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Soft Real-Time Scheduling on Simultaneous Multithreaded Processors

Soft Real-Time Scheduling on Simultaneous Multithreaded Processors. Outline. Introduction Resource Sharing Algorithms Co-Scheduling Algorithms Experimental Results Conclusions. What is SMT?. Simultaneous multithreading (SMT) combines wide issue superscalar with multithreading,

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Soft Real-Time Scheduling on Simultaneous Multithreaded Processors

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  1. Soft Real-Time Scheduling on Simultaneous Multithreaded Processors

  2. Outline • Introduction • Resource Sharing Algorithms • Co-Scheduling Algorithms • Experimental Results • Conclusions

  3. What is SMT? • Simultaneous multithreading (SMT) • combines wide issue superscalar with multithreading, • issues instructions from several threads simultaneously.

  4. SMT vs. CMP SMT CMP

  5. Introduction • SMT (Simultaneous Multithreading) improves processor throughput by processing instructions form multiple threads each cycle. • This paper concerns the use of SMT processors for soft real-time multimedia applications. • On Scheduling: • 1. determine which tasks to co-schedule • 2. how to partition processor resources among co-scheduled tasks

  6. Resource Sharing Algorithms • Two classes: • Maximize overall throughput • Guarantee a level of performance for all threads

  7. Resource Sharing Algorithms • Throughput-driven resource sharing • Resource sharing with performance guarantees • Resources controlled by threaded-specific resource sharing algorithms

  8. Co-Scheduling Algorithms • Design space • Prediction • Partition algorithms • Global scheduling algorithms

  9. Design space for co-scheduling algorithms • Partition vs. global scheduling algorithms • Partitioning approach provides for admission control • Symbiosis-aware vs. symbiosis-oblivious algorithms • Symbiosis factor = ∑ (realized IPC of jobi / single threaded IPC of jobi) N i=1

  10. Benefit of symbiosis-aware scheduling

  11. τ1 & τ3 τ2 & τ4 • EDF schedule • Symbiosis-aware schedule 0 50 100 150 200 τ1 & τ2 τ3 & τ4 0 50 100 150 200

  12. Predicting execution time, utilization, and symbiosis • To predict execution time/utilization, we need to predict both the IPC and the number of instructions • Take average job IPC and average instruction count as the prediction • To predict symbiosis, task IPCs in single-threaded mode is also required.

  13. Partitioning algorithms partitioning (PART) Symbiosis oblivious (NOSYM) Symbiosis aware (SYM) Dynamic (DYN) Static (STAT) Dynamic (DYN) 3 Base (b) Enhanced (e) Base (b) Enhanced (e) 5 1 2 4

  14. PART-NOSYM-DYN-b • First-fit-decreasing-utilization & EDF admission test • PART-NOSYM-DYN-e • Modifies the admission test • Simulates the schedule for a hyperperiod • PART-NOSYM-STAT • FFDU & EDF admission test • It does not need additional approximations for predicting execution time

  15. PART-SYM-DYN-b • Maximizes average symbiosis among tasks in different partition • PART-SYM-DYN-e • Corrects for the utilization approximation in PART-SYM-DYN-b

  16. Global scheduling algorithms Global scheduling (GLOB) Symbiosis oblivious (NOSYM) Symbiosis aware (SYM) PAIIN (dynamic) US (dynamic) PAIIN (dynamic) US (dynamic) 6 7 8 9

  17. GLOB-NOSYM-PLAIN • EDF • GLOB-NOSYM-US • Giving the highest priority to high utilization tasks in the task set • GLOB-SYM-PLAIN • Slightly more complex than EDF • GLOB-SYM-US • Defaults to GLOB-NOSYM-US, if Ui>N/(2N-1) for Ti • Defaults to GLOB-SYM-PLAIN, otherwise

  18. Experimental Results - synthetic • metric : success ratio = the percentage of all generated task-sets successfully scheduled by an algorithm • Best algorithm : GLOB-SYM-US

  19. Experimental Results - real • metric : critical serial utilization (CSU) • Serial utilization (S) = ΣCi/Pi • Ci = computation time of τi in single-threaded mode • Pi = period of τi • Best algorithm : GLOB-SYM-US

  20. Conclusions • In terms of schedulability, the best algorithm uses global scheduling, exploits symbiosis, prioritizes high utilization tasks, and uses dynamic resource sharing. • Partitioning algorithm that utilizes static resource sharing • provides a strict admission control • Requires less profiling

  21. Conclusions • Earliest deadline first global algorithm • Does not provide a strict admission control • Requires no profiling • individual design decision • Dynamic resource sharing • Partitioning algorithm • Symbiosis-awareness

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