wcet analysis for a java processor

WCET Analysis for a WCET Analysis for a Java ProcessorJava Processor

Martin SchoeberlMartin SchoeberlTU Vienna, AustriaTU Vienna, Austria

Rasmus PedersenRasmus PedersenCBS, DenmarkCBS, Denmark

Martin Schoeberl – WCET Analysis for a Java Martin Schoeberl – WCET Analysis for a Java ProcessorProcessor

JTRES 2006JTRES 2006

OutlineOutline

MotivationMotivation WCET analysisWCET analysis The Java processor JOPThe Java processor JOP ResultsResults Conclusion, future workConclusion, future work DemoDemo



MotivationMotivation

Schedule analysisSchedule analysis For (hard) real-timeFor (hard) real-time Execution time numbers neededExecution time numbers needed

Static WCET analysisStatic WCET analysis No measurements!No measurements!

Can not guarantee the WCCan not guarantee the WC Still usedStill used



Issues with static WCET Issues with static WCET AnalysisAnalysis

Why is WCET Analysis so Why is WCET Analysis so seldom used?seldom used?

High-level part is easyHigh-level part is easy Low-level is the hard partLow-level is the hard part

Instruction timingInstruction timing CachesCaches Advanced, speculative processorsAdvanced, speculative processors



Static WCET AnalysisStatic WCET Analysis

Mature researchMature research High-level based in ILPHigh-level based in ILP

Construct CFGConstruct CFG Add execution time to BBAdd execution time to BB Build ILP equations:Build ILP equations:

Sum of BB tSum of BB texeexe will be maximized will be maximized Frequency of in-edges = out-edgesFrequency of in-edges = out-edges Add loop constraintsAdd loop constraints



Low-level WCET AnalysisLow-level WCET Analysis

Execution time of basic blocksExecution time of basic blocks Sum of instruction timing?Sum of instruction timing?

Not in modern CPUsNot in modern CPUs What about memory access?What about memory access?

Instruction cacheInstruction cache Instruction prefetchInstruction prefetch Data cacheData cache

Very hard for general purpose Very hard for general purpose CPUsCPUs



The Proposed SolutionThe Proposed Solution

Build a processor to simplify Build a processor to simplify WCET AnalysisWCET Analysis Avoid non analyzable featuresAvoid non analyzable features Find better solutionsFind better solutions

Java processor JOPJava processor JOP Built from ground up for WCETBuilt from ground up for WCET FPGA implementation (small)FPGA implementation (small) Not slow on averageNot slow on average



JOP FeaturesJOP Features

A A RISCRISC stack machine stack machine 4 stage pipeline4 stage pipeline

No dependenciesNo dependencies No shared state (e.g. memory)No shared state (e.g. memory)

Stack cacheStack cache Method cacheMethod cache



SizeSize

ResourceResource

(LC)(LC)

MemoryMemory

(KB)(KB)

FmaxFmax

(MHz)(MHz)

JOP min.JOP min. 10771077 3.253.25 9898

JOP typ.JOP typ. 20492049 3.253.25 100100

LightfootLightfoot 34003400 44 4040

LEON3LEON3 79787978 10.910.9 3535



PerformancePerformance

0

2000

4000

6000

8000

10000

12000

J OP Cjip Komodo aJ 80 aJ 100

Perf

orm

ance [itera

tions/s

]



Low-level TimingLow-level Timing

Bytecode execution time knownBytecode execution time known Analysis of microcode (DATE’06)Analysis of microcode (DATE’06) No dependenciesNo dependencies DocumentedDocumented



Memory AccessMemory Access

Assume SRAMAssume SRAM Constant access time (Constant access time (rrwsws and and wwwsws))

Access time partially hidden:Access time partially hidden:

Method cache load:Method cache load:

Partially hiddenPartially hidden



Method CacheMethod Cache

Full method loadedFull method loaded Misses only on invoke/returnMisses only on invoke/return Cache contains several Cache contains several

methodsmethods Simpler to analyzeSimpler to analyze

At call tree levelAt call tree level Other instructions are a hitOther instructions are a hit



Method Cache AnalysisMethod Cache Analysis

Only call tree leavesOnly call tree leaves Return is always a hitReturn is always a hit Invoke in a loopInvoke in a loop

for (int i=0; i<10; ++i) {for (int i=0; i<10; ++i) {

foo();foo();

}}

One miss and 9 hitsOne miss and 9 hits Miss times added to the CFGMiss times added to the CFG



Miss Times in the CFGMiss Times in the CFG

=>



EvaluationEvaluation

MeasuredMeasured

(cycle)(cycle)

EstimatedEstimated

(cycle)(cycle)

PessimismPessimism

(ratio)(ratio)

RobotRobot 736736 775775 1.051.05

LiftLift 72147214 1124911249 1.561.56

KflKfl 1333413334 2876328763 2.162.16

UdpIpUdpIp 1182311823 219569219569 18.5718.57



ConclusionConclusion

We need static WCET analysisWe need static WCET analysis COTS processors don’t workCOTS processors don’t work Design HW for WCETDesign HW for WCET

A RT computer architectureA RT computer architecture JOP is a first step to RT JOP is a first step to RT

processorsprocessors Analysis at bytecode levelAnalysis at bytecode level



Future WorkFuture Work

Method cache cont.Method cache cont. Detection of loop boundsDetection of loop bounds Integration into EclipseIntegration into Eclipse WC memory consumptionWC memory consumption



Demo TimeDemo Time



Thank You!Thank You!

QuestionsQuestions

&&

SuggestionsSuggestions

wcet analysis for a java processor

Documents

wcet analysisavoid

static wcet analysiswhy

wwsaccess time

method cache load

hitmethod cache analysisonly

rt processorsanalysis

tree leavesreturn

mean appmhz104