Hyper-Threading Technology  

• Hyper-Threading Technology brings the simultaneous multi-threading approach to the Intel architecture.
• Hyper-Threading Technology makes a single physical processor appear as two or more logical processor.

 Hyper-Threading Technology  

• Hyper-Threading Technology provides thread-level-parallelism on each processor resulting in increased utilization of processor and execution resources.
• Each logical processor maintain one copy of the architecture state

 Hyper-Threading Technology  

• With processor and application parallelism becoming more prevalent, today’s server platforms are increasingly turning to threading as a way of increasing overall system performance.

 Hyper-Threading Technology  

• A multiprocessor system with Hyper-Threading technology duplicates the architectural state on each physical processor, providing two “logical” processors per physical processor.
• In Figure 1, the left-hand configuration represents a traditional multiprocessor system with two discrete physical processors.

 Hyper-Threading Technology  

• Each processor has its own set of processor execution resources and its own single architectural state.
• The right-hand configuration in Figure 1 represents an Intel® Xeon™ processor familybased multiprocessor system where each processor features Hyper-Threading technology.

Threading Algorithms

• Switch-on-event:
  Task switching in case of long pauses.
  Waiting for data coming from a relatively slow source, CPU resources are given to other processes

Threading Algorithms

• Multiprocessing:
  Distribute the load over many processors.
  Adds extra cost .

Threading Algorithms

• Simultaneous multi-threading:
  Multiple threads execute on a single processor without switching.
  Basis of Intel’s Hyper-Threading technology.

Traditional Multiprocessing

Here you see a multiprocessing system with two physical processors.

One processor is executing the yellow thread, and another processor is executing the dark Green thread. 

The peak execution bandwidth here is 6 instructions every cycle, 3 on each processor. The system may operate at less than peak bandwidth, as indicated by the abundance of idle(white) execution units.

Hyper-Threading Technology

Here you see a multiprocessing system with two physical processors featuring Hyper-Threading technology.
One processor is simultaneously executing the yellow and dark Green threads, while another processor executes the two patterned threads.
On such a system,you can see that the system may execute at closer to peak bandwidth.

Hyper-Threading Technology

Figure 2. On a traditional multi-processor system, the idle (white) execution units indicate execution resource utilization is not optimum, while Hyper-
Threading technology not only allows multiple threads to execute simultaneously, but also reduces the number of idle execution units.

Benefits of H-T

Disadvantages

• To take advantage of hyper-threading performance, serial execution can not be used.
• Threads are non-deterministic and involve extra design
• Threads have increased overhead
• Shared resource conflicts