1. A Precision Timed Architecture for Predictable and Repeatable Timing
Hiren D. Patel
Isaac Liu
Ben Lickly
Edward A. Lee
A Precision Timed Architecture for Predictable and Repeatable Timing
PRET Philosophy
Most abstractions in computing hide timing properties of software. As a result, computer architects, and compiler and language designers use clever techniques to improve the average-case performance. This, however, is at the expense of predictable and repeatable timing. We find these techniques to be problematic for real-time embedded computing because they result in unpredictable and non-repeatable behavior, and brittle systems
Our Approach
Our approach treats time as a first-class property of embedded computing. In doing so, we prototype a precision timed (PRET) embedded processor architecture that introduces temporal semantics at the instruction-set architecture, and one that carefully selects architectural optimization techniques to deliver predictable performance enhancements. We believe that timing predictability and repeatability are not at odds with performance
Processor Architecture
By employing a thread-interleaved pipeline, we remove data hazards and dependencies in the pipeline.
Use software-managed caches – scratchpad memories.
Memory wheel to arbitrate access to main memory in a time-triggered fashion.
ISA & C Extensions
Timing instructions providing control over the execution time of a sequence of instructions.
Open-source compiler frameworks Clang and LLVM for code translation, code generation, and analysis.
References
1. Ben Lickly, Isaac Liu, Sungjun Kim, Hiren D. Patel, Stephen A. Edwards and Edward A. Lee, Predictable Programming on a Precision Timed Architecture, in proceedings of International Conference on Compilers, Architecture, and Synthesis from Embedded Systems (CASES), October,
2. Hiren D. Patel, Ben Lickly, Bas Burgers and Edward A. Lee, A Timing Requirements-Aware Scratchpad Memory Allocation Scheme for a Precision Timed Architecture, Technical Report No. UCB/EECS , September, 2008.
3. Shanna-Shaye Forbes, Hugo A. Andrade, Hiren D. Patel and Edward A. Lee. An Automated Mapping of Timed Functional Specification to A Precision Timed Architecture, In proceedings of the 12-th IEEE International Symposium on Distributed Simulation and Real Time Applications, (DSRT), October, 2008.
Acknowledgement
This work was supported in part by the Center for Hybrid and Embedded Software Systems (CHESS) at UC Berkeley, which receives support from the National Science Foundation (NSF awards # (CSR-EHS:PRET) and # (CSR-CPS)), the U. S. Army Research Office (ARO#W911NF ), the U. S. Air Force Office of Scientific Research (MURI #FA ), the Air Force Research Lab (AFRL), the State of California Micro Program, and the following companies: Agilent, Bosch, HSBC, Lockheed-Martin, National Instruments, and Toyota.
Scratchpad memories
Thread-interleaved pipeline
Time-triggered arbitration
Round-robin thread scheduling
ISA with timing instructions
thread0
thread1
thread2
thread3
thread4
thread5
90 cycles until thread0 completes
On time
Predictable timing behavior when accessing main memory
Worst-case bound on access time:
13* = 90 cycles
Block 1
Block 2
Block 3
Timing Instructions
Behavior
Deadload
Load new deadline value into the given timer register
Dead
Stall until previous timer expires, then load new value
Deadbranch
Same as dead, but raises an exception if new deadline expires
Deadloadbranch
Combination of functionality of deadload and deadbranch
Stall pipeline
Dependencies result in complex timing behaviors
Predictable timing behavior of instructions
Thread-interleaved pipeline:
Traditional pipeline:
13 cycles latency
thread0
thread2
thread4
1 cycle latency
April 16, 2009
Center for Hybrid and Embedded Software Systems