27 research outputs found
Distributed Sparse Cut Approximation
We study the problem of computing a sparse cut in an undirected network graph G=(V,E). We measure the sparsity of a cut (S,VS) by its conductance phi(S), i.e., by the ratio of the number of edges crossing the cut and the sum of the degrees on the smaller of the two sides. We present an efficient distributed algorithm to compute a cut of low conductance. Specifically, given two parameters b and phi, if there exists a cut of balance at least b and conductance at most phi, our algorithm outputs a cut of balance at least b/2 and conductance at most ~O(sqrt{phi}), where ~O(.) hides polylogarithmic factors in the number of nodes n. Our distributed algorithm works in the congest model, i.e., it only requires to send messages of size at most O(log(n)) bits. The time complexity of the algorithm is ~O(D + 1/b*phi), where D is the diameter of G. This is a significant improvement over a result by Das Sarma et al. [ICDCN 2015], where it is shown that a cut of the same quality can be computed in time ~O(n + 1/b*phi). The improved running time is in particular achieved by devising and applying an efficient distributed algorithm for the all-prefix-sums problem in a distributed search tree. This algorithm, which is based on the classic parallel all-prefix-sums algorithm, might be of independent interest
Local Mixing Time: Distributed Computation and Applications
The mixing time of a graph is an important metric, which is not only useful
in analyzing connectivity and expansion properties of the network, but also
serves as a key parameter in designing efficient algorithms. We introduce a new
notion of mixing of a random walk on a (undirected) graph, called local mixing.
Informally, the local mixing with respect to a given node , is the mixing of
a random walk probability distribution restricted to a large enough subset of
nodes --- say, a subset of size at least for a given parameter
--- containing . The time to mix over such a subset by a random walk
starting from a source node is called the local mixing time with respect to
. The local mixing time captures the local connectivity and expansion
properties around a given source node and is a useful parameter that determines
the running time of algorithms for partial information spreading, gossip etc.
Our first contribution is formally defining the notion of local mixing time
in an undirected graph. We then present an efficient distributed algorithm
which computes a constant factor approximation to the local mixing time with
respect to a source node in rounds, where is
the local mixing time w.r.t in an -node regular graph. This bound holds
when is significantly smaller than the conductance of the local mixing
set (i.e., the set where the walk mixes locally); this is typically the
interesting case where the local mixing time is significantly smaller than the
mixing time (with respect to ). We also present a distributed algorithm that
computes the exact local mixing time in rounds,
where and is the diameter of the graph. We
further show that local mixing time tightly characterizes the complexity of
partial information spreading.Comment: 16 page
The Cost of Global Broadcast in Dynamic Radio Networks
We study the single-message broadcast problem in dynamic radio networks. We
show that the time complexity of the problem depends on the amount of stability
and connectivity of the dynamic network topology and on the adaptiveness of the
adversary providing the dynamic topology. More formally, we model communication
using the standard graph-based radio network model. To model the dynamic
network, we use a generalization of the synchronous dynamic graph model
introduced in [Kuhn et al., STOC 2010]. For integer parameters and
, we call a dynamic graph -interval -connected if for every
interval of consecutive rounds, there exists a -vertex-connected stable
subgraph. Further, for an integer parameter , we say that the
adversary providing the dynamic network is -oblivious if for constructing
the graph of some round , the adversary has access to all the randomness
(and states) of the algorithm up to round .
As our main result, we show that for any , any , and any
, for a -oblivious adversary, there is a distributed
algorithm to broadcast a single message in time
. We further show that even for large interval -connectivity,
efficient broadcast is not possible for the usual adaptive adversaries. For a
-oblivious adversary, we show that even for any (for any constant ) and for any , global broadcast in -interval -connected networks requires at least
time. Further, for a oblivious adversary,
broadcast cannot be solved in -interval -connected networks as long as
.Comment: 17 pages, conference version appeared in OPODIS 201
Run for Cover: Dominating Set via Mobile Agents
Research involving computing with mobile agents is a fast-growing field,
given the advancement of technology in automated systems, e.g., robots, drones,
self-driving cars, etc. Therefore, it is pressing to focus on solving classical
network problems using mobile agents. In this paper, we study one such problem
-- finding small dominating sets of a graph using mobile agents. Dominating
set is interesting in the field of mobile agents as it opens up a way for
solving various robotic problems, e.g., guarding, covering, facility location,
transport routing, etc. In this paper, we first present two algorithms for
computing a {\em minimal dominating set}: (i) an time algorithm if the
robots start from a single node (i.e., gathered initially), (ii) an
time algorithm, if the robots start from
multiple nodes (i.e., positioned arbitrarily), where is the number of edges
and is the maximum degree of , is the number of clusters of
the robot initially and is the maximum ID-length of the robots. Then
we present a approximation algorithm for the {\em minimum}
dominating set which takes rounds