1. Fast Simulation Techniques for Design Space Exploration Daniel Knorreck, Ludovic Apvrille, Renaud Pacalet daniel.knorreck@telecom-paristech.fr

2. slide 2 2 Outline DIPLODOCUS basics New simulation strategy Case study MPEG decoder Conclusions and Future Work

3. slide 3 DIPLODOCUS basics

4. slide 4 DIPLODOCUS on one slide Platform for efficient Design Space Exploration of SoCs Clear separation between -Applications -Architecture -Mapping Data abstraction Control flow oriented Simulation and formal analysis on abstract models Our environment is based on UML as modeling language LOTOS and UPPAAL for formal analysis SystemC/C++ for simulation

5. slide 5 Methodology Application modeling Architecture modeling DSE mapping Simulation Static analysis Simulation Static analysis

6. slide 6 Toolkit: TTool

7. slide 7 DIPLODOCUS: Task Diagram Declaration of a task Event. Used for inter-task signaling. Type may be infinite FIFO or finite FIFO. When a finite FIFO is full, the older event is erased. Events may carry values. Event. Used for inter-task signaling. Type may be infinite FIFO or finite FIFO. When a finite FIFO is full, the older event is erased. Events may carry values. Request. Use to spawn a task if an instance of that task is not currently executing. Channel. Do not convey value: they are meant to model a number of exchanged samples. cha1 = event name Three channel types: BR-BW: Blocking Read – Blocking write (= Finite FIFO) BR-NBW: Blocking Read – Non Blocking Write (= infinite FIFO) NBR-NBW: Non Blocking Read - Non Blocking Write (= shared memory) Channel. Do not convey value: they are meant to model a number of exchanged samples. cha1 = event name Three channel types: BR-BW: Blocking Read – Blocking write (= Finite FIFO) BR-NBW: Blocking Read – Non Blocking Write (= infinite FIFO) NBR-NBW: Non Blocking Read - Non Blocking Write (= shared memory)

8. slide 8 DIPLODOCUS: Application Modeling A behavior must be provided for each task UML activity diagram Usual control operators -Loops -Choices Channels -Write x samples on a channel -Read x samples from a channel Events -Send, receive an event -Test whether an event may be received -Select between events Requests -Send a request

9. slide 9 DIPLODOCUS: Task Behavior Sending of request req1 with “1” as natural parameter Loop Sending of event done Receiving of one data sample on channel cha1 Loop condition is false Loop condition is true Receiving of one data sample on channel cha1 Modeling between 1 and 2 execution instructions on an integer unit. It has no meaning at application modeling level.

10. slide 10 DIPLODOCUS: Mapping

11. slide 11 New Simulation Strategy

12. slide 12 Motivations for a new Simulator Existing SystemC based simulator Relies on the freely available SystemC kernel One SystemC Task per CPU, Bus, Memory Simulation on cycle accurate level And so… that simulator is quite slow New simulator implemented in pure C++: No overhead due to the SystemC kernel Coarse grained simulation strategy based on transactions comprising several cycles Simulation granularity is automatically adapted to the application

13. slide 13 Architecture of the Simulator For the sake of comprehensibility, many sub-classes have been omitted and merely inheritance relationships are shown.

14. slide 14 Transactions Merges several clock cycles, contains penalties Important parameters: virtualLengh: number of virtual execution units length: duration of the transaction in time units runnableTime: time when it becomes runnable startTime: execution starts at this time penalties: task switching, branching, idle Transaction travels through simulator: Command Channel CPU Bus Slave

15. slide 15 Basic Simulation Strategy in one slide CPU 2 CPU 1 T11 T12 T21T22 T11 T21 T22 T11 T21  T23 CPU 1 CPU 2 T22 T11 T21 CPU 1 CPU 2 T12 CPU 1 CPU 2 activate

16. slide 16 Hierarchical scheduling process

17. slide 17 Simulation phases Three phases are entered alternately Preparation Phase -Check if current command has been processed entirely, proceed to next command if necessary -Create next transaction, register transaction at channel if necessary Scheduling procedure Execution phase -Issue read/write operations on channels -Update progress of command -Add transaction to schedule

18. slide 18 Case study MPEG decoder

19. slide 19 10/24/2015 DIPLODOCUS: System Level Design Space Exploration Task diagram (Data dependencies) (processing sequence)

20. slide 20 DIPLODOCUS: System Level Design Space Exploration Task: Parser Sequence header Picture, Slice, Macroblock header, to be refined Launch processing No of coded blocks Picture type decision Picture format Picture type decision

21. slide 21 Conclusions and Future Work

22. slide 22 Simulation Strategy: Summary Strength: simulation time increases with the number of transactions and NOT with the number of clock cycles Thus in general, and take the same execution time. Scenario 1: Task 1 executes, after that, Task 2 executes a million times result: 1,000,001 transactions Scenario 2: same as before, but the tasks execute the read/write commands concurrently: result: 2,000,000 transactions split of write transaction is necessary to leave the decision which task executes next up to the CPU scheduler

23. slide 23 Conclusions Implementation of a simulation environment Simulation granularity automatically adapts to the application model Based on pure C++ Simulation speed up by 6x up to 30x or more depending on the application granularity MPEG case study

24. slide 24 Future Work Extension of the simulation environment: Refinement of bus and memory model Refinement of the hardware accelerator component MPEG case study, using meta-data to direct control flow Longer-term objectives: Verification of functional requirements during simulation Exploration of several branches of control flow, possibility to return to a previous system state Technical improvements of the simulator

25. slide 25 Thank You! Questions?