CPUs rev new engines

Computerworld - In 1971, Intel Corp. introduced its first microprocessor chip, the 4004. The 4-bit processor chugged along at a mere 104 KHz. In the 35 years since then, processor clock speeds—and performance—have doubled about every 18 months.

Today, however, it's becoming more difficult and expensive to boost the speed of processors while keeping them cool. Chip designers use many techniques to wring more throughput from a processor chip without increasing its clock rate. Those techniques include multithreading, instruction-branch prediction and clever uses of cache. But the most promising approach is to put more than one processing engine on a chip.

In 2001, IBM introduced the first mainstream "dual-core" chip, the Power4, for its IBM eServer pSeries and iSeries servers. Early this year, Sun Microsystems Inc. shipped its UltraSparc IV with two cores for its Sun Fire V series servers, and Hewlett-Packard Co. unveiled its own dual-core PA-RISC 8800 processor. Advanced Micro Devices Inc. responded last summer by demonstrating an x86-based, 64-bit, dual-core Opteron processor. Intel Corp. subsequently announced plans to ship its Itanium 2-based, dual-core Montecito CPU in 2005.

The chip makers say that within two years, most processor chips—from desktop systems on up—will have two or more processing units. The reasons for this are compelling. A dual-core chip might provide twice the performance of a single-core chip at a much lower cost than two single-core chips can. Communication between two processors is faster when they're on one chip, and cache sharing can make processing more efficient. Dual-core processors also use less space, consume less power and generate less heat than separate processors do.

Reality Check

Vendors claim that multicore chips are well suited for transaction processing and for database and scientific applications.

"It's probably fair to say that the realistic range is 40% to 80% faster," says Kevin Krewell, editor in chief of the "Microprocessor Report" newsletter and an analyst at In-Stat/MDR in San Jose. They're less effective on single-application machines and for applications whose instructions can't be broken into parallel streams, he adds.

While the number of transistors on a chip is still doubling every 18 months, how that extra capacity is used is about to change. "This is the end of the clock-speed race," Krewell says. "As more transistors are available, do you go for higher instructions per cycle? Most people think we have come close to the limit of what can be done there." So those extra transistors are used to build another processing engine—and to enable multithreading, in which multiple instruction streams, or threads, execute in parallel. Indeed, earlier this month, Intel scrapped its plan to boost the speed of its Pentium 4 chip from 3.6 GHz to 4 GHz in favor of enlarging on-chip cache.