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