3 research outputs found
A Generally Applicable, Highly Scalable Measurement Computation and Optimization Approach to Sequential Model-Based Diagnosis
Model-Based Diagnosis deals with the identification of the real cause of a
system's malfunction based on a formal system model and observations of the
system behavior. When a malfunction is detected, there is usually not enough
information available to pinpoint the real cause and one needs to discriminate
between multiple fault hypotheses (called diagnoses). To this end, Sequential
Diagnosis approaches ask an oracle for additional system measurements.
This work presents strategies for (optimal) measurement selection in
model-based sequential diagnosis. In particular, assuming a set of leading
diagnoses being given, we show how queries (sets of measurements) can be
computed and optimized along two dimensions: expected number of queries and
cost per query. By means of a suitable decoupling of two optimizations and a
clever search space reduction the computations are done without any inference
engine calls. For the full search space, we give a method requiring only a
polynomial number of inferences and show how query properties can be guaranteed
which existing methods do not provide. Evaluation results using real-world
problems indicate that the new method computes (virtually) optimal queries
instantly independently of the size and complexity of the considered diagnosis
problems and outperforms equally general methods not exploiting the proposed
theory by orders of magnitude
Evaluating Active Learning Heuristics for Sequential Diagnosis
Given a malfunctioning system, sequential diagnosis aims at identifying the
root cause of the failure in terms of abnormally behaving system components. As
initial system observations usually do not suffice to deterministically pin
down just one explanation of the system's misbehavior, additional system
measurements can help to differentiate between possible explanations. The goal
is to restrict the space of explanations until there is only one (highly
probable) explanation left. To achieve this with a minimal-cost set of
measurements, various (active learning) heuristics for selecting the best next
measurement have been proposed.
We report preliminary results of extensive ongoing experiments with a set of
selection heuristics on real-world diagnosis cases. In particular, we try to
answer questions such as "Is some heuristic always superior to all others?",
"On which factors does the (relative) performance of the particular heuristics
depend?" or "Under which circumstances should I use which heuristic?
DynamicHS: Streamlining Reiter's Hitting-Set Tree for Sequential Diagnosis
Given a system that does not work as expected, Sequential Diagnosis (SD) aims
at suggesting a series of system measurements to isolate the true explanation
for the system's misbehavior from a potentially exponential set of possible
explanations. To reason about the best next measurement, SD methods usually
require a sample of possible fault explanations at each step of the iterative
diagnostic process. The computation of this sample can be accomplished by
various diagnostic search algorithms. Among those, Reiter's HS-Tree is one of
the most popular due its desirable properties and general applicability.
Usually, HS-Tree is used in a stateless fashion throughout the SD process to
(re)compute a sample of possible fault explanations in each iteration, each
time given the latest (updated) system knowledge including all so-far collected
measurements. At this, the built search tree is discarded between two
iterations, although often large parts of the tree have to be rebuilt in the
next iteration, involving redundant operations and calls to costly reasoning
services.
As a remedy to this, we propose DynamicHS, a variant of HS-Tree that
maintains state throughout the diagnostic session and additionally embraces
special strategies to minimize the number of expensive reasoner invocations. In
this vein, DynamicHS provides an answer to a longstanding question posed by
Raymond Reiter in his seminal paper from 1987.
Extensive evaluations on real-world diagnosis problems prove the
reasonability of the DynamicHS and testify its clear superiority to HS-Tree
wrt. computation time. More specifically, DynamicHS outperformed HS-Tree in 96%
of the executed sequential diagnosis sessions and, per run, the latter required
up to 800% the time of the former. Remarkably, DynamicHS achieves these
performance improvements while preserving all desirable properties as well as
the general applicability of HS-Tree