Hard-Real-Time Multithreading: A Combined Microarchitectural and Scheduling Approach.

Abstract

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 fine-grain 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 worst-case 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.