Moore’s law

• The number of transistors on a die doubles every 18-24 months
  • Exponential growth in available transistor count
  • If transistor utilization is constant, this would lead to exponential performance growth; but life is slightly more complicated
  • Wires don’t scale with transistor technology: wire delay becomes the bottleneck
  • Short wires are good: dictates localized logic design
  • But superscalar processors exercise a “centralized” control requiring long wires (or pipelined long wires)
  • However, to utilize the transistors well, we need to overcome the memory wall problem
  • To hide memory latency we need to extract more independent instructions i.e. more ILP
• Extracting more ILP directly requires more available in-flight instructions
  • But for that we need bigger ROB which in turn requires a bigger register file
  • Also we need to have bigger issue queues to be able to find more parallelism
  • None of these structures scale well: main problem is wiring
  • So the best solution to utilize these transistors effectively with a low cost must not require long wires and must be able to leverage existing technology: CMP satisfies these goals exactly (use existing processors and invest transistors to have more of these on-chip instead of trying to scale the existing processor for more ILP)

Power consumption?

• Hey, didn’t I just make my power consumption roughly N-fold by putting N cores on the die?
  • Yes, if you do not scale down voltage or frequency
  • Usually CMPs are clocked at a lower frequency
    • Oops! My games run slower!
  • Voltage scaling happens due to smaller process technology
  • Overall, roughly cubic dependence of power on voltage or frequency
  • Need to talk about different metrics
    • Performance/Watt (same as reciprocal of energy)
    • More general, Performancek+1/Watt (k > 0)
  • Need smarter techniques to further improve these metrics
    • Online voltage/frequency scaling