When does 1 GHz + 1 GHz + 1 GHz + 1 GHz not necessarily equal 4 GHz? When you're calculating the performance potential of a multicore chip.

Freescale Semiconductor recently introduced a DSP chip, the MSC8144, that contains four 1-GHz SC3400 processor cores. Freescale characterizes the new chip as being "performance equivalent" to one 4-GHz core. But is it really? As usual, the answer is, "It depends."

If you're running small programs that have minimal interaction with one another--as is common in voice-over-Internet Protocol infrastructure applications--then yes, it's possible that four 1-GHz cores will be "performance equivalent" to a single 4-GHz core.

But what if your program is a big one that needs to be partitioned across cores and requires significant intercore communication, such as high-resolution video compression? Or, what if your chip is running a dynamic mixture of software--for example, a mixture of voice and video channels that changes based on demand?

In cases like those, you probably won't get full utilization of the cores. And figuring out what you will get is tricky; it can be surprisingly difficult to answer questions like, "Which chip is faster?" or, "How much processing headroom will I have?"

Concepts like "busy" and "idle" become murky when you're working with multicore chips. What if three cores are busy and one is idle? Do you count all of those idle cycles as "headroom," as you typically would in a single-core chip? Maybe yes, maybe no.

If the idle core is competing with the busy cores for resources (such as L2 cache), then those "idle" cycles may be hard to use. And if the core is idle for only a few cycles at a time, those cycles may be useless.

It would be a mistake to simply add up all the free cycles, regardless of their temporal proximity to one another or the states of the other cores, and call that headroom. Assessing multicore-chip performance is a complex process, and it requires much more than simple addition.

Jeff Bier is the president of Berkeley Design Technology Inc. (www.BDTI.com), a consulting firm providing analysis and advice on DSP technology. Jennifer Eyre of BDTI contributed to this column.