it.unimi.dsi.webgraph.algo
Class ParallelBreadthFirstVisit

java.lang.Object
  extended by it.unimi.dsi.webgraph.algo.ParallelBreadthFirstVisit

public class ParallelBreadthFirstVisit
extends Object

Performs breadth-firsts visits of a graph exploiting multicore parallelism.

To use this class you first create an instance, and then invoke visit(int). If you want perform more visits preserving the marker state you can invoke visit(int) again. By calling clear(), instead, you can reset marker (i.e., forget about visited nodes).

Alternatively, visitAll() will start a visit from all the nodes of the graph in a more efficient way.

After the visit, you can peek at the field marker to discover details about the visit. Depending on the parent value provided at construction time, the array marker will be filled with parent information (e.g., with the index of the parent node in the visit tree) or with a round number increased at each nonempty visit, which act as a connected-component index if the graph is symmetric.

In the case of visit(int, int), queue and cutPoints, too, provide useful information. In particular, the nodes in queue from the d-th to the (d +1)-th cutpoint are exactly the nodes at distance d from the source.

Performance issues

This class needs three integers per node. If there are several available cores, breadth-first visits will be decomposed into relatively small tasks (small blocks of nodes in the queue at the same distance from the starting node) and each task will be assigned to the first available core. Since all tasks are completely independent, this ensures a very high degree of parallelism.

Note, however, that if the degree distribution is extremely skewed some cores might get stuck in the enumeration of the successors of some nodes with a very high degree.


Field Summary
 IntArrayList cutPoints
          At the end of a visit, the cutpoints of queue.
 ImmutableGraph graph
          The graph under examination.
 AtomicIntegerArray marker
          The marker array; contains -1 for nodes that have not still been enqueued, the parent of the visit tree if parent is true, or an index increased at each visit if parent is false, which in the symmetric case is the index of the connected component of the node.
 boolean parent
          Whether marker contains parent nodes or round numbers.
 IntArrayList queue
          The queue of visited nodes.
 int round
          A number increased at each nonempty visit (used to mark marker if parent is false).
 
Constructor Summary
ParallelBreadthFirstVisit(ImmutableGraph graph, int requestedThreads, boolean parent, ProgressLogger pl)
          Creates a new class for keeping track of the state of parallel breadth-first visits.
 
Method Summary
 void clear()
          Clears the internal state of the visit, setting all marker entries and round to -1.
 int maxDistance()
          Returns the maximum distance computed during the last visit (e.g., the eccentricity of the source).
 int nodeAtMaxDistance()
          Returns a node at maximum distance during the last visit (e.g., a node realising the positive eccentricity of the starting node).
 int visit(int start)
          Performs a breadth-first visit of the given graph starting from the given node.
 int visit(int start, int expectedSize)
          Performs a breadth-first visit of the given graph starting from the given node.
 void visitAll()
          Visits all nodes.
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 

Field Detail

graph

public final ImmutableGraph graph
The graph under examination.


queue

public final IntArrayList queue
The queue of visited nodes.


cutPoints

public final IntArrayList cutPoints
At the end of a visit, the cutpoints of queue. The d-th cutpoint is the first node in the queue at distance d. The last cutpoint is the queue size.


parent

public final boolean parent
Whether marker contains parent nodes or round numbers.


marker

public final AtomicIntegerArray marker
The marker array; contains -1 for nodes that have not still been enqueued, the parent of the visit tree if parent is true, or an index increased at each visit if parent is false, which in the symmetric case is the index of the connected component of the node.


round

public int round
A number increased at each nonempty visit (used to mark marker if parent is false).

Constructor Detail

ParallelBreadthFirstVisit

public ParallelBreadthFirstVisit(ImmutableGraph graph,
                                 int requestedThreads,
                                 boolean parent,
                                 ProgressLogger pl)
Creates a new class for keeping track of the state of parallel breadth-first visits.

Parameters:
graph - a graph.
requestedThreads - the requested number of threads (0 for Runtime.availableProcessors()).
parent - if true, marker will contain parent nodes; otherwise, it will contain round numbers.
pl - a progress logger, or null.
Method Detail

clear

public void clear()
Clears the internal state of the visit, setting all marker entries and round to -1.


visit

public int visit(int start)
Performs a breadth-first visit of the given graph starting from the given node.

This method will increment round.

Parameters:
start - the starting node.
Returns:
the number of visited nodes.
See Also:
visit(int,int)

visit

public int visit(int start,
                 int expectedSize)
Performs a breadth-first visit of the given graph starting from the given node.

This method will increment round if at least one node is visited.

Parameters:
start - the starting node.
expectedSize - the expected size (number of nodes) of the visit (for logging), or -1 to use the number of nodes of the graph.
Returns:
the number of visited nodes.

visitAll

public void visitAll()
Visits all nodes. Calls clear() initially.

This method is more efficient than invoking visit(int, int) on all nodes as threads are created just once.


nodeAtMaxDistance

public int nodeAtMaxDistance()
Returns a node at maximum distance during the last visit (e.g., a node realising the positive eccentricity of the starting node).

Returns:
the maximum distance computed during the last visit.

maxDistance

public int maxDistance()
Returns the maximum distance computed during the last visit (e.g., the eccentricity of the source).

Returns:
the maximum distance computed during the last visit.