same holds true for the cores in multicore processors.

Depending on how your code is distributed across seemingly identical cores, the speed at which that code is executed on a multicore processor can vary by as much as 10 percent.

If you've ever had a program perform slower than expected or perform quickly on one day and not as spritely the next, you might want to examine how that CPU is executing the job.

"The solution to this is to dynamically map processes to the right cores," said Thomas Scogland, a Virginia Tech graduate student who summarized this quirk at the SC08 conference in Austin, Texas, last month. Scogland and fellow researchers, with help from the Energy Department's Argonne National Laboratory, developed prototype software that could one day help balance performance more equally across all cores. DOE also helped fund the work.

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In all fairness, it's not the cores' fault, technically speaking. Although the cores are identical, how a program is distributed among the cores can affect how quickly it runs. And in most cases, the operating system and hardware spread a program across multiple cores rather arbitrarily, which leads to varying performance.

A number of factors contribute to that variance, the researchers said. One factor is how the CPU hardware handles interrupts. In many cases, they could be directed to a single core, which could slow other applications on that core. However, if the interrupts are distributed across all the cores dynamically, there is no guarantee that the core handling the interrupt will be the same one that is running the program for which that interrupt was intended. Therefore, additional communication time is needed between the two cores.