Multicore
A multicore processor is a chip with more than one processing units (cores). Mostly, each unit can run multiple instructions at the same time and each has its own cache.
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The limitations of single-processor architecture
- Narrow data bandwidth, big gap between CPU speed and memory speed: It is reported that on average, 75% of CPU time is wasted in waiting for memory access results.
- High frequency has a ultimate size limit: at light speed, a 100GHz chip can only be in size of 3 mm.
- long pipeline introduces big penalty for mis-prediction/wrong speculation
- High energy consumption and hard to cool off
- Bad cost/performance ratio
Multicore architecture is a solution
A multicore architecture is actually a SMP implemented on a single VLSI integrated circuit. The goal is to allow greater utilization of thread-level parallelism (TLP), especially for applications that lack sufficient instruction-level parallelism (ILP) to make good use of superscalar processors. It is called Chip-level multiprocessing (also known as CMP), or Chip-level multiprocessing (CMT)
The characteristics of a CMP system
- A slow but wide approach: improve the throughput of the whole computer system.
- Good for transaction processing, database and scientific computing applications.
- No benefits for single application that cannot be parallelized (divided and run on several tasks or threads)
- Better data locality than regular multi-processor architectures
- Better communication between processing units
- saves space, save energy.
- better cost/performance ratio than single-core processor
Example multicore processors
The whole microprocessor industry is jumping into multicore today. The latest versions of most RISC architectures use CMP, including
- PA-RISC (PA-8800),
- IBM POWER (POWER4 and POWER5)
- Sun Microsystems SPARC (UltraSPARC IV).
- AMD Opteron (shipping in May of 2005)
- Intel Pentium D (shipping in May 2005)
Other microprocessor families are also expected to use CMP in future versions.
- Intel's Itanium is expected to do so in the second half of 2005, with a release codenamed Montecito; then even more extensively in 2007 with a product codenamed Tukwila.