Hyper Threading Technology
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Hyper-Threading Technology allows two logical processors to simultaneously share one physical processor's execution resources. It appears to the operating system as two processors. While the die size increase is small, adding Hyper-Threading Technology presented large complexity challenges. Different microarchitecture components use different resource sharing approaches, such as partitioning resources between logical processors, using thresholds to limit maximum usage, or fully sharing caches between logical processors. Performance tests showed good gains from Hyper-Threading Technology, especially when running dissimilar applications simultaneously.
Hyper Threading Technology
- 2. Outline What is Hyper-Threading Technology ? Hyper-Threadig Technology in Intel microprocessors Microarchitecture Choices & Tradeoffs Performance Results Conclusion
- 3. Outline What is Hyper-Threading Technology ? Hyper-Threading Technology in Intel microprocessors Microarchitecture Choices & Tradeoffs Performance Results Conclusion
- 4. Hyper-Threading Technology Simultaneous Multi-threading – 2 logical processors (LP) simultaneously share one physical processor’s execution resources Appears to software as 2 processors (2-way shared memory multiprocessor) – O perating S ystem schedules software threads/processes to both logical processors – Fully compatible to existing multi-processor system software and hardware. Integral part of Intel Netburst Microarchitecture
- 5. Die Size Increase is Small Total die area added is small – A few small structures duplicated – Some additional control logic and pointers
- 6. Complexity is Large Challenged many basic assumptions New microarchitecture algorithms – To address new uop (micro-operation) prioritization issues – To solve potential new livelock scenarios High logic design complexity Validation Effort – Explosion of validation space
- 7. Outline What is Hyper-Threading Technology ? Hyper-Threading Technology in Intel microprocessors Microarchitecture Choices & Tradeoffs Performance Results Conclusion
- 8. HT Technology in Intel microprocessors Hyper-Threading is the Intel implementation of simultanious multi-threading Integral part of Intel Netburst Microarchitecture – e.g. Intel Xeon Processors
- 9. Intel Processors with Netburst Microarchitecture Intel Xeon MP Processor Intel Xeon Processor Intel Xeon Processor 256 KB 2nd-Level Cache 256 KB 2nd-Level Cache 512 KB 2nd-Level Cache 1 MB 3rd-Level Cache
- 10. What was added
- 11. Outline What is Hyper-Threading Technology ? Hyper-Threading Technology in Intel microprocessors Microarchitecture Choices & Tradeoffs Performance Results Conclusion
- 12. Managing Resources Choices – Partition – Half of resource dedicated to each logical processor – Threshold – Flexible resource sharing with limit on maximum resource usage – Full Sharing – Flexible resource sharing with no limit on maximum resource usage Considerations – Throughput and fairness – Die size and Complexity
- 13. Partitioning Half of resource dedicated to each logical processor – Simple, low complexity Good for structures where – Occupancy time can be high and unpredictable – High average utilization Major pipeline queues are a good example – Provide buffering to avoid pipeline stalls – Allow slip between logical processors
- 15. Execution Pipeline Partition queues between major pipestages of pipeline
- 16. Partitioned Queue Example W ith full sharing, a slow thread can get unfair share of resources ! So, Partitioning c an prevent a faster thread from making rapid progress.
- 20. Partitioned Queue Example Partitioning resource ensures fairness and ensures progress for both logical processors !
- 21. Thresholds Flexible resource sharing with limit on maximum resource usage Good for small structures where – Occupancy time is low and predictable – Low average utilization with occasional high peaks Schedulers are a good example – Throughput is high because of data speculation (get data regardless of cache hit) – uOps pass through scheduler very quickly – Schedulers are small for speed
- 22. Schedulers, Queues 5 schedulers: MEM ALU0 ALU1 FP Move FP/MMX/SSE Threshold prevents one logical processor from consuming all entries ( Round Robin until reach threshold)
- 23. Variable partitioning allows a logical processor to use most resources when the other doesn’t need them
- 24. Full Sharing Flexible resource sharing with no limit on maximum resource usage Good for large structures where – Working set sizes are variable – Sharing between logical processors possible – Not possible for one logical processor to starve Caches are a good example – All caches are shared – Better overall performance vs. partitioned caches – Some applications share code and/or data – High set associativity minimizes conflict misses. – Level 2 and 3 caches are 8-way set associative
- 25. On average, a shared cache has 40% better hit rate and 12% better performance for these applications.
- 26. Outline What is Hyper-Threading Technology ? Hyper-Threading Technology in Intel microprocessors Microarchitecture Choices & Tradeoffs Performance Results Conclusion
- 27. Server Performance Good performance benefit from small die area investment
- 28. Multi-tasking Larger gains can be realized by running dissimilar applications due to different resource requirements
- 29. Outline What is Hyper-Threading Technology ? Hyper-Threading Technology in Intel microprocessors Microarchitecture Choices & Tradeoffs Performance Results Conclusion
- 30. Conclusions Hyper-Threading Technology is an integral part of the part of the Netburst Microarchitecture – Very little additional die area needed – Compelling performance – Currently enabled for both server and desktop processors Microarchitecture design choices – Resource sharing policy matched to traffic and performance requirements New challenging microarchitecture direction