The What, Why, and How of Probabilistic Verification...Probabilistic Programming Joost-Pieter...
Transcript of The What, Why, and How of Probabilistic Verification...Probabilistic Programming Joost-Pieter...
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The What, Why, and How of Probabilistic VerificationPart 1: Motivation and Models
Joost-Pieter Katoen
CAV Invited Tutorial 2015, San Francisco
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Roadmap of This Tutorial
Part 1. Motivation and Models▸ More Than 5 Reasons for Probabilistic Analysis▸ Elementary Models and Properties
Part 2. Algorithmic Foundations▸ Reachability and Beyond in Discrete Markov Models▸ Timed Reachability in Continuous Markov Models
Part 3. Treating Gigantic Markov Models▸ Abstraction: Precise, Aggressive, and Compositional
Part 4. Recent Research Developments▸ Parameter Synthesis and Model Repair▸ Counterexample Generation▸ Probabilistic Programming
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Overview
The Relevance of Probabilities
Markov Models and Properties
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The Relevance of Probabilities
Overview
The Relevance of Probabilities
Markov Models and Properties
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The Relevance of Probabilities
More Than Five Reasons for Probabilities
1. Randomised Algorithms2. Reducing Complexity3. Probabilistic Programming4. Reliability5. Performance6. Optimization7. Systems Biology
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The Relevance of Probabilities
Randomised Algorithms: Simulating a Die [Knuth & Yao, 1976]
Heads = “go left”; tails = “go right”.
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The Relevance of Probabilities
Randomised Algorithms: Simulating a Die [Knuth & Yao, 1976]
Heads = “go left”; tails = “go right”. Does this model a six-sided die?
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The Relevance of Probabilities
Distributed Computing
FLP impossibility result [Fischer et al., 1985]
In an asynchronous setting, where only one processor might crash, there is nodistributed algorithm that solves the consensus problem—getting a distributednetwork of processors to agree on a common value.
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The Relevance of Probabilities
Distributed Computing
FLP impossibility result [Fischer et al., 1985]
In an asynchronous setting, where only one processor might crash, there is nodistributed algorithm that solves the consensus problem—getting a distributednetwork of processors to agree on a common value.
Ben-Or’s possibility result [Ben-Or, 1983]
If a process can make a decision based on its internal state, the messagestate, and some probabilistic state, consensus in an asynchronous setting isalmost surely possible.
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The Relevance of Probabilities
Reducing Complexity: Matrix Multiplication [Freivald, 1977]
Input: three O(N2) square matrices A, B , and C
Output: yes, if A ×B = C ; no, otherwise
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The Relevance of Probabilities
Reducing Complexity: Matrix Multiplication [Freivald, 1977]
Input: three O(N2) square matrices A, B , and C
Output: yes, if A ×B = C ; no, otherwise
Deterministic: compute A × B and compare with C
Complexity: in O(N3), best known complexity O(N2.37)
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The Relevance of Probabilities
Reducing Complexity: Matrix Multiplication [Freivald, 1977]
Input: three O(N2) square matrices A, B , and C
Output: yes, if A ×B = C ; no, otherwise
Deterministic: compute A × B and compare with C
Complexity: in O(N3), best known complexity O(N2.37)
Randomised: 1. take a random bit-vector x⃗ of size N2. compute A × (B x⃗) − C x⃗3. output yes if this yields the null vector; no otherwise4. repeat these steps k times
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The Relevance of Probabilities
Reducing Complexity: Matrix Multiplication [Freivald, 1977]
Input: three O(N2) square matrices A, B , and C
Output: yes, if A ×B = C ; no, otherwise
Deterministic: compute A × B and compare with C
Complexity: in O(N3), best known complexity O(N2.37)
Randomised: 1. take a random bit-vector x⃗ of size N2. compute A × (B x⃗) − C x⃗3. output yes if this yields the null vector; no otherwise4. repeat these steps k times
Complexity: in O(k ⋅N2), with false positive with probability ⩽ 2−k
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The Relevance of Probabilities
Probabilistic Programming
2013, DARPA launched a 48M (US dollar) program on
“Probabilistic Programming (PP) for Advanced Machine Learning (ML)”
“PP is a new programming paradigm for managing uncertain information.By incorporating it into ML, we seek to greatly increase the number of people
who can successfully build ML applications,and make ML experts radically more effective”.
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The Relevance of Probabilities
Probabilistic Programming: Once Upon a Time . . . . . .
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The Relevance of Probabilities
Duelling Cowboys [McIver and Morgan, 2005]
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The Relevance of Probabilities
Duelling Cowboys [McIver and Morgan, 2005]
Claim: cowboy A wins the duel with probability at least (1−b)⋅aa+b−a⋅b
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The Relevance of Probabilities
Survivor Probability
Claim: cowboy A wins the duel with probability at least (1−b)⋅aa+b−a⋅b
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The Relevance of Probabilities
Survivor Probability
Claim: cowboy A wins the duel with probability at least (1−b)⋅aa+b−a⋅b
Usage: security, machine learning, approximate computing . . . . . .
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The Relevance of Probabilities
Reliability Engineering
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The Relevance of Probabilities
Reliability: (Dynamic) Fault Trees [Dugan et al., 1990]
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The Relevance of Probabilities
A Fault Tree Example
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The Relevance of Probabilities
A Fault Tree Example
(D)FTs: one of —if not the— most prominent models for risk analysisAims: quantify system reliability and availability, MTTF, . . . . . .
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The Relevance of Probabilities
Reliability: Architectural Languages [Feiler et al., 2010]
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The Relevance of Probabilities
Reliability: Architectural Languages [Feiler et al., 2010]
[System] Power.Imp
Nominal
pr imary
b a c k u p
b a t t 1 . e m p t yb a t t 2 . e m p t y
pr imary b a c k u p
[Device] Battery.imp: batt1
NominalError Data
c h a r g e d
ene rgy ‘ = -0 .02 e n e r g y ‘ > = 2 0
e n e r g y > = 1 5 vo l t age := f ( ene rgy)
d e p l e t e d
ene rgy ‘ = -0 .03
e m p t y = > e n e r g y < 2 0
vo l t age := f ( ene rgy )
ok
d e a d
e m p t y
e n e r g y ini t 100
pr imary
vo l t age
vo l t age
[Device] Battery.imp: batt2
NominalError Data
c h a r g e d
ene rgy ‘ = -0 .02 e n e r g y ‘ > = 2 0
e n e r g y > = 1 5 vo l t age := f ( ene rgy)
d e p l e t e d
ene rgy ‘ = -0 .03
e m p t y = > e n e r g y < 2 0
vo l t age := f ( ene rgy )
ok
d e a d
e m p t y
e n e r g y ini t 100
b a c k u p
vo l t age
vo l t age
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The Relevance of Probabilities
Reliability: Architectural Languages [Feiler et al., 2010]
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The Relevance of Probabilities
Performance: GSPNs [Ajmone Marsan et al., 1984]
The early days:
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The Relevance of Probabilities
Performance: GSPNs [Ajmone Marsan et al., 1984]
The early days:
More modern times: Petri nets with
▸ Timed transitions▸ Immediate transitions▸ Natural weights
t0
k0t1
k1
t2 k2
λ
µ
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The Relevance of Probabilities
Performance: GSPNs [Ajmone Marsan et al., 1984]
The early days:
More modern times: Petri nets with
▸ Timed transitions▸ Immediate transitions▸ Natural weights
t0
k0t1
k1
t2 k2
λ
µ
Aims: quantify arrivals, waiting times, QoS, soft deadlines, . . . . . .GSPNs: very—if not the most—popular in performance modeling
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The Relevance of Probabilities
Stochastic Scheduling
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The Relevance of Probabilities
Encyclopedia of Optimization 2008
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The Relevance of Probabilities
Stochastic Scheduling
▸ Job processing times are subject to random variability▸ machine breakdowns and repairs, job parameters, . . .▸ N independent jobs with mean duration 1
µi
▸ M identical machines▸ job processing with (or without) pre-emption
▸ Objective = minimal expected makespan—finishing time of last job
▸ SEPT policy yields minimal expected makespan (Bruno et al., JACM 1981)
“it is hard to calculate these expected values”
Which policy maximises the probability to finish all jobs on time?
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The Relevance of Probabilities
Stochastic Scheduling (N = 4;M = 2)
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The Relevance of Probabilities
Stochastic Scheduling (N = 4;M = 2)
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The Relevance of Probabilities
Stochastic Scheduling (N = 4;M = 2)
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The Relevance of Probabilities
Stochastic Model
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The Relevance of Probabilities
Systems Biology
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The Relevance of Probabilities
Systems Biology
Enzyme-catalysed substrate conversion
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The Relevance of Probabilities
Stochastic Chemical Kinetics
▸ Types of reaction described by stochiometric equations:
E + Sk1
⇌k2
Ck3
→ E + P
▸ N different types of molecules that randomly collidewhere state X(t) = (x1, . . . , xN) with xi =# molecules of sort i
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The Relevance of Probabilities
Stochastic Chemical Kinetics
▸ Types of reaction described by stochiometric equations:
E + Sk1
⇌k2
Ck3
→ E + P
▸ N different types of molecules that randomly collidewhere state X(t) = (x1, . . . , xN) with xi =# molecules of sort i
▸ Reaction probability within infinitesimal interval [t, t+∆):αm(x⃗) ⋅∆ = Pr{reaction m in [t, t+∆) ∣ X(t) = x⃗}
where αm(x⃗) = km ⋅ # possible combinations of reactant molecules in x⃗
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The Relevance of Probabilities
Stochastic Chemical Kinetics
▸ Types of reaction described by stochiometric equations:
E + Sk1
⇌k2
Ck3
→ E + P
▸ N different types of molecules that randomly collidewhere state X(t) = (x1, . . . , xN) with xi =# molecules of sort i
▸ Reaction probability within infinitesimal interval [t, t+∆):αm(x⃗) ⋅∆ = Pr{reaction m in [t, t+∆) ∣ X(t) = x⃗}
where αm(x⃗) = km ⋅ # possible combinations of reactant molecules in x⃗
▸ Process has the Markov property and is time-homogeneous
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The Relevance of Probabilities
Substrate Conversion in the Small
2400
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States: init goalenzymes 2 2substrates 4 0complex 0 0products 0 4
Transitions: E + S1⇌1C
0.001→ E +P
e.g., (xE , xS , xC , xP)0.001⋅xC→ (xE + 1, xS , xC − 1, xP + 1) for xC > 0
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Markov Models and Properties
Overview
The Relevance of Probabilities
Markov Models and Properties
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Markov Models and Properties
Common Feature
All these applications consider Markov models1
1Non-exponential distributions are approximated by phase-type distributions.Joost-Pieter Katoen What, Why, and How of Probabilistic Verification 32/38
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Markov Models and Properties
Discrete-Time Markov Models
A Markov chainfor Knuth-Yao’s algorithm A Markov decision process
for the cowboy program
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Markov Models and Properties
Continuous-Time Markov Models
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A Markov chainfor substrate conversion
t0
t1
t2
λ
µ
p1,3
p2,3
p1,5
p4 p6
p2,5 p2,7
τ
τ
τ
λ
µ1
A Markov decision processfor the GSPN
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Markov Models and Properties
Fault Trees are Continuous-Time MDPs
Markov models of a cold, warm and hot basic event(dormancy factor µ = α⋅λ)
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Markov Models and Properties
Continuous-Time Markov Models
Markov decision process for stochasticscheduling Markov decision processa for a SPARE gate
aIn fact, an interactive Markov chain.
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Markov Models and Properties
Markov Models
Nondeterminism Nondeterminismno yes
Discrete time discrete-time Markov decisionMarkov chain (DTMC) process (MDP)
Continuous time CTMC CTMDP
Other models: e.g., probabilistic variants of (priced) timed automata
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Markov Models and Properties
Properties
Logic Monitors
Discrete time probabilistic deterministic automataCTL (safety and LTL)
Continuous time probabilistic deterministictimed CTL timed automata
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Markov Models and Properties
Properties
Logic Monitors
Discrete time probabilistic deterministic automataCTL (safety and LTL)
Continuous time probabilistic deterministictimed CTL timed automata
Core problem: computing (timed) reachability probabilities
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