Class ParallelMultistarter<T extends Copyable<T>>

  • Type Parameters:
    T - The type of object being optimized.
    All Implemented Interfaces:
    AutoCloseable, Splittable<TrackableSearch<T>>, Metaheuristic<T>, TrackableSearch<T>
    Direct Known Subclasses:
    ParallelReoptimizableMultistarter

    public class ParallelMultistarter<T extends Copyable<T>>
    extends Object
    implements Metaheuristic<T>, AutoCloseable

    This class is used for implementing parallel multistart metaheuristics. It can be used to restart any class that implements the Metaheuristic interface. A multistart metaheuristic returns the best result from among all of the restarts. In the case of a parallel multistart metaheuristic, the search returns the best result from among all restarts across all threads.

    There are several constructors enabling different ways to configure the search. You can initialize the search with a combination of a Metaheuristic and number of threads along with either a RestartSchedule for the purpose of specifying run lengths for the restarts, or a run length if all runs are to be of the same length. You can also initialize the search with a Collection of RestartSchedule objects, one for each thread (with number of threads implied by size of Collection. Or you can initialize the search with a Collection of Metaheuristic objects and a Collection of RestartSchedule objects (both Collections of the same size). You can also initialize the search with a Multistarter configured with your restart schedule, along with the number of threads, or a Collection of Multistarter objects.

    • Constructor Detail

      • ParallelMultistarter

        public ParallelMultistarter​(Metaheuristic<T> search,
                                    int runLength,
                                    int numThreads)
        Constructs a parallel multistart metaheuristic that executes multiple runs of a specified metaheuristic in parallel across multiple threads. All restarts are the same in length.
        Parameters:
        search - The metaheuristic to restart multiple times in parallel.
        runLength - The length of every restarted run of the metaheuristic.
        numThreads - The number of threads to use.
        Throws:
        IllegalArgumentException - if numThreads is less than 1.
        IllegalArgumentException - if nunLength is less than 1.
      • ParallelMultistarter

        public ParallelMultistarter​(Metaheuristic<T> search,
                                    RestartSchedule r,
                                    int numThreads)
        Constructs a parallel multistart metaheuristic that executes multiple runs of a specified metaheuristic in parallel across multiple threads. All parallel instances follow the same restart schedule of run lengths.
        Parameters:
        search - The metaheuristic to restart multiple times in parallel.
        r - The schedule of run lengths. Note that the threads do not share a single RestartSchedule. Rather, each thread will be initialized with its own copy of r.
        numThreads - The number of threads to use.
        Throws:
        IllegalArgumentException - if numThreads is less than 1.
      • ParallelMultistarter

        public ParallelMultistarter​(Metaheuristic<T> search,
                                    Collection<? extends RestartSchedule> schedules)
        Constructs a parallel multistart metaheuristic that executes multiple runs of a specified metaheuristic in parallel across multiple threads. Each parallel instance follows its own restart schedule of run lengths.
        Parameters:
        search - The metaheuristic to restart multiple times in parallel.
        schedules - The schedules of run lengths, one for each thread. The number of threads will be equal to the number of restart schedules.
        Throws:
        IllegalArgumentException - if schedules.size() is less than 1.
      • ParallelMultistarter

        public ParallelMultistarter​(Collection<? extends Metaheuristic<T>> searches,
                                    Collection<? extends RestartSchedule> schedules)
        Constructs a parallel multistart metaheuristic that executes multiple runs of a set of specified metaheuristics in parallel across multiple threads. Each parallel search follows its own restart schedule of run lengths.
        Parameters:
        searches - A collection of the metaheuristics to restart multiple times in parallel. The number of threads will be equal to the size of this collection.
        schedules - The schedules of run lengths, one for each thread.
        Throws:
        IllegalArgumentException - if searches.size() is not equal to schedules.size().
        IllegalArgumentException - if the Collection of Metaheuristics don't all share the same problem (i.e., requires that s1.getProblem() == s2.getProblem() for all s1, s2 in searches).
        IllegalArgumentException - if the Collection of Metaheuristics don't all share a single ProgressTracker (i.e., requires that s1.getProgressTracker() == s2.getProgressTracker() for all s1, s2 in searches).
      • ParallelMultistarter

        public ParallelMultistarter​(Collection<? extends Metaheuristic<T>> searches,
                                    int runLength)
        Constructs a parallel multistart metaheuristic that executes multiple runs of a set of specified metaheuristics in parallel across multiple threads. All runs of all parallel instances follows a constant run length.
        Parameters:
        searches - A collection of the metaheuristics to restart multiple times in parallel. The number of threads will be equal to the size of this collection.
        runLength - The length of all restarted runs of all parallel metaheuristics.
        Throws:
        IllegalArgumentException - if runLength < 1.
        IllegalArgumentException - if the Collection of Metaheuristics don't all share the same problem (i.e., requires that s1.getProblem() == s2.getProblem() for all s1, s2 in searches).
        IllegalArgumentException - if the Collection of Metaheuristics don't all share a single ProgressTracker (i.e., requires that s1.getProgressTracker() == s2.getProgressTracker() for all s1, s2 in searches).
      • ParallelMultistarter

        public ParallelMultistarter​(Multistarter<T> multistartSearch,
                                    int numThreads)
        Constructs a parallel multistart metaheuristic that executes multiple runs of a specified metaheuristic in parallel across multiple threads. All parallel instances follow the same restart schedule of run lengths.
        Parameters:
        multistartSearch - A Multistarter configured with the metaheuristic and restart schedule. Each of the threads will be an identical copy of this Multistarter.
        numThreads - The number of threads to use.
        Throws:
        IllegalArgumentException - if numThreads is less than 1.
      • ParallelMultistarter

        public ParallelMultistarter​(Collection<? extends Multistarter<T>> multistarters)
        Constructs a parallel multistart metaheuristic that executes multiple runs of a set of specified metaheuristics in parallel across multiple threads. Each of the Multistarters will run in its own thread. The number of threads will be equal to the number of Multistarters passed to the constructor.
        Parameters:
        multistarters - A collection of Multistarters configured with the metaheuristics and restart schedules for the threads.
        Throws:
        IllegalArgumentException - if the Collection of Multistarters don't all share the same problem (i.e., requires that s1.getProblem() == s2.getProblem() for all s1, s2 in multistarters).
        IllegalArgumentException - if the Collection of Multistarters don't all share a single ProgressTracker (i.e., requires that s1.getProgressTracker() == s2.getProgressTracker() for all s1, s2 in multistarters).
    • Method Detail

      • optimize

        public final SolutionCostPair<T> optimize​(int numRestarts)

        Executes a parallel multistart search. The number of threads, the specific metaheuristic executed by each thread, the restart schedules, etc are determined by how the ParallelMultistarter was configured at the time of construction. The optimize method runs the optimize method of each of the parallel instances of the search the specified number of times, keeping track of the best solution across the multiple parallel runs of the search. Each restart of each parallel search begins at a new randomly generate initial state.

        If this method is called multiple times, the restart schedules of the parallel metaheuristics are not reinitialized, and the run lengths for the additional restarts will continue where the schedules left off.

        Specified by:
        optimize in interface Metaheuristic<T extends Copyable<T>>
        Parameters:
        numRestarts - The number of times to restart each of the parallel metaheuristics.
        Returns:
        The best end of run solution (and its cost) of this set of parallel restarts, which may or may not be the same as the solution contained in this metaheuristic's ProgressTracker, which contains the best of all runs. Returns null if the run did not execute, such as if the ProgressTracker already contains the theoretical best solution.
        Throws:
        IllegalStateException - if the close() method was previously called.
      • close

        public final void close()

        Initiates an orderly shutdown of the thread pool used by this ParallelMultistarter. The ParallelMultistarter utilizes a fixed thread pool so that multiple calls to the optimize(int) method can reuse threads to minimize the expensive task of thread creation. When you no longer need the ParallelMultistarter, you should call the close method to ensure that unneeded threads do not persist. Once close is called, all subsequent calls to optimize(int) will throw an exception.

        This method is invoked automatically on objects managed by the try-with-resources statement.

        Specified by:
        close in interface AutoCloseable
      • isClosed

        public final boolean isClosed()
        Checks whether the thread pool has been shutdown.
        Returns:
        true if and only if the close() method has been called previously.
      • split

        public ParallelMultistarter<T> split()
        Description copied from interface: Splittable
        Generates a functionally identical copy of this object, for use in multithreaded implementations of search algorithms. The state of the object that is returned may or may not be identical to that of the original. Thus, this is a distinct concept from the functionality of the Copyable interface. Classes that implement this interface must ensure that the object returned performs the same functionality, and that it does not share any state data that would be either unsafe or inefficient for concurrent access by multiple threads. The split method is allowed to simply return the this reference, provided that it is both safe and efficient for multiple threads to share a single copy of the Splittable object. The intention is to provide a multithreaded search with the capability to provide spawned threads with their own distinct search operators. Such multithreaded algorithms can call the split method for each thread it spawns to generate a functionally identical copy of the operator, but with independent state.
        Specified by:
        split in interface Metaheuristic<T extends Copyable<T>>
        Specified by:
        split in interface Splittable<T extends Copyable<T>>
        Returns:
        A functionally identical copy of the object, or a reference to this if it is both safe and efficient for multiple threads to share a single instance of this Splittable object.
      • getProgressTracker

        public final ProgressTracker<T> getProgressTracker()
        Description copied from interface: TrackableSearch
        Gets the ProgressTracker object that is in use for tracking search progress. The object returned by this method contains the best solution found during the search (including across multiple concurrent runs if the search is multithreaded, or across multiple restarts if the run methods were called multiple times), as well as cost of that solution, among other information. See the ProgressTracker documentation for more information about the search data tracked by this object.
        Specified by:
        getProgressTracker in interface TrackableSearch<T extends Copyable<T>>
        Returns:
        the ProgressTracker in use by this metaheuristic.
      • setProgressTracker

        public final void setProgressTracker​(ProgressTracker<T> tracker)
        Description copied from interface: TrackableSearch
        Sets the ProgressTracker object that is in use for tracking search progress. Any previously set ProgressTracker is replaced by this one.
        Specified by:
        setProgressTracker in interface TrackableSearch<T extends Copyable<T>>
        Parameters:
        tracker - The new ProgressTracker to set. The tracker must not be null. This method does nothing if tracker is null.
      • getTotalRunLength

        public final long getTotalRunLength()

        Gets the total run length of all restarts of all parallel instances of the underlying metaheuristics combined. This may differ from what may be expected based on run lengths passed to the optimize and reoptimize methods of the underlying metaheuristics. For example, the optimize method terminates if it finds the theoretical best solution, and also immediately returns if a prior call found the theoretical best. In such cases, the total run length may be less than the requested run length.

        The meaning of run length may vary based on what metaheuristic is being restarted.

        Specified by:
        getTotalRunLength in interface TrackableSearch<T extends Copyable<T>>
        Returns:
        the total run length of all restarts of the underlying metaheuristic, which includes across multiple calls to the restart mechanism and across all parallel instances.