Multicore - Definition and Overview

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.

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 nm.
• long pipeline introduces big penalty for mis-prediction/wrong speculation
• High energe 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 Multithreading (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 SPARC (UltraSPARC IV).

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.
• Intel's Pentium is expected to start using CMP in 2006
• AMD's Opteron is expected to incorporate the technique in mid-2005.

See also

• Dual-core
• CMP, SMP
• SMT