NC STATE UNIVERSITY Center for Embedded Systems Research (CESR) Department of Electrical & Computer Eng’g North Carolina State University Ali El-Haj-Mahmoud, Ahmed S. AL-Zawawi, Aravindh Anantaraman, and Eric Rotenberg Virtual Multiprocessor: An Analyzable, High-Performance Microarchitecture for Real-Time Computing

NC STATE UNIVERSITY CASES 20052El-Haj-Mahmoud © 2005 Embedded Processor Trends  Inheriting desktop high-performance features  Examples ARM11: 8-stage pipeline, caches, dynamic br. pred. Ubicom IP3023: 8 hardware threads PowerPC 750: 2-way superscalar, OOO execution

NC STATE UNIVERSITY CASES 20053El-Haj-Mahmoud © 2005 Real-Time Systems and Analyzability  Schedulability of task-set determined a priori Requires worst-case execution times (WCET)  statically analyzable microarchitecture  Dynamic microarchitecture features complicate real-time design A trade-off between performance and analyzability A B

NC STATE UNIVERSITY CASES 20054El-Haj-Mahmoud © 2005 Multiple Simple Processors + Analyzable + Natural fit with real-time systems − Rigid resource partitioning − Higher cost/performance metric proc 1proc 2 A B C

NC STATE UNIVERSITY CASES 20055El-Haj-Mahmoud © 2005 Simultaneous Multithreading (SMT) + Flexible resource sharing + Better cost/performance metric − Unanalyzable SMT AB C

NC STATE UNIVERSITY CASES 20056El-Haj-Mahmoud © 2005 Unanalyzability of SMT − Violates single-task WCET assumption (tasks analyzed separately) − Arbitrary periods  arbitrary overlap of tasks − Dynamic interference Cannot derive WCETs  Cannot perform schedulability

NC STATE UNIVERSITY CASES 20057El-Haj-Mahmoud © 2005 Real-Time Virtual Multiprocessor  Combine MP analyzability and SMT flexibility  Key idea: Interference-free multithreading SMT performance WCET of each task independent of task-set  RVMP substrate: two parts Highly reconfigurable multithreaded superscalar  Space: multiple arbitrary interference-free partitions  Time: rapidly reconfigure partitions Static schedule orchestrates partitioning

NC STATE UNIVERSITY CASES 20058El-Haj-Mahmoud © 2005 Big Picture Co-design processor and real-time scheduling for analyzable high-performance

NC STATE UNIVERSITY CASES 20059El-Haj-Mahmoud © 2005 RVMP Architecture  Superscalar “ways” are natural partitioning granularity  Different-sized virtual processors carved out of single superscalar

NC STATE UNIVERSITY CASES El-Haj-Mahmoud © 2005 Processor Architecture  Starting point Alpha 21164: 4-way in-order superscalar Ubicom IP3023: 8 hardware threads (4 in RVMP  4 VPs)  Simplifications for analyzability In-order issue within VPs Software-managed scratchpads Static branch prediction Not limitations of RVMP!

NC STATE UNIVERSITY CASES El-Haj-Mahmoud © 2005 Processor Architecture Fetch Unit PC Interleaved Instruction Scratchpad Decode Slotter and Scoreboard (Issue Logic) Int RF Shadow Buffers FP RF Data Scratchpad FU4: FPU FU0: INT FU1: INT/MUL/DIV FU2: INT/AGEN FU3: INT/AGEN 44 1 Shadow Buffers RD WR HRT Fetch Buffer

NC STATE UNIVERSITY CASES El-Haj-Mahmoud © 2005 Fetch Buffer Instruction Scratchpad Fetch Unit Decode Issue Backend FV PV Shadow Buffers FV PV HRT Instruction Fetch

NC STATE UNIVERSITY CASES El-Haj-Mahmoud © 2005 Real-Time Scheduling  Too complicated Must schedule entire hyper-period Overwhelming # of possible space/time schedules High dedicated-storage cost for schedule A B C D

NC STATE UNIVERSITY CASES El-Haj-Mahmoud © 2005 WCET 2 WCET 1 Task A WCET 3 WCET 4 period # of ways 4 ways 3 ways 2 ways 1 way … … … … … Round

NC STATE UNIVERSITY CASES El-Haj-Mahmoud © 2005 Task B period # of ways 4 ways 3 ways 2 ways 1 way … … … … …

NC STATE UNIVERSITY CASES El-Haj-Mahmoud © 2005 Task C period # of ways 4 ways 3 ways 2 ways 1 way … … … … …

NC STATE UNIVERSITY CASES El-Haj-Mahmoud © 2005 Task D period # of ways 4 ways 3 ways 2 ways 1 way … … … … …

NC STATE UNIVERSITY CASES El-Haj-Mahmoud © 2005 FAILEDSUCCEEDED Cyclic schedule

NC STATE UNIVERSITY CASES El-Haj-Mahmoud © 2005 Interaction: Scheduling and Architecture HRT LTC FU0 FU1 FU2 FU3 FU4 FU0 FU1 FU2 FU3 FU cycles 6040 FVPVCVs 0 EOT 1 INVALID

NC STATE UNIVERSITY CASES El-Haj-Mahmoud © 2005 Experiments  Tasks from C-lab and MiBench benchmarks  100 task-sets 4 tasks per task-set (also 8 tasks in paper) grouped according to scalar utilization (U_scalar)  Two experiments Worst-case schedulability analysis Run-time experiments

NC STATE UNIVERSITY CASES El-Haj-Mahmoud © 2005 Worst-Case Schedulability Tests

NC STATE UNIVERSITY CASES El-Haj-Mahmoud © 2005 RVMP Configurations

NC STATE UNIVERSITY CASES El-Haj-Mahmoud © 2005 Run-Time Experiments

NC STATE UNIVERSITY CASES El-Haj-Mahmoud © 2005 Unsafe Behavior of SMT

NC STATE UNIVERSITY CASES El-Haj-Mahmoud © 2005 Summary  Novel contributions Virtualize a single processor − Space: variable-size interference-free partitions − Time: rapid reconfiguration Simple real-time scheduling approach  Analyzability of MP with flexibility of SMT  Co-design processor and real-time scheduling for analyzable high-performance