Compaq W8000 Hyper-Threading Technology, New Feature of Intel Xeon Processor - Page 20

Hyper-Threading Technology, New Feature of Intel Xeon Processor White Paper 20 VCS Benchmark on Windows XP 8 7.1 7.2 7 6.53 6 5.2 5 Lower is better 4 3 2 1 0 2P 1P Disable 5.2 7.1 Enable 6.53 7.2 Figure 16 Summary Hyper-Threading technology represents a new approach to improving the instruction throughput of processors (instruction-level parallelism) that is targeted for servers and high-performance workstations. One of the principal benefits of Hyper-Threading technology is that it enables the processor to use on-die resources that may otherwise be idle. Although existing operating system and application code will run correctly on a processor with Hyper-Threading technology, to obtain performance gain, some relatively simple code modifications are needed to get the optimum benefit from Hyper-Threading technology. To refrain from wasting resources with the idle loops, the OS needs to issue immediately the Halt instruction to the logical processor that has a thread going idle. A thread that is using spin lock code, which aggressively captures processor resources, may need to add the pause instruction within the loop to allow the other threads a fair access to the processor resources. The limited trace cache size requires tight code loops in the trace cache for Hyper-Threading technology to be effective. Programmers now avoid loop unrolling, as it increases the amount of code created and increases trace cache misses. A trace cache miss forces the execution to the X86 instruction decoder, which can only decode one X86 instruction per cycle. 167T-0202A-WWEN