Hard - Real - Time Multithreading : a Combined Microarchitectural and Scheduling Approach

EL-HAJ MAHMOUD, ALI AHMAD. Hard-Real-Time Multithreading: A Combined Microarchitectural and Scheduling Approach. (Under the direction of Dr. Eric Rotenberg). Simultaneous Multithreading (SMT) enables fine-grain resource sharing of a single superscalar processor among multiple tasks, improving cost-performance. However, SMT cannot be safely exploited in hard-real-time systems. These systems require analytical frameworks for making worst-case performance guarantees. SMT violates simplifying assumptions for deriving worst-case execution times (WCET) of tasks. Classic real-time theory uses single-task WCET analysis, where a task is assumed to have access to dedicated processor resources, hence, its WCET can be derived independent of its task-set context. This is not true for SMT, where tasks interfere due to resource sharing. Modeling interference requires whole task-set WCET analysis, but this approach is futile since co-scheduled tasks vary and compete for resources arbitrarily. Thus, formally proving real-time guarantees for SMT is intractable. This dissertation proposes flexible interference-free multithreading. Interference-free partitioning guarantees that the performance of a single task is not affected by its workload context (hence, preserving single-task WCET analysis), while flexible resource sharing emulates finegrain resource sharing of SMT to achieve similar cost-performance efficiency. The Real-time Virtual Multiprocessor (RVMP) paradigm virtualizes a single superscalar processor into multiple interference-free different-sized virtual processors. This provides a flexible spatial dimension. In the time dimension, the number and sizes of virtual processors can be rapidly reconfigured. A simple real-time scheduling approach concentrates scheduling within a small time interval (the “round”), producing a simple repeating space/time schedule that orchestrates virtualization. Worst-case schedulability experiments show that more task-sets are provably schedulable on RVMP than on conventional rigid multiprocessors with equal aggregate resources, and the advantage only intensifies with more demanding task-sets. Run-time experiments show RVMP’s statically-controlled coarser-grain resource sharing is as effective as unsafe SMT, and provides a real-time formalism that SMT does not currently provide. RVMP’s round-based scheduling enables other optimizations for safely improving performance even more. A framework is developed on top of RVMP to safely, tractably, and tightly bound overlap between computation and memory accesses of different tasks to improve worstcase performance. This framework captures the throughput gain of dynamic switch-on-event multithreading, but in a way that is compatible with hard-real-time formalism. HARD-REAL-TIME MULTITHREADING: A COMBINED MICROARCHITECTURAL AND SCHEDULING APPROACH by ALI AHMAD EL-HAJ MAHMOUD A dissertation submitted to the Graduate Faculty of North Carolina State University in partial fulfillment of the requirements for the Degree of Doctor of Philosophy COMPUTER ENGINEERING