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# Why are stochastic networks so hard to simulate?

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http://arxiv.org/abs/0906.4514 …

http://arxiv.org/abs/0906.4514

Strange behavior of simulation of queues and other "skip free" stochastic models, including the R-W Hastings Metropolis algorithm.

Presented at the Workshop on Markov chains and MCMC, in honor of Persi Diaconis

http://http//pages.cs.aueb.gr/users/yiannisk/AWMCMC.html

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• 1. Why are reflected random walks so hard to simulate ? Sean Meyn Department of Electrical and Computer Engineering and the Coordinated Science Laboratory University of Illinois Joint work with Ken Duffy - Hamilton Institute, National University of Ireland NSF support: ECS-0523620
• 3. Outline Motivation Reflected Random Walks Why So Lopsided? 0 T 20 Theory: Observed: X Most Likely Paths 16 12 8 4 0 0 10 20 30 40 50 Summary and Conclusions
• 4. MM1 queue - the nicest of Markov chains For the stable queue, with load strictly less than one, it is • Reversible • Skip-free • Monotone • Marginal distribution geometric • Geometrically ergodic
• 5. MM1 queue - the nicest of Markov chains For the stable queue, with load strictly less than one, it is • Reversible • Skip-free • Monotone • Marginal distribution geometric • Geometrically ergodic Why then, can’t I simulate my queue?
• 6. MM1 queue - the nicest of Markov chains Ch 11 CTCN 1000 800 Gaussian density 600 Histogram 100,000 1 400 √ X(t) − η 100,000 t=1 200 MM1 queue 0 ρ = 0.9 −1000 −500 0 500 1000 1500 2000 Why then, can’t I simulate my queue?
• 7. II Reflected Random Walk t
• 8. Reflected Random Walk A reflected random walk where is the initial condition, and the increments ∆ are i.i.d.
• 9. Reflected Random Walk A reflected random walk where is the initial condition, and the increments ∆ are i.i.d. Basic stability assumption: and finite second moment
• 10. MM1 queue - the nicest of Markov chains Reflected random walk with increments, Load condition
• 11. Reflected Random Walk A reflected random walk where is the initial condition, and the increments ∆ are i.i.d. Basic stability assumption: Basic question: Can I compute sample path averages to estimate the steady-state mean?
• 12. Simulating the RRW: Asymptotic Variance The CLT requires a third moment for the increments E[∆(0)] < 0 and E[∆(0)3 ] < ∞ 1 Asymptotic variance = O (1 − ρ)4 Whitt 1989 Asmussen 1992
• 13. Simulating the RRW: Asymptotic Variance The CLT requires a third moment for the increments E[∆(0)] < 0 and E[∆(0)3 ] < ∞ 1 Asymptotic variance = O (1 − ρ)4 Ch 11 CTCN 1000 800 Gaussian density 600 Histogram 100,000 1 400 √ X(t) − η 100,000 t=1 200 MM1 queue 0 ρ = 0.9 −1000 −500 0 500 1000 1500 2000
• 14. Simulating the RRW: Lopsided Statistics Assume only negative drift, and finite second moment: E[∆(0)] < 0 and E[∆(0)2 ] < ∞ Lower LDP asymptotics: For each r < η , 1 lim log P η(n) ≤ r n→∞ n = −I(r) < 0 M 2006
• 15. Simulating the RRW: Lopsided Statistics Assume only negative drift, and finite second moment: E[∆(0)] < 0 and E[∆(0)2 ] < ∞ Lower LDP asymptotics: For each r < η , 1 lim log P η(n) ≤ r n→∞ n = −I(r) < 0 M 2006 Upper LDP asymptotics are null: For each r ≥ η , 1 lim log P η(n) ≥ r = 0 M 2006 n→∞ n CTCN even for the MM1 queue!
• 16. III Why So Lopsided?
• 17. Lower LDP Construct the family of twisted transition laws ˇ ˇ ˇ (x, dy) = eθx−Λ(θ)+F (x) P (x, dy)eF (y) P θ<0
• 18. Lower LDP Construct the family of twisted transition laws ˇ ˇ ˇ (x, dy) = eθx−Λ(θ)+F (x) P (x, dy)eF (y) P θ<0 Geometric ergodicity of the twisted chain implies multiplicative ergodic theorems, and thence Bahadur-Rao LDP asymptotics Kontoyiannis & M 2003, 2005 M 2006
• 19. Lower LDP Construct the family of twisted transition laws ˇ ˇ ˇ (x, dy) = eθx−Λ(θ)+F (x) P (x, dy)eF (y) P θ<0 Geometric ergodicity of the twisted chain implies multiplicative ergodic theorems, and thence Bahadur-Rao LDP asymptotics Kontoyiannis & M 2003, 2005 M 2006 This is only possible when θ is negative
• 20. Null Upper LDP: What is the area of a triangle? 1 A= 2 bh h b
• 21. Null Upper LDP: What is the area of a triangle? 1 A= 2 bh X(t) η(n) ≈ n−1 A h t 0 T b
• 22. Null Upper LDP: Area of a Triangle? Consider the scaling T =n η(n) ≈ n−1 A h = γn Base obtained from most likely path to this height: b = β∗n e.g., Ganesh, O’Connell, and Wischik, 2004 X(t) Large deviations for reflected random walk: h t 0 T b
• 23. Null Upper LDP: Area of a Triangle Consider the scaling T =n η(n) ≈ n−1 A h = γn An = 1 γβ ∗ n2 2 b = β∗n X(t) Upper LDP is null: For any γ, h t 0 T M 2006 b CTCN 2008 (see also Borovkov, et. al. 2003)
• 24. IV Most Likely Paths
• 25. What Is The Most Likely Area? Are triangular excursions optimal? Most likely path? t 0 T
• 26. What Is The Most Likely Area? Triangular excursions are not optimal: Better... t 0 T A concave perturbation: greatly increased area at modest “cost”
• 27. Better... Most Likely Paths t 0 T Scaled process ψ n (t) = n−1 X(nt) LDP question translated to the scaled process Duffy and M 2009, ...
• 28. Better... Most Likely Paths t 0 T Scaled process ψ n (t) = n−1 X(nt) LDP question translated to the scaled process 1 η(n) ≈ An ⇐⇒ ψ n (t) dt ≈ A 0
• 29. Better... Most Likely Paths t 0 T Scaled process ψ n (t) = n−1 X(nt) LDP question translated to the scaled process 1 η(n) ≈ An ⇐⇒ ψ n (t) dt ≈ A 0 Basic assumption: The sample paths for the unconstrained random walk with increments ∆ satisfy the LDP in D[0,1] with good rate function IX
• 30. Better... Most Likely Paths t 0 T Scaled process ψ n (t) = n−1 X(nt) LDP question translated to the scaled process 1 η(n) ≈ An ⇐⇒ ψ n (t) dt ≈ A 0 Basic assumption: The sample paths for the unconstrained random walk with increments ∆ satisfy the LDP in D[0,1] with good rate function IX 1 ¯ IX (ψ) = ϑ+ ψ(0+) + ˙ I∆ (ψ(t)) dt For concave ψ, with no downward jumps 0
• 31. Better... Most Likely Path Via Dynamic Programming t 0 T Dynamic programming formulation 1 min ¯ ϑ+ ψ(0+) + ˙ I∆ (ψ(t)) dt 0 1 s.t. ψ(t) dt = A 0
• 32. Better... Most Likely Path Via Dynamic Programming t 0 T Dynamic programming formulation 1 min ¯ ϑ+ ψ(0+) + ˙ I∆ (ψ(t)) dt 0 1 s.t. ψ(t) dt = A 0 Solution: Integration by parts + Lagrangian relaxation: 1 1 ¯ min ϑ+ ψ(0+) + ˙ I∆ (ψ(t)) dt + λ ψ(1) − A − ˙ tψ(t) dt 0 0
• 33. Better... Most Likely Path Via Dynamic Programming t 0 T 1 1 ¯ min ϑ+ ψ(0+) + ˙ I∆ (ψ(t)) dt + λ ψ(1) − A − ˙ tψ(t) dt 0 0 Variational arguments where ψ is strictly concave: ψ(t) ψ linear here ψ(0+) t 0 Strictly concave on (T00 , T1 ) 0 T0 T1 1
• 34. Better... Most Likely Path Via Dynamic Programming t 0 T 1 1 ¯ min ϑ+ ψ(0+) + ˙ I∆ (ψ(t)) dt + λ ψ(1) − A − ˙ tψ(t) dt 0 0 Variational arguments where ψ is strictly concave: ψ(t) ψ linear here ψ(0+) t 0 Strictly concave on (T00 , T1 ) 0 T0 T1 1 ˙ I(ψ(t)) = b − λ∗ t 0 for a.e. t ∈ (T0 , T1 )
• 35. ψ(t) Most Likely Paths - Examples ψ linear here ψ(0+) t 0 0 T0 T1 1 1 Increment Exponent: I ψ (0+) + + ∗ ˙ I∆ (ψ ∗ (t)) dt Optimal paths ψ ∗ (t) for various A 0 1 0.8 (MM1 Queue) 1 0.6 0.8 ±1 0.6 0.4 0.4 0.2 0.2 0 0 0.1 0.2 0.3 0.4 0.5 A 0 t 0 0.2 0.4 0.6 0.8 1 0.5 0.4 Gaussian 3 0.3 2 0.2 1 0.1 0 A t 0 0.2 0.4 0.6 0.8 1 0 0 0.2 0.4 0.6 0.8 1 0.6 0.5 0.8 0.4 Poisson 0.6 0.3 0.4 0.2 0.2 0.1 0 0 0.1 0.2 0.3 0.4 0.5 A t 0 0 0.2 0.4 0.6 0.8 1 4 Geometric 2 3 1.5 2 1 0.5 1 0 A 0 0.2 0.4 0.6 0.8 1 0 t 0 0.2 0.4 0.6 0.8 1
• 36. ψ(t) Most Likely Paths - Examples ψ linear here ψ(0+) t 0 0 T0 T1 1 Selected Sample Paths: 20 Theory: 20 Theory: Observed: X Observed: X 15 15 10 10 5 5 0 0 10 20 30 40 50 0 10 20 30 40 RRW: Gaussian Increments RRW: MM1 queue The observed path has the largest simulated mean out of 108 sample paths
• 37. Conclusions
• 38. Summary It is widely known that simulation variance is high in “heavy traffic” for queueing models Large deviation asymptotics are exotic, regardless of load Sample-path behavior is identified for RRW when the sample mean is large. This behavior is very different than previously seen in “buffer overflow” analysis of queues
• 39. Summary It is widely known that simulation variance is high in “heavy traffic” for queueing models Large deviation asymptotics are exotic, regardless of load Sample-path behavior is identified for RRW when the sample mean is large. This behavior is very different than previously seen in “buffer overflow” analysis of queues What about other Markov models?
• 40. MM1 queue - the nicest of Markov chains What about other Markov models? • Reversible • Skip-free • Monotone • Marginal distribution geometric • Geometrically ergodic
• 41. MM1 queue - the nicest of Markov chains What about other Markov models? • Reversible • Skip-free • Monotone • Marginal distribution geometric • Geometrically ergodic The skip-free property makes the fluid model analysis possible. Similar behavior in • Fixed-gain SA • Some MCMC algorithms
• 42. What Next? • Heavy-tailed and/or long-memory settings?
• 43. What Next? • Heavy-tailed and/or long-memory settings? • Variance reduction, such as control variates η(n) 30 25 η + (n) η − (n) 20 15 10 5 n 0 0 100 200 300 400 500 600 700 800 900 1000 M 2006 CTCN
• 44. What Next? • Heavy-tailed and/or long-memory settings? • Variance reduction, such as control variates Performance as a function of safety stocks 23 23 22 22 21 21 20 20 19 19 18 18 10 10 10 20 10 20 20 30 20 30 30 40 30 40 40 50 50 w 40 50 50 w w 60 60 w 60 60 (i) Simulation using smoothed estimator (ii) Simulation using standard estimator ˆ Smoothed estimator using fluid value function in CV: g = h − P h = −Dh, h = J Henderson, M., and Tadi´c 2003 CTCN
• 45. What Next? • Heavy-tailed and/or long-memory settings? • Variance reduction, such as control variates • Original question? Conjecture, 1 lim √ log P η(n) ≥ r = −Jη (r) < 0, r>η n→∞ n for some range of r
• 46. [1] S. P. Meyn. Large deviation asymptotics and control variates for simulating large functions. Ann. Appl. Probab., 16(1):310–339, 2006. [2] S. P. Meyn. Control Techniques for Complex Networks. Cambridge University Press, Cambridge, 2007. [3] K. R. Duﬀy and S. P. Meyn. Most likely paths to error when estimating the mean of a reﬂected random walk. http://arxiv.org/abs/0906.4514, June 2009. References [1] I. Kontoyiannis and S. P. Meyn. Spectral theory and limit theorems for geometrically ergodic Markov processes. Ann. Appl. Probab., 13:304–362, 2003. Presented at the INFORMS Applied Probability Conference, NYC, July, 2001. [2] I. Kontoyiannis and S. P. Meyn. Large deviations asymptotics and the spectral theory of multiplicatively regular Markov processes. Electron. J. Probab., 10(3):61–123 (electronic), 2005. [1] G. Fort, S. Meyn, E. Moulines, and P. Priouret. ODE methods for skip-free Markov chain stability with applications to MCMC. Ann. Appl. Probab., 18(2):664–707, 2008. [2] V. S. Borkar and S. P. Meyn. The O.D.E. method for convergence of stochastic approximation and reinforcement learning. SIAM J. Control Optim., 38(2):447–469, 2000. LDPs for RRW [1] S. P. Meyn. Large deviation asymptotics and control variates for simulating large functions. Ann. Appl. Probab., 16(1):310–339, 2006. [2] S. P. Meyn. Control Techniques for Complex Networks. Cambridge University Press, Cambridge, 2007. [3] K. R. Duﬀy and S. P. Meyn. Most likely paths to error when estimating the mean of a reﬂected random walk. http://arxiv.org/abs/0906.4514, June 2009. Exact LDPs [1] I. Kontoyiannis and S. P. Meyn. Spectral theory and limit theorems for geometrically ergodic Markov processes. Ann. Appl. Probab., 13:304–362, 2003. [2] I. Kontoyiannis and S. P. Meyn. Large deviations asymptotics and the spectral theory of multiplicatively regular Markov processes. Electron. J. Probab., 10(3):61–123 (electronic), 2005. Simulation [1] S. G. Henderson, S. P. Meyn, and V. B. Tadi´. Performance evaluation and policy selection in multiclass c networks. Discrete Event Dynamic Systems: Theory and Applications, 13(1-2):149–189, 2003. Special issue on learning, optimization and decision making (invited). [2] S. P. Meyn. Control Techniques for Complex Networks. Cambridge University Press, Cambridge, 2007. [1] G. Fort, S. Meyn, E. Moulines, and P. Priouret. ODE methods for skip-free Markov chain stability with applications to MCMC. Ann. Appl. Probab., 18(2):664–707, 2008. [2] V. S. Borkar and S. P. Meyn. The ODE method for convergence of stochastic approximation and reinforcement learning. SIAM J. Control Optim., 38(2):447–469, 2000.