HyperThreading

1
HyperThreading

• Improves processor performance under certain
  workloads by providing useful work for execution
  units that would otherwise be idle
• Duplicates sections of the processor
• those that store the architectural state
• does not duplicate main execution resources
• allows a physical processor to be two logical
  processors
• can schedule two threads or processes
  simultaneously

2
When processor stalls

• as a result of
• cache miss
• branch misprediction
• data dependency
• processor may use those execution resources to
  execute other processes or threads

3
Architectural State

• Control Registers
• instruction flag registers
• interrupt mask registers
• memory management registers
• status registers
• General purpose registers (AX, BX, CX, DX, ...)

4
Improved Performance

• Uses 5 of the die area over the normal
  processor
• Performance improvements of up to 30

5
Dual Core

• One which combines two or more independent
  processors into a single package
• Contains two independent microprocessors
• Allows thread-level parallelism w/o including
  multiple microprocessors in separate physical
  packages
• CMP chip level multiprocessing

6
Advantages vs SMP

• Cache coherency
• Can operate at a much higher clock rate than is
  possible if the signals have to travel off-chip
• Less space required
• Less power usage

7
Disadvantages

• Requires OS support
• Requires threaded applications
• Integration of multi-core chip drives production
  yields down, they are more difficult to manage
  thermally than lower-density single-chiop