3,971 research outputs found
Bad Luck When Joining the Shortest Queue
A frequent observation in service systems with queues in parallel is that customers in other queues tend to be served faster than those in one’s own queue. This paper quantifies the probability that one’s service would have started earlier if one had joined another queue than the queue that was actually chosen, for exponential multiserver systems with queues in parallel in which customers join one of the shortest queues upon arrival and in which jockeying is not possible.Queueing;Join-the-shortest-queue;Probability of bad luck;Power-series algorithm;Overtaking customers;Dedicated customers
Improved Distributed Algorithms for Exact Shortest Paths
Computing shortest paths is one of the central problems in the theory of
distributed computing. For the last few years, substantial progress has been
made on the approximate single source shortest paths problem, culminating in an
algorithm of Becker et al. [DISC'17] which deterministically computes
-approximate shortest paths in time, where
is the hop-diameter of the graph. Up to logarithmic factors, this time
complexity is optimal, matching the lower bound of Elkin [STOC'04].
The question of exact shortest paths however saw no algorithmic progress for
decades, until the recent breakthrough of Elkin [STOC'17], which established a
sublinear-time algorithm for exact single source shortest paths on undirected
graphs. Shortly after, Huang et al. [FOCS'17] provided improved algorithms for
exact all pairs shortest paths problem on directed graphs.
In this paper, we present a new single-source shortest path algorithm with
complexity . For polylogarithmic , this improves
on Elkin's bound and gets closer to the
lower bound of Elkin [STOC'04]. For larger values of
, we present an improved variant of our algorithm which achieves complexity
, and
thus compares favorably with Elkin's bound of in essentially the entire range of parameters. This
algorithm provides also a qualitative improvement, because it works for the
more challenging case of directed graphs (i.e., graphs where the two directions
of an edge can have different weights), constituting the first sublinear-time
algorithm for directed graphs. Our algorithm also extends to the case of exact
-source shortest paths...Comment: 26 page
Steady-state analysis of shortest expected delay routing
We consider a queueing system consisting of two non-identical exponential
servers, where each server has its own dedicated queue and serves the customers
in that queue FCFS. Customers arrive according to a Poisson process and join
the queue promising the shortest expected delay, which is a natural and
near-optimal policy for systems with non-identical servers. This system can be
modeled as an inhomogeneous random walk in the quadrant. By stretching the
boundaries of the compensation approach we prove that the equilibrium
distribution of this random walk can be expressed as a series of product-forms
that can be determined recursively. The resulting series expression is directly
amenable for numerical calculations and it also provides insight in the
asymptotic behavior of the equilibrium probabilities as one of the state
coordinates tends to infinity.Comment: 41 pages, 13 figure
Fast emergency paths schema to overcome transient link failures in ospf routing
A reliable network infrastructure must be able to sustain traffic flows, even
when a failure occurs and changes the network topology. During the occurrence
of a failure, routing protocols, like OSPF, take from hundreds of milliseconds
to various seconds in order to converge. During this convergence period,
packets might traverse a longer path or even a loop. An even worse transient
behaviour is that packets are dropped even though destinations are reachable.
In this context, this paper describes a proactive fast rerouting approach,
named Fast Emergency Paths Schema (FEP-S), to overcome problems originating
from transient link failures in OSPF routing. Extensive experiments were done
using several network topologies with different dimensionality degrees. Results
show that the recovery paths, obtained by FEPS, are shorter than those from
other rerouting approaches and can improve the network reliability by reducing
the packet loss rate during the routing protocols convergence caused by a
failure.Comment: 18 page
Data Structures for Task-based Priority Scheduling
Many task-parallel applications can benefit from attempting to execute tasks
in a specific order, as for instance indicated by priorities associated with
the tasks. We present three lock-free data structures for priority scheduling
with different trade-offs on scalability and ordering guarantees. First we
propose a basic extension to work-stealing that provides good scalability, but
cannot provide any guarantees for task-ordering in-between threads. Next, we
present a centralized priority data structure based on -fifo queues, which
provides strong (but still relaxed with regard to a sequential specification)
guarantees. The parameter allows to dynamically configure the trade-off
between scalability and the required ordering guarantee. Third, and finally, we
combine both data structures into a hybrid, -priority data structure, which
provides scalability similar to the work-stealing based approach for larger
, while giving strong ordering guarantees for smaller . We argue for
using the hybrid data structure as the best compromise for generic,
priority-based task-scheduling.
We analyze the behavior and trade-offs of our data structures in the context
of a simple parallelization of Dijkstra's single-source shortest path
algorithm. Our theoretical analysis and simulations show that both the
centralized and the hybrid -priority based data structures can give strong
guarantees on the useful work performed by the parallel Dijkstra algorithm. We
support our results with experimental evidence on an 80-core Intel Xeon system
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