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25673 Scalable multi-core model checking
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Laarman, A.W. (2014) Scalable multi-core model checking. PhD thesis, University of Twente. CTIT Ph.D.-thesis series No. 14-308 ISBN 978-90-365-3656-1

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Our modern society relies increasingly on the sound performance of digital systems.
Guaranteeing that these systems actually behave correctly according to their specification
is not a trivial task, yet it is essential for mission-critical systems like auto-pilots,
(nuclear) power-plant controllers and your car’s ABS.
The highest degree of certainty about a system’s correctness can be obtained via
mathematical proof, a tedious manual process of formally describing and analyzing the
system’s behavior. Especially the latter step is tedious and requires the creativity of
a mathematician to demonstrate that certain properties are preserved under the strict
mathematical rule system. With the invention of “model checking”, this part of this
process became automated, by letting a computer exhaustively explore the behavior of
the system.
However, the size of the systems that can be “model checked” is severely limited
by the available computational resources. This is caused by the so called state explosion,
a consequence of the fact that a machine can only perform small mechanized
computations and does not exhibit the creativity to make generalizing (thinking) steps.
Therefore, the goal of the current thesis is to enable the full use of computational power
of modern multi-core computers for model checking. The parallel model checking procedures
that we present, utilize all available processor cores and obtain a speedup proportional
to the number of cores, i.e. they are “scalable”.
The current thesis achieves efficient parallelization of a broad set of model checking
problems in three steps, each described in one part of the thesis:
First, we adapt lockless hash tables for multi-core, explicit-state reachability, the
underlying search method that realizes the exhaustive exploration of the system’s behavior.
With a concurrent tree data structure we realize state compression, and reduce
memory requirements significantly. Incremental updates to this tree further ensure sim-
ilar performance and scalability as the lockless hash table, while the combination with
a compact hash table realizes small compressed sizes of around 4 bytes per state, even
when storing more than 10 billion states. Empirical evidence shows that the compression
rates most often lie within 110% of this optimal.
Second, we devise parallel nested depth-first search algorithms to support model
checking of LTL properties in linear time. Building on the multi-core reachability, we
let worker threads progress semi-independently through the search space. This swarmbased
technique leverages low communication costs through the use of optimistic, yet
possibly redundant work scheduling. It could therefore become more important in future
multi-core systems, where communication costs rise with the increasing steepness
of memory hierarchies. Experiments on current hardware already demonstrate little
redundancy and good scalability.
Third, to support verification of real-time systems as well, we extend multi-core
reachability and LTL checking to the domain of timed automata. We develop a lockless
multimap to record time-abstracted states, and also present algorithms that deal with
coarse subsumption abstraction for the verification of LTL for solving larger problem
instances. The scalability, memory compression and performance are all maintained
in the timed setting, and experiments therefore show great gains with respect to the
state-of-the-art timed model checker uppaal.
The above techniques were all implemented in the model checking toolset LTSmin,
which is language-independent, allowing a direct comparison to other model checkers.
We present an experimental comparison with the state-of-the-art explicit-state model
checkers spin and DiVinE. Both implement multi-core algorithms, while DiVinE also
heavily focuses on distributed verification. These experiments show that our proposed
techniques offer significant improvements in terms of scalability, absolute performance
and memory usage.
Current trends and future predictions tell us that the available processing cores increase
exponentially over time (Moore’s Law). Hence, our results may stand to gain
from this trend. Whether our proposed methods will withstand the ravages of time is to
be seen, but so far the speedup of our algorithms has kept up with the 3-fold increase in
cores that we have witnessed during this 4-year project.

Item Type:PhD Thesis
Supervisors:van de Pol, J.C.
Research Group:EWI-FMT: Formal Methods and Tools
Research Program:CTIT-DSN: Dependable Systems and Networks
Research Project:CEDICT: 3TU Center of Excellence for ICT
ID Code:25673
Deposited On:10 February 2015
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