February 2007
The Promise and Perils of the Coming Multicore Revolution and Its Impact
John McCalpin, Advanced Micro Devices, Inc.
Chuck Moore, Advanced Micro Devices, Inc.
Phil Hester, Advanced Micro Devices, Inc.

4. Longer-Term Extrapolations
SMP on a chip

The RISC SMP market of the mid to late 1990's was dominated by 4, 8, and 16-way SMPs. This "sweet spot" in the market provided enough extra CPU power to justify efforts at parallelizing applications, without incurring unacceptable price overheads or providing too many processors for applications to use effectively. Current trends suggest that similar SMP sizes will be available on a single chip within a few years, leading one to speculate in several directions:

  • Will multicore chips create a new market for parallel codes in the same way that RISC SMPs created the UNIX server market in the 1990's?
  • Will effective exploitation of multicore processor chips require fundamental changes in architecture or programming models, or will growth in parallel applications happen on its own – perhaps supported by more modest architectural enhancements (e.g., transactional memory)?
  • Will the increasing number of cores/threads available on a single chip obviate the need for larger SMP systems for the majority of users?

Even at the scale of one socket and two socket systems, the increasing number of cores per chip will likely lead to users running combinations of multithreaded and single-threaded jobs (none using all the CPU cores) – more like the large SMP servers of the last decade than like traditional usage models. Increasing numbers of cores is also likely to lead to widespread adoption of virtualization even in these small systems, with multiple guest operating systems having dedicated cores, but competing for memory space, memory bandwidth, shared caches, and other shared resources.

Design Space Explosion

The simple examples at the beginning of this article demonstrated the complexity of defining appropriate figures of merit for microprocessor chips with fairly limited degrees of freedom (e.g., one or two cores plus small or large cache). Dual-core processors first shipped in volume from 90 nm fabrication processes, with quad-core processors expected in quantity in 2007 based on 65 nm process technology. Going to 45 nm provides the possibility for another doubling (8 cores), 32 nm provides the possibility for another doubling (16 cores), and 22 nm technology providing yet another doubling (32 cores) is considered feasible.

Recent studies11 have shown that the CMP design space is multidimensional both from the engineering and application performance perspectives. The parameter space associated with all the design possibilities inherent in the flexibility of having so many independent "modules" on chip is huge, but the real problem is that the definition of performance and performance/price metrics grows to have almost as many degrees of freedom. If every application of interest has different optimal design points for single-threaded performance, multi-threaded performance design point, single-threaded performance/price design point, and multi-threaded performance/price, then making design decisions is going to become even more difficult, and the struggle between commodity volumes and a proliferation of independent "optimal" design points will become one of the key challenges facing the industry.

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Reference this article
McCalpin, J., Moore, C., Hester, P. "The Role of Multicore Processors in the Evolution of General-Purpose Computing," CTWatch Quarterly, Volume 3, Number 1, February 2007. http://www.ctwatch.org/quarterly/articles/2007/02/the-role-of-multicore-processors-in-the-evolution-of-general-purpose-computing/

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