1535 graph algorithms

Graph Algorithms
Ananth Grama, Anshul Gupta, George
Karypis, and Vipin Kumar
To accompany the text ``Introduction to Parallel Computing'', Addison Wesley, 2003

Topic Overview
• Definitions and Representation
• Minimum Spanning Tree: Prim's Algorithm
• Single-Source Shortest Paths: Dijkstra's Algorithm
• All-Pairs Shortest Paths
• Transitive Closure
• Connected Components
• Algorithms for Sparse Graphs

Definitions and Representation
• An undirected graph G is a pair (V,E), where V is a finite
set of points called vertices and E is a finite set of edges.
• An edge e ∈ E is an unordered pair (u,v), where u,v ∈ V.
• In a directed graph, the edge e is an ordered pair (u,v).
An edge (u,v) is incident from vertex u and is incident to
vertex v.
• A path from a vertex v to a vertex u is a sequence
<v0,v1,v2,…,vk> of vertices where v0 = v, vk = u, and (vi, vi+1)
∈ E for I = 0, 1,…, k-1.
• The length of a path is defined as the number of edges
in the path.

a) An undirected graph and (b) a directed graph.

• An undirected graph is connected if every pair of vertices
is connected by a path.
• A forest is an acyclic graph, and a tree is a connected
acyclic graph.
• A graph that has weights associated with each edge is
called a weighted graph.

• Graphs can be represented by their adjacency matrix or
an edge (or vertex) list.
• Adjacency matrices have a value ai,j = 1 if nodes i and j
share an edge; 0 otherwise. In case of a weighted graph,
ai,j = wi,j, the weight of the edge.
• The adjacency list representation of a graph G = (V,E)
consists of an array Adj[1..|V|] of lists. Each list Adj[v] is
a list of all vertices adjacent to v.
• For a grapn with n nodes, adjacency matrices take Θ(n2
)
space and adjacency list takes Θ(|E|) space.

An undirected graph and its adjacency matrix representation.
An undirected graph and its adjacency list representation.

Minimum Spanning Tree
• A spanning tree of an undirected graph G is a subgraph
of G that is a tree containing all the vertices of G.
• In a weighted graph, the weight of a subgraph is the sum
of the weights of the edges in the subgraph.
• A minimum spanning tree (MST) for a weighted
undirected graph is a spanning tree with minimum
weight.

Minimum Spanning Tree
An undirected graph and its minimum spanning tree.

Minimum Spanning Tree: Prim's
Algorithm
• Prim's algorithm for finding an MST is a greedy
algorithm.
• Start by selecting an arbitrary vertex, include it into the
current MST.
• Grow the current MST by inserting into it the vertex
closest to one of the vertices already in current MST.

Minimum Spanning Tree: Prim's Algorithm
Prim's minimum spanning tree algorithm.

Minimum Spanning Tree: Prim's
Algorithm
Prim's sequential minimum spanning tree algorithm.

Prim's Algorithm: Parallel Formulation
• The algorithm works in n outer iterations - it is hard to execute these
iterations concurrently.
• The inner loop is relatively easy to parallelize. Let p be the number
of processes, and let n be the number of vertices.
• The adjacency matrix is partitioned in a 1-D block fashion, with
distance vector d partitioned accordingly.
• In each step, a processor selects the locally closest node, followed
by a global reduction to select globally closest node.
• This node is inserted into MST, and the choice broadcast to all
processors.
• Each processor updates its part of the d vector locally.

The partitioning of the distance array d and the adjacency matrix A
among p processes.

• The cost to select the minimum entry is O(n/p + log p).
• The cost of a broadcast is O(log p).
• The cost of local updation of the d vector is O(n/p).
• The parallel time per iteration is O(n/p + log p).
• The total parallel time is given by O(n2
/p + n log p).
• The corresponding isoefficiency is O(p2
log2
p).

Single-Source Shortest Paths
• For a weighted graph G = (V,E,w), the single-source
shortest paths problem is to find the shortest paths from
a vertex v ∈ V to all other vertices in V.
• Dijkstra's algorithm is similar to Prim's algorithm. It
maintains a set of nodes for which the shortest paths are
known.
• It grows this set based on the node closest to source
using one of the nodes in the current shortest path set.

Single-Source Shortest Paths: Dijkstra's
Algorithm
Dijkstra's sequential single-source shortest paths algorithm.

Dijkstra's Algorithm: Parallel
Formulation
• Very similar to the parallel formulation of Prim's
algorithm for minimum spanning trees.
• The weighted adjacency matrix is partitioned using the 1-
D block mapping.
• Each process selects, locally, the node closest to the
source, followed by a global reduction to select next
node.
• The node is broadcast to all processors and the l-vector
updated.
• The parallel performance of Dijkstra's algorithm is
identical to that of Prim's algorithm.

All-Pairs Shortest Paths
• Given a weighted graph G(V,E,w), the all-pairs shortest
paths problem is to find the shortest paths between all
pairs of vertices vi, vj ∈ V.
• A number of algorithms are known for solving this
problem.

All-Pairs Shortest Paths: Matrix-
Multiplication Based Algorithm
• Consider the multiplication of the weighted adjacency
matrix with itself - except, in this case, we replace the
multiplication operation in matrix multiplication by
addition, and the addition operation by minimization.
• Notice that the product of weighted adjacency matrix
with itself returns a matrix that contains shortest paths of
length 2 between any pair of nodes.
• It follows from this argument that An
contains all shortest
paths.

Matrix-Multiplication Based Algorithm

Matrix-Multiplication Based Algorithm
• An
is computed by doubling powers - i.e., as A, A2
, A4
, A8
,
and so on.
• We need log n matrix multiplications, each taking time
O(n3
).
• The serial complexity of this procedure is O(n3
log n).
• This algorithm is not optimal, since the best known
algorithms have complexity O(n3
).

Matrix-Multiplication Based Algorithm:
Parallel Formulation
• Each of the log n matrix multiplications can be performed
in parallel.
• We can use n3
/log n processors to compute each matrix-
matrix product in time log n.
• The entire process takes O(log2
n) time.

Dijkstra's Algorithm
• Execute n instances of the single-source shortest path
problem, one for each of the n source vertices.
• Complexity is O(n3
).

Dijkstra's Algorithm: Parallel
Formulation
• Two parallelization strategies - execute each of the n
shortest path problems on a different processor (source
partitioned), or use a parallel formulation of the shortest
path problem to increase concurrency (source parallel).

Dijkstra's Algorithm: Source Partitioned
Formulation
• Use n processors, each processor Pi finds the shortest
paths from vertex vi to all other vertices by executing
Dijkstra's sequential single-source shortest paths
algorithm.
• It requires no interprocess communication (provided that
the adjacency matrix is replicated at all processes).
• The parallel run time of this formulation is: Θ(n2
).
• While the algorithm is cost optimal, it can only use n
processors. Therefore, the isoefficiency due to
concurrency is p3
.

Dijkstra's Algorithm: Source Parallel
Formulation
• In this case, each of the shortest path problems is further
executed in parallel. We can therefore use up to n2
processors.
• Given p processors (p > n), each single source shortest
path problem is executed by p/n processors.
• Using previous results, this takes time:
• For cost optimality, we have p = O(n2
/log n) and the
isoefficiency is Θ((p log p)1.5
).

Floyd's Algorithm
• For any pair of vertices vi, vj ∈ V, consider all paths from
vi to vj whose intermediate vertices belong to the set
{v1,v2,…,vk}. Let pi
(
,
k
j
)
(of weight di
(
,
k
j
)
be the minimum-weight
path among them.
• If vertex vk is not in the shortest path from vi to vj, then pi
(
,
k
j
)
is the same as pi
(
,
k
j
-1)
.
• If f vk is in pi
(
,
k
j
)
, then we can break pi
(
,
k
j
)
into two paths - one
from vi to vk and one from vk to vj . Each of these paths
uses vertices from {v1,v2,…,vk-1}.

Floyd's Algorithm
From our observations, the following recurrence relation
follows:
This equation must be computed for each pair of nodes
and for k = 1, n. The serial complexity is O(n3
).

Floyd's Algorithm
Floyd's all-pairs shortest paths algorithm. This program
computes the all-pairs shortest paths of the graph G =
(V,E) with adjacency matrix A.

Floyd's Algorithm: Parallel Formulation
Using 2-D Block Mapping
• Matrix D(k)
is divided into p blocks of size (n / √p) x (n /
√p).
• Each processor updates its part of the matrix during
each iteration.
• To compute dl
(
,
k
k
-1)
processor Pi,j must get dl
(
,
k
k
-1)
and dk
(
,
k
r
-1)
.
• In general, during the kth
iteration, each of the √p
processes containing part of the kth
row send it to the √p
- 1 processes in the same column.
• Similarly, each of the √p processes containing part of the
kth
column sends it to the √p - 1 processes in the same
row.

(a) Matrix D(k)
distributed by 2-D block mapping into √p x √p subblocks,
and (b) the subblock of D(k)
assigned to process Pi,j.

(a) Communication patterns used in the 2-D block mapping. When computing di
(
,
k
j
)
,
information must be sent to the highlighted process from two other processes along
the same row and column. (b) The row and column of √p processes that contain the
kth
row and column send them along process columns and rows.

Floyd's parallel formulation using the 2-D block mapping. P*,j denotes all
the processes in the jth
column, and Pi,* denotes all the processes in
the ith
row. The matrix D(0)
is the adjacency matrix.

• During each iteration of the algorithm, the kth
row and kth
column of processors perform a one-to-all broadcast
along their rows/columns.
• The size of this broadcast is n/√p elements, taking time
Θ((n log p)/ √p).
• The synchronization step takes time Θ(log p).
• The computation time is Θ(n2
/p).
• The parallel run time of the 2-D block mapping
formulation of Floyd's algorithm is

• The above formulation can use O(n2
/ log2
n) processors
cost-optimally.
• The isoefficiency of this formulation is Θ(p1.5
log3
p).
• This algorithm can be further improved by relaxing the
strict synchronization after each iteration.

Floyd's Algorithm: Speeding Things Up
by Pipelining
• The synchronization step in parallel Floyd's algorithm
can be removed without affecting the correctness of the
algorithm.
• A process starts working on the kth
iteration as soon as it
has computed the (k-1)th
iteration and has the relevant
parts of the D(k-1)
matrix.

by Pipelining
Communication protocol followed in the pipelined 2-D block mapping formulation of
Floyd's algorithm. Assume that process 4 at time t has just computed a segment of
the kth
column of the D(k-1)
matrix. It sends the segment to processes 3 and 5. These
processes receive the segment at time t + 1 (where the time unit is the time it takes
for a matrix segment to travel over the communication link between adjacent
processes). Similarly, processes farther away from process 4 receive the segment
later. Process 1 (at the boundary) does not forward the segment after receiving it.

by Pipelining
• In each step, n/√p elements of the first row are sent from process Pi,j
to Pi+1,j.
• Similarly, elements of the first column are sent from process Pi,j to
process Pi,j+1.
• Each such step takes time Θ(n/√p).
• After Θ(√p) steps, process P√p ,√p gets the relevant elements of the first
row and first column in time Θ(n).
• The values of successive rows and columns follow after time Θ(n2
/p)
in a pipelined mode.
• Process P√p ,√p finishes its share of the shortest path computation in
time Θ(n3
/p) + Θ(n).
• When process P√p ,√p has finished the (n-1)th
iteration, it sends the
relevant values of the nth
row and column to the other processes.

by Pipelining
• The overall parallel run time of this formulation is
• The pipelined formulation of Floyd's algorithm uses up to
O(n2
) processes efficiently.
• The corresponding isoefficiency is Θ(p1.5
).

All-pairs Shortest Path: Comparison
• The performance and scalability of the all-pairs shortest
paths algorithms on various architectures with bisection
bandwidth. Similar run times apply to all cube
architectures, provided that processes are properly
mapped to the underlying processors.

Transitive Closure
• If G = (V,E) is a graph, then the transitive closure of G is
defined as the graph G* = (V,E*), where E*
= {(vi,vj) |
there is a path from vi to vj in G}
• The connectivity matrix of G is a matrix A*
= (ai
*
,j) such
that ai
*
,j = 1 if there is a path from vi to vj or i = j, and ai
*
,j =
∞ otherwise.
• To compute A*
we assign a weight of 1 to each edge of E
and use any of the all-pairs shortest paths algorithms on
this weighted graph.

Connected Components
• The connected components of an undirected graph are
the equivalence classes of vertices under the ``is
reachable from'' relation.
A graph with three connected components: {1,2,3,4},
{5,6,7}, and {8,9}.

Connected Components: Depth-First
Search Based Algorithm
• Perform DFS on the graph to get a forest - eac tree in
the forest corresponds to a separate connected
component.
Part (b) is a depth-first forest obtained from depth-first
traversal of the graph in part (a). Each of these trees is a
connected component of the graph in part (a).

Connected Components: Parallel
Formulation
• Partition the graph across processors and run
independent connected component algorithms on each
processor. At this point, we have p spanning forests.
• In the second step, spanning forests are merged
pairwise until only one spanning forest remains.

Formulation
Computing connected components in parallel. The adjacency matrix of the graph G in (a)
is partitioned into two parts (b). Each process gets a subgraph of G ((c) and (e)).
Each process then computes the spanning forest of the subgraph ((d) and (f)). Finally,
the two spanning trees are merged to form the solution.

Formulation
• To merge pairs of spanning forests efficiently, the
algorithm uses disjoint sets of edges.
• We define the following operations on the disjoint sets:
• find(x)
– returns a pointer to the representative element of the set
containing x . Each set has its own unique representative.
• union(x, y)
– unites the sets containing the elements x and y. The two sets
are assumed to be disjoint prior to the operation.

Formulation
• For merging forest A into forest B, for each edge (u,v) of
A, a find operation is performed to determine if the
vertices are in the same tree of B.
• If not, then the two trees (sets) of B containing u and v
are united by a union operation.
• Otherwise, no union operation is necessary.
• Hence, merging A and B requires at most 2(n-1) find
operations and (n-1) union operations.

Connected Components: Parallel 1-D
Block Mapping
• The n x n adjacency matrix is partitioned into p blocks.
• Each processor can compute its local spanning forest in
time Θ(n2
/p).
• Merging is done by embedding a logical tree into the
topology. There are log p merging stages, and each
takes time Θ(n). Thus, the cost due to merging is Θ(n log
p).
• During each merging stage, spanning forests are sent
between nearest neighbors. Recall that Θ(n) edges of
the spanning forest are transmitted.

Connected Components: Parallel 1-D
Block Mapping
• The parallel run time of the connected-component
algorithm is
• For a cost-optimal formulation p = O(n / log n). The
corresponding isoefficiency is Θ(p2
log2
p).

Algorithms for Sparse Graphs
• A graph G = (V,E) is sparse if |E| is much smaller than |
V|2.
Examples of sparse graphs: (a) a linear graph, in which each vertex has two incident
edges; (b) a grid graph, in which each vertex has four incident vertices; and (c) a
random sparse graph.

• Dense algorithms can be improved significantly if we
make use of the sparseness. For example, the run time
of Prim's minimum spanning tree algorithm can be
reduced from Θ(n2
) to Θ(|E| log n).
• Sparse algorithms use adjacency list instead of an
adjacency matrix.
• Partitioning adjacency lists is more difficult for sparse
graphs - do we balance number of vertices or edges?
• Parallel algorithms typically make use of graph structure
or degree information for performance.

A street map (a) can be represented by a graph (b). In the graph
shown in (b), each street intersection is a vertex and each edge is a
street segment. The vertices of (b) are the intersections of (a)
marked by dots.

Finding a Maximal Independent Set
• A set of vertices I ⊂ V is called independent if no pair of
vertices in I is connected via an edge in G. An
independent set is called maximal if by including any
other vertex not in I, the independence property is
violated.
Examples of independent and maximal independent sets.

Finding a Maximal Independent Set (MIS)
• Simple algorithms start by MIS I to be empty, and
assigning all vertices to a candidate set C.
• Vertex v from C is moved into I and all vertices adjacent
to v are removed from C.
• This process is repeated until C is empty.
• This process is inherently serial!

• Parallel MIS algorithms use randimization to gain
concurrency (Luby's algorithm for graph coloring).
• Initially, each node is in the candidate set C. Each node
generates a (unique) random number and communicates
it to its neighbors.
• If a nodes number exceeds that of all its neighbors, it
joins set I. All of its neighbors are removed from C.
• This process continues until C is empty.
• On average, this algorithm converges after O(log|V|)
such steps.

The different augmentation steps of Luby's randomized maximal
independent set algorithm. The numbers inside each vertex
correspond to the random number assigned to the vertex.

Finding a Maximal Independent Set
(MIS): Parallel Formulation
• We use three arrays, each of length n - I, which stores
nodes in MIS, C, which stores the candidate set, and R,
the random numbers.
• Partition C across p processors. Each processor
generates the corresponding values in the R array, and
from this, computes which candidate vertices can enter
MIS.
• The C array is updated by deleting all the neighbors of
vertices that entered MIS.
• The performance of this algorithm is dependent on the
structure of the graph.

Single-Source Shortest Paths
• Dijkstra's algorithm, modified to handle sparse graphs is
called Johnson's algorithm.
• The modification accounts for the fact that the
minimization step in Dijkstra's algorithm needs to be
performed only for those nodes adjacent to the
previously selected nodes.
• Johnson's algorithm uses a priority queue Q to store the
value l[v] for each vertex v ∈ (V – VT).

Single-Source Shortest Paths: Johnson's
Algorithm
Johnson's sequential single-source shortest paths algorithm.

Single-Source Shortest Paths: Parallel
Johnson's Algorithm
• Maintaining strict order of Johnson's algorithm generally
leads to a very restrictive class of parallel algorithms.
• We need to allow exploration of multiple nodes
concurrently. This is done by simultaneously extracting p
nodes from the priority queue, updating the neighbors'
cost, and augmenting the shortest path.
• If an error is made, it can be discovered (as a shorter
path) and the node can be reinserted with this shorter
path.

Johnson's Algorithm
An example of the modified Johnson's algorithm for processing unsafe vertices
concurrently.

Johnson's Algorithm
• Even if we can extract and process multiple nodes from
the queue, the queue itself is a major bottleneck.
• For this reason, we use multiple queues, one for each
processor. Each processor builds its priority queue only
using its own vertices.
• When process Pi extracts the vertex u ∈ Vi, it sends a
message to processes that store vertices adjacent to u.
• Process Pj, upon receiving this message, sets the value
of l[v] stored in its priority queue to min{l[v],l[u] + w(u,v)}.

Johnson's Algorithm
• If a shorter path has been discovered to node v, it is
reinserted back into the local priority queue.
• The algorithm terminates only when all the queues
become empty.
• A number of node paritioning schemes can be used to
exploit graph structure for performance.

1535 graph algorithms

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