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An Efficient High-dimensional Gradient Estimator for Stochastic Differential Equations
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Proposes an unbiased generator gradient estimator for parameter gradients of expected SDE (with jumps) value functions whose cost scales with O(d^2)/O(d^3) simulated SDEs instead of d+dn, and validates it on neural-network-parameterized linear-quadratic control.
Problem
Estimating gradients of path-dependent SDE expectations with respect to a high-dimensional parameter θ (n large, e.g., neural network weights) via pathwise differentiation requires simulating a d+d·n dimensional SDE, which becomes prohibitive as n grows.
Approach
Rewrites the θ-gradient as an expectation involving only space derivatives of the value function (via the PDE/PIDE generator), then estimates those space derivatives by simulating the state, its spatial Jacobian and Hessian; the cost is independent of n and the estimator is unbiased with polynomially growing variance.
Key results
- Generator gradient estimator's runtime stays near-constant as n grows, outperforming pathwise differentiation by more than an order of magnitude for large n (up to n ~ 10^8).
- Averaged standard error ratio (generator gradient / pathwise) is below 1 across all tested n (e.g., 0.971 at n=102, 0.903 at n=21002).
- The generator gradient estimator is unbiased with variance growing at most polynomially in the initial state x (Var ≤ C(|x|+1)^{2m+4}).
Measured effects reported in the paper
| What was changed | Where | Measured | Effect |
|---|---|---|---|
| Use the generator gradient estimator instead of pathwise differentiation | Neural-network-parameterized linear-quadratic control · Linear-quadratic control, tested values of n | Runtime per gradient sample | Helped |
| Use the generator gradient estimator instead of pathwise differentiation | Neural-network-parameterized linear-quadratic control · Linear-quadratic control, larger tested values of n | Runtime per gradient sample | Helped |
| Use the generator gradient estimator instead of pathwise differentiation | Neural-network-parameterized linear-quadratic control · Linear-quadratic control, n=102 | Avg SE: 6.424 -> 5.253; Avg SE ratio 0.971 | Helped |
| Use the generator gradient estimator instead of pathwise differentiation | Neural-network-parameterized linear-quadratic control · Linear-quadratic control, n=1002 | Avg SE: 5.710 -> 5.785; Avg SE ratio 0.932 | Hurt |
| Use the generator gradient estimator instead of pathwise differentiation | Neural-network-parameterized linear-quadratic control · Linear-quadratic control, n=5502 | Avg SE: 4.453 -> 3.533; Avg SE ratio 0.946 | Helped |
| Use the generator gradient estimator instead of pathwise differentiation | Neural-network-parameterized linear-quadratic control · Linear-quadratic control, n=21002 | Avg SE: 1.191 -> 1.205; Avg SE ratio 0.903 | Hurt |
| Use the generator gradient estimator instead of pathwise differentiation | Neural-network-parameterized linear-quadratic control · Linear-quadratic control, n=3.24e5 | Avg SE: 1.110 -> 0.965; Avg SE ratio 0.902 | Helped |
| Use the generator gradient estimator instead of pathwise differentiation | Neural-network-parameterized linear-quadratic control · Linear-quadratic control, n=1.29e6 | Avg SE: 0.935 -> 0.729; Avg SE ratio 0.914 | Helped |
| Use the generator gradient estimator instead of pathwise differentiation | Neural-network-parameterized linear-quadratic control · Linear-quadratic control, n=5.14e6 | Avg SE: 0.786 -> 0.600; Avg SE ratio 0.926 | Helped |
| Use the generator gradient estimator instead of pathwise differentiation | Neural-network-parameterized linear-quadratic control · Linear-quadratic control, n=1.15e7 | Avg SE: 0.466 -> 0.407; Avg SE ratio 0.961 | Helped |
Surprising findings
- The estimator also works in some settings violating smoothness assumptions (e.g., ReLU networks, CIR model), supported only by numerical checks.
Limitations
- Requires second-order continuous differentiability and uniform boundedness of space derivatives of coefficients.
- Dimension of the simulated system grows as O(d^3) when volatility depends on θ, which can be costly for large state dimension d.
Experimental setup
Benchmarks: linear-quadratic control with neural-network-parameterized controls
Metrics: average runtime per sample standard error of gradient estimate standard error ratio
Baselines: pathwise differentiation estimator
Builds on: infinitesimal perturbation analysis Feynman-Kac formula stochastic flows (Kunita) likelihood ratio gradient estimation
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