8th Biennial Ptolemy Miniconference Berkeley, CA April 16, 2009 Precision Timed (PRET) Architecture Hiren D. Patel, Ben Lickly, Isaac Liu and Edward A. Lee University of California, Berkeley

Hiren D. Patel, Berkeley 2 of 14Ptolemy Miniconference, April 16, 2009 Timing Properties in Computing Abstractions Most traditional computing abstractions hide timing properties of software Advantages Focus on functionality Push for higher average-case performance Disadvantages Real-time embedded systems Unpredictable Non-repeatable Brittle Programming models and languages Multithreading Speculative execution, caches, and deep pipelines Compilers, and ISAs

Hiren D. Patel, Berkeley 3 of 14Ptolemy Miniconference, April 16, 2009 Resulting Real-time Embedded Systems Unpredictability Difficulty in determining timing behavior through analysis Non-repeatability Different executions may yield different timing behavior Brittleness Small changes have big effects on timing behavior Time as a first class citizen of embedded computing

Hiren D. Patel, Berkeley 4 of 14Ptolemy Miniconference, April 16, 2009 Precision Timed (PRET) Architectures Stephen. A. Edwards and Edward. A. Lee, “The case for the Precision Timed (PRET) machine.” In Proceedings of the 44th Annual Conference on Design Automation (San Diego, California, June , 2007). DAC '07. ACM, New York, NY, Predictable and repeatable timing

Hiren D. Patel, Berkeley 5 of 14Ptolemy Miniconference, April 16, 2009 Precision Timed Architecture Scratchpad memories Thread- interleaved pipeline Time-triggered arbitration Round-robin thread scheduling ISA with timing instructions

Hiren D. Patel, Berkeley 6 of 14Ptolemy Miniconference, April 16, 2009 Timing Instructions: Deadline ISA extensions dead [Ip & Edwards in 2006] deadload Deadline instructions Denote the required execution time of a block When decoded Stall instruction until timer value is 0 Then set timer value to new value Block 1 Block 2 Block 3

Hiren D. Patel, Berkeley 7 of 14Ptolemy Miniconference, April 16, 2009 Timing Instructions: Exceptions ISA extensions deadbranch deadloadbranch What happens when missing deadlines? Raise exception and perform pre-specified actions To control timing behaviors in software, we need a predictable underlying architecture

Hiren D. Patel, Berkeley 8 of 14Ptolemy Miniconference, April 16, 2009 Pipeline Architecture with Predictable Timing Stall pipeline Dependencies result in complex timing behaviors Predictable timing behavior of instructions Thread-interleaved pipeline: Traditional pipeline:

Hiren D. Patel, Berkeley 9 of 14Ptolemy Miniconference, April 16, 2009 Thread-interleaved Pipeline with Timing Instructions Thread stalls Main memory access Deadline instructions Replay mechanism Execute same PC next iteration Decrement deadline timers Stall if deadline instruction If not stalled, increment PC

Hiren D. Patel, Berkeley 10 of 14Ptolemy Miniconference, April 16, 2009 Memory Hierarchy with Predictable Timing Scratchpad memories Software managed caches Each thread has a uniquely defined address space 13 cycles latency thread0 thread2 thread4 1 cycle latency Predictable timing behavior during cache accesses Shared data goes through to main memory

Hiren D. Patel, Berkeley 11 of 14Ptolemy Miniconference, April 16, 2009 Time-triggered Access to Main Memory Predictable timing behavior when accessing main memory Worst-case bound on access time: 13* = 90 cycles Each thread must make and complete access within its window Memory wheel Time-triggered access

Hiren D. Patel, Berkeley 12 of 14Ptolemy Miniconference, April 16, 2009 Examples Video rendering for a computer game Real-time requirements through deadline instructions Autonomous robot finding moving target Anytime algorithms using timing exceptions Eliminating time-exploiting attacks in cryptosystems Repeatable timing behavior through deadline instructions RSA Encryption (RSAREF 2.0)DSA Encryption from OpenSSL (0.9.8j)

Hiren D. Patel, Berkeley 13 of 14Ptolemy Miniconference, April 16, 2009 Predictable Timing and High Performance Scratchpad memory allocation schemes Thread scheduling and synchronizations Real-time network on- chip Multi-PRET architecture Timing analysis PRET Machine Programming models and languages with time semantics Code generation from Giotto, SDF, and PTIDES.

Hiren D. Patel, Berkeley 14 of 14Ptolemy Miniconference, April 16, 2009 Conclusion Treat time as a first class property of embedded computing Predictable and repeatable timing behaviors PRET cycle-accurate simulator ISA extensions with timing instructions Architecture with predictable timing behaviors Download:

Hiren D. Patel, Berkeley 15 of 14Ptolemy Miniconference, April 16, 2009 End