Precision Timed Embedded Systems Using TickPAD Memory Matthew M Y Kuo* Partha S Roop* Sidharta Andalam † Nitish Patel* *University of Auckland, New Zealand † TUM CREATE, Singapore

Introduction Hard real time systems ◦ Need to meet real time deadlines ◦ Catastrophic events may occur when missed Synchronous execution approach ◦ Good for hard real time systems  Deterministic  Reactive ◦ Aids static timing analysis  Well bounded programs  No unbounded loops or recursions

Synchronous Languages Executes in logical time ◦ Ticks  Sample input → computation → emit output Synchronous hypothesis ◦ Tick are instantaneous  Assumes system is executes infinitely fast  System is faster than environment response ◦ Worst case reaction time  Time between two logical ticks Languages ◦ Esterel ◦ Scade ◦ PRET-C  Extension to C

Synchronous Languages Executes in logical time ◦ Ticks  Sample input → computation → emit output Synchronous hypothesis ◦ Tick are instantaneous  Assumes system is executes infinitely fast  System is faster than environment response ◦ Worst case reaction time  Time between two logical ticks Languages ◦ Esterel ◦ Scade ◦ PRET-C  Extension to C

PRET-C Light-weight multithreading in C Provides thread safe memory access C extension implemented as C macros StatementMeaning ReactiveInput IDeclares I as a reactive environment input ReactiveOutput ODeclares O as a reactive environment output PAR(T1, …. Tn)Synchronously executes n threads in parallel, where thread t i has a higher priority than t i+1 EOTMarks the end of tick [weak] abort P when C Preempt p when c is true

Introduction Practical System require larger memory ◦ Not all applications fit on on-chip memory Require memory hierarchy ◦ Processor memory gap [1] Hennessy, John L., and David A. Patterson. Computer Architecture: A Quantitative Approach. San Francisco, CA: Morgan Kaufmann, 2011.

Introduction Traditional approaches ◦ Caches ◦ Scratchpads However, ◦ Scant research for memory architectures tailored for synchronous execution and concurrency.

Caches CPU Main Memory

Caches Traditionally Caches ◦ Small fast piece of memory  Temporal locality  Spatial locality ◦ Hardware Controlled  Replacement policy CPU Main Memory Cache

Caches Hard real time systems ◦ Needs to model the architecture  Compute the WCRT ◦ Caches models  Trade off between length of computation time and tightness  Very tight worse case estimate is not scalable CPU Main Memory Cache

Scratchpad Scratchpad Memory (SPM) ◦ Software controlled ◦ Statically allocated  Statically or dynamically loaded ◦ Requires an allocation algorithm  e.g. ILP, Greedy CPU Main Memory SPM

Scratchpad Hard real time systems ◦ Easy to compute tight the WCRT ◦ Reduces the worst case performance ◦ Balance between amount of reload points and overheads  May perform worst than cache in the worst case performance CPU Main Memory SPM

TickPAD CPU Main Memory SPMCache  Good at overall performance  Hardware controlled  Good at worst case performance  Easy for fast and tight static analysis

TickPAD CPU Main Memory SPMCache  Good at overall performance  Hardware controlled  Good at worst case performance  Easy for fast and tight static analysis TPM

TickPAD CPU Main Memory TPM TickPAD Memory ◦ TickPAD - Tick Precise Allocation Device ◦ Memory controller  Hybrid between caches and scratchpads  Hardware controlled features  Static software allocation ◦ Tailored for synchronous languages ◦ Instruction memory

TickPAD Design flow

PRET-C int main() { init(); PAR(t1,t2,t3);... } void thread t1() { compute; EOT; compute; EOT; } main t1 t3 t2

PRET-C int main() { init(); PAR(t1,t2,t3);... } void thread t1() { compute; EOT; compute; EOT; } Computation main t1 t3 t2

PRET-C int main() { init(); PAR(t1,t2,t3);... } void thread t1() { compute; EOT; compute; EOT; } Spawn children threads main t1 t3 t2

PRET-C int main() { init(); PAR(t1,t2,t3);... } void thread t1() { compute; EOT; compute; EOT; } End of tick – Synchronization boundaries main t1 t3 t2

PRET-C int main() { init(); PAR(t1,t2,t3);... } void thread t1() { compute; EOT; compute; EOT; } Child thread terminate main t1 t3 t2

PRET-C int main() { init(); PAR(t1,t2,t3);... } void thread t1() { compute; EOT; compute; EOT; } Main thread resume main t1 t3 t2

PRET-C Execution Time main t1 t3 t2 Sample inputs

PRET-C Execution main t1 t3 t2 main Time

PRET-C Execution main t1 t3 t2 main Time t1

PRET-C Execution main t1 t3 t2 main Time t1t2

PRET-C Execution main t1 t3 t2 main Time t1t2

PRET-C Execution main t1 t3 t2 main Time t1t2 Emit Outputs

PRET-C Execution main t1 t3 t2 main Time t1t2 1 tick (reaction time)

PRET-C Execution main t1 t3 t2 main Time t1t2 local tick

Assumptions 0x000x040x080x0C 4 Instructions 1 Cache Line Takes 1 burst transfer from main memory Cache miss, takes 38 clock cycles [2] 0x00Each instructions takes 2 cycles to execute buffer Buffers are 1 cache line in size 2. J. Whitham and N. Audsley. The Scratchpad Memory Management Unit for Microblaze: Implémentation, Testing, and Case Study. Technical Report YCS , University of York, 2009.

TickPAD - Overview TickPAD - Overview

Spatial memory pipeline  To accelerate linear code TickPAD - Overview TickPAD - Overview

Associative loop memory ◦ For predictable temporal locality  Statically allocated and Dynamically loaded TickPAD - Overview TickPAD - Overview

Tick address queue ◦ Stores the resumptions address of active threads TickPAD - Overview TickPAD - Overview

Tick instruction buffer ◦ Stores the instructions at the resumption of the next active thread ◦ To reduce context switching overhead at state/tick boundaries TickPAD - Overview TickPAD - Overview

Command table  Stores a set of commands to be executed by the TickPAD controller. TickPAD - Overview TickPAD - Overview

Command buffer  A buffer to store operands fetched from main memory  Command requiring 2+ operands TickPAD - Overview TickPAD - Overview

Spatial Memory Pipeline Cache – on miss ◦ Fetches from main memory on to cache  First instruction miss, subsequence instructions on that line hits ◦ Requires history of cache needed for timing analysis Scratchpad – unallocated ◦ Executes from main memory  Miss cost for all instructions ◦ Simple timing analysis

Spatial Memory Pipeline Memory controller ◦ Single line buffer Simple analysis ◦ Analyse previous instruction  First instruction miss, subsequence instructions on that line hits CPU Main Memory

Spatial Memory Pipeline Computation required many lines of instructions Exploit spatial locality ◦ Predictability prefetch the next line of instructions ◦ Add another buffer

Spatial Memory Pipeline To preserve determinism ◦ Prefetch only active if no branch

Spatial Memory Pipeline

Timing analysis ◦ Simple to analyse ◦ Analysis next instruction line  If has a branch next target line will miss  e.g. 38 clock cycles  Else – will be prefetched  e.g. 38 – 8 = 30 clock cycles

Spatial Memory Pipeline Timing analysis ◦ Simple to analyse ◦ Analysis next instruction line  If has a branch next target line will miss  e.g. 38 clock cycles  Else – will be prefetched  e.g. 38 – 8 = 30 clock cycles

Spatial Memory Pipeline Timing analysis ◦ Simple to analyse ◦ Analysis next instruction line  If has a branch next target line will miss  e.g. 38 clock cycles  Else – will be prefetched  e.g. 38 – 8 = 30 clock cycles

Tick Address Queue Tick Instruction Buffer Reduce cost of context switching Maintains a priority queue ◦ Thread execution order Prefetches instructions from next thread Make context switching points appear as linear code ◦ Paired using Spatial Memory Pipeline

Tick Address Queue Tick Instruction Buffer

Context switching – memory cost same as linear code

Tick Address Queue Tick Instruction Buffer

Timing analysis ◦ Same prefetch lines for allocated context switching points

Associative Loop Memory Statically Allocated ◦ Greedy  Allocates inner most look first Fetches Loop Before Executing ◦ Predictable – easy and tight to model ◦ Exploits temporal locality

Command Table Statically Allocated A Look Up table to dynamically load ◦ Tick Instruction Buffer ◦ Tick Queue ◦ Associative Loop Memory Command are executed when the PC matches the address stored on the command ◦ Allows the TickPAD to function without modification to source code  Libraries  Propriety programs

Command Table Three fields ◦ Address  The PC address to execute the command ◦ Command  Discard Loop Associative Memory  Store Loop Associative Memory  Fill Tick Instruction Buffer  Load Tick Address Queue ◦ Operand  Data used by the command

Command Table Allocation NodeCommandAddress FORKLoad Tick Address Queue x N Fill Tick Instruction Buffer Address of FORK EOTLoad Tick Address Queue Fill Tick Instruction Buffer Address of EOT KILLFill Tick Instruction BufferAddress of Kill LoopsDiscard Loop Associative Memory Store Loop Associative Memory Address at start of Loop

Command Table Allocation NodeCommandAddress FORKLoad Tick Address Queue x N Fill Tick Instruction Buffer Address of FORK EOTLoad Tick Address Queue Fill Tick Instruction Buffer Address of EOT KILLFill Tick Instruction BufferAddress of Kill LoopsDiscard Loop Associative Memory Store Loop Associative Memory Address at start of Loop

Command Table Allocation NodeCommandAddress FORKLoad Tick Address Queue x N Fill Tick Instruction Buffer Address of FORK EOTLoad Tick Address Queue Fill Tick Instruction Buffer Address of EOT KILLFill Tick Instruction BufferAddress of Kill LoopsDiscard Loop Associative Memory Store Loop Associative Memory Address at start of Loop

Command Table Allocation NodeCommandAddress FORKLoad Tick Address Queue x N Fill Tick Instruction Buffer Address of FORK EOTLoad Tick Address Queue Fill Tick Instruction Buffer Address of EOT KILLFill Tick Instruction BufferAddress of Kill LoopsDiscard Loop Associative Memory Store Loop Associative Memory Address at start of Loop

Command Table Allocation NodeCommandAddress FORKLoad Tick Address Queue x N Fill Tick Instruction Buffer Address of FORK EOTLoad Tick Address Queue Fill Tick Instruction Buffer Address of EOT KILLFill Tick Instruction BufferAddress of Kill LoopsDiscard Loop Associative Memory Store Loop Associative Memory Address at start of Loop

Results

Results WCRT reduction 8.5% Locked SPMs 12.3% Thread multiplexed SPM 13.4% Direct Mapped Caches

Results

Results - Synthesis

Conclusion Presented a new memory architecture ◦ Tailored for synchronous programs Has better worst case performance Analysis time is scalable ◦ Between scratchpad and abstract cache analysis The presented architecture is also suitable for other synchronous languages Future work ◦ Data TickPAD ◦ TickPAD on multicores

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