\cortext[cor]{Corresponding author.}
\begin{abstract}
- Most of the time, asynchronous load balancing algorithms have extensively been
- studied in a theoretical point of view. The Bertsekas and Tsitsiklis'
+ Most of the time, asynchronous load balancing algorithms are extensively
+ studied from a theoretical point of view. The Bertsekas and Tsitsiklis'
algorithm~\cite
%[section~7.4]
- {bertsekas+tsitsiklis.1997.parallel} is undeniably
- the most well known algorithm for which the asymptotic convergence proof is given.
+ {bertsekas+tsitsiklis.1997.parallel} is undeniably the best known algorithm for which the asymptotic convergence proof is given.
From a
practical point of view, when a node needs to balance a part of its load to
some of its neighbors, the algorithm's description is unfortunately too succinct, and no details are given on what is really sent and how the load balancing decisions are taken. In this paper, we
of a node to all its less loaded neighbors while ensuring that all involved nodes by the load balancing phase have the same amount of load. Moreover, since
asynchronous iterative algorithms are less sensitive to communications delays
and their variations \cite{bcvc07:bc}, both load transfer and load information messages are dissociated.
- To speedup the convergence time of the load balancing process, we propose {\it a clairvoyant virtual load} heuristic which allows
- %asynchronous iterative algorithms, in which an asynchronous load balancing
- %algorithm is implemented, can dissociate, most of the time, messages concerning
- %load transfers and message concerning load information. In order to increase
- %the converge of a load balancing algorithm, we propose a simple heuristic
- %called \emph{virtual load}. This heuristic allows
- a node receiving a load
+ To speedup the convergence time of the load balancing process, we propose {\it a clairvoyant virtual load} heuristic. This heuristic allows a node receiving a load
information message to integrate the future virtual load (if any) in its load's list, even if the load has not been received yet. This leads to have predictive snapshots of nodes' loads at each iteration of the load balancing process. Consequently, the notified node sends a real part of its load to some of
- its neighbors taking into account the virtual load it will receive in the subsequent time-steps. Based on SimGrid simulator, series of test-bed scenarios are considered and many QoS metrics are evaluated to show the usefulness of the proposed algorithm. %In order to validate our approaches, we have defined a
+ its neighbors taking into account the virtual load it will receive in the subsequent time-steps. Based on the SimGrid simulator, some series of test-bed scenarios are considered and many QoS metrics are evaluated to show the usefulness of the proposed algorithm. %In order to validate our approaches, we have defined a
% simulator based on SimGrid which allowed us to conduct many experiments.
\end{abstract}
computing systems to wireless sensor networks (WSN).
The objective of load balancing is to orchestrate the distribution of the global load so that
the load difference between the computational resources of the network is
-minimized as low as possible. Unfortunately, this problem is known to be {\bf NP-hard} in its
-general forms and heuristics are required to achieve sub-optimal solutions but in
+minimized as much as possible. Unfortunately, this problem is known to be {\bf NP-hard} in its
+general form and heuristics are required to achieve sub-optimal solutions but in
polynomial time complexity.
In this paper, we focus on asynchronous load balancing of non negative real numbers of {\it divisible loads}
in homogeneous distributed systems. Loads can be divided in arbitrary {\it fine-grain} parallel parts size
that can be processed independently of each other. This model of divisible loads arises in
-a wide range of real-world applications. Common examples among many, include signal processing,
-feature extraction and edge detection in image processing, records search in a huge databases,
-average consensus in WSN, pattern search in Big data and so on. % c'est pout toi raphael ;-)
+a wide range of real-world applications. Common examples, among many, include signal processing,
+feature extraction and edge detection in image processing, records search in huge databases,
+average consensus in WSN, pattern search in Big data and so on.
In the literature, the problem of load balancing has been formulated and studied in various ways. The first pioneering work is due to Bertsekas and Tsitsiklis~\cite{bertsekas+tsitsiklis.1997.parallel}. Under some specific hypothesis and {\it ping-pong} awareness conditions (see section~\ref{sec.bt-algo} for more details), an asymptotic convergence proof is derived.
+
+%%RAPH Attention cette partie n'apparait plus
\begin{comment}
This algorithm has been borrowed and adapted in many works. For instance, in~\cite{CortesRCSL02} a static load balancing (called DASUD) for non negative integer number of divisible loads in arbitrary networks topologies is investigated. The term {\it "static"} stems from the fact that no loads are added or consumed during the load balancing process. The theoretical correctness proofs of the convergence property are given. Some generalizations of the same authors' own work for partially asynchronous discrete load balancing model are presented in~\cite{cedo+cortes+ripoll+al.2007.convergence}. The authors prove that the algorithm's convergence is finite and bounded by the straightforward network's diameter of the global equilibrium threshold in the network. In~\cite{bahi+giersch+makhoul.2008.scalable}, a fault tolerant communication version is addressed to deal with average consensus in wireless sensor networks. The objective is to have all nodes converged to the average of their initial measurements based only on nodes' local information. A slight adaptation is also considered in~\cite{BahiCG10} for dynamic networks with bounded delays asynchronous diffusion. The dynamical aspect stands at the communication level as links between the network's resources may be intermittent.
\end{comment}
-%in order to reduce the execution times. They can be applied in
-%different scientific fields from high performance computation to micro sensor
-%networks. In a distributed context (i.e. without centralization), they are iterative by nature.
-%In literature many kinds of load
-%balancing algorithms have been studied. They can be classified according
-%different criteria: centralized or decentralized, in static or dynamic
-%environment, with homogeneous or heterogeneous load, using synchronous or
-%asynchronous iterations, with a static topology or a dynamic one which evolves
-%during time. In this work, we focus on asynchronous load balancing algorithms
-%where computing nodes are considered homogeneous and with homogeneous load with
-%no external load.
-%In this context, Bertsekas and Tsitsiklis have proposed an
-%algorithm which is definitively a reference for many works. In their work, they
-%proved that under classical hypotheses of asynchronous iterative algorithms and
-%a special constraint avoiding \emph{ping-pong} effect, an asynchronous
-%iterative algorithm converges to the uniform load distribution. This work has
-%been extended by many authors. For example, Cortés et al., with
-%DASUD~\cite{cortes+ripoll+cedo+al.2002.asynchronous}, propose a
-%version working with integer load. This work was later generalized by
-%the same authors in \cite{cedo+cortes+ripoll+al.2007.convergence}.
-%\FIXME{Rajouter des choses ici. Lesquelles ?}
+
Although Bertsekas and Tsitsiklis describe the necessary conditions to
-ensure the algorithm's convergence, there is no indication or any strategy to really implement
+ensure the algorithm's convergence, there is no indication nor any strategy to really implement
the load distribution. %In other word, a node can send some amount of its load to one or many of its neighbors while all the convergence conditions are followed.
Consequently, we propose a new strategy called \besteffort{}
that tries to balance the load of a node to all its less loaded neighbors while
amount of load. Moreover, %when real-world asynchronous applications are considered,
%using asynchronous load balancing algorithms can reduce the execution
%times.
-most of the times, it is simpler to dissociate load information messages
+most of the time, it is simpler to dissociate load information messages
from data migration messages. Former ones allow a node to inform its
-neighbors about its current load. These messages are in fact very small and can be sent
-often very quickly. For example, if a computing iteration takes a significant time
+neighbors about its current load. These messages are in fact very small and can often be sent
+very quickly. For example, if a computing iteration takes a significant time
(ranging from seconds to minutes), it is possible to send a new load information
-message to each involved neighbor at each iteration. Then, the load is sent, but the reception may take time when the amount of load is huge and when communication links are slow. Depending on the application, it may have
-sense or not that nodes try to balance a part of their load at each computing
+message to each involved neighbor at each iteration. Then, the load is sent, but the reception may take time when the amount of load is huge and when communication links are slow. Depending on the application, it may make sense or not for the nodes to try to balance a part of their load at each computing
iteration. But the time to transfer a load message from a node to another one is
-often much more longer that the time to transfer a load information message. So,
+often much longer than the time to transfer a load information message. So,
when a node is notified
%receives the information
that later it will receive a data message,
\begin{itemize}
\item We propose a {\it best effort strategy} which proceeds greedily to achieve efficient local neighborhoods equilibrium. Upon local load imbalance detection, a {\it significant amount} of load is moved from a highly loaded node (initiator) to less loaded neighbors.
-\item Unlike earlier works, we use a new concept of virtual loads transfers which allows nodes to predict the future loads they will receive in the subsequent iterations.
+\item Unlike earlier works, we use a new concept of virtual loads transfer which allows nodes to predict the future loads they will receive in the subsequent iterations.
This leads to a noticeable speedup of the global convergence time of the load balancing process.
\item We use SimGrid simulator which is known to be able to characterize and modelize realistic models of computation and communication in different types of platforms. We show that taking into account both loads transfers' costs and network contention is essential and has a real impact on the quality of the load balancing performances.
-%\item We improve the straightforward network's diameter bound of the global equilibrium threshold in the network. % not sure, it depends on the remaining time before the paper submission ...
\end{itemize}
-%{\bf The contributions of this paper are the following:}
-%\begin{itemize}
-%\item We propose a new strategy to improve the distribution of the
-%load and a simple but efficient trick that also improves the load
-%balancing.
-%\item we have conducted many simulations with SimGrid in order to
-%validate that our improvements are really efficient. Our simulations consider
-%that in order to send a message, a latency delays the sending and according to
-%the network performance and the message size, the time of the reception of the
-%message also varies.
-%\end{itemize}
-
-The reminder of the paper is organized as follows.
-In Section~\ref{sec.related.works}, we review the relevant approaches in the literature. Section~\ref{sec.bt-algo} describes the
-Bertsekas and Tsitsiklis' asynchronous load balancing algorithm. %Moreover, we present a possible problem in the convergence conditions.
+
+The reminder of the paper is organized as follows. Section~\ref{sec.related.works} offers a review of the relevant approaches in the literature. Section~\ref{sec.bt-algo} describes the
+Bertsekas and Tsitsiklis' asynchronous load balancing algorithm.
Section~\ref{sec.besteffort} presents the best effort strategy which provides
-efficient local loads equilibrium. %This strategy will be compared with the one presented in Section~\ref{sec.other}.
+efficient local loads equilibrium.
In Section~\ref{sec.virtual-load}, the clairvoyant virtual load scheme is proposed to speedup the convergence time of the load balancing process.
-We provide in Section~\ref{sec.simulations}, a comprehensive set of numerical results that exhibit the usefulness of our proposal when we deal with realistic models of computation and communication. Finally, we give some concluding remarks in Section~\ref{conclusions-remarks}.
+In Section~\ref{sec.simulations}, a comprehensive set of numerical results that exhibit the usefulness of our proposal when dealing with realistic models of computation and communication is provided. Finally, some concluding remarks are made in Section~\ref{conclusions-remarks}.
\section{Related works}
\label{sec.related.works}
-In this section, we fairly review the relevant techniques proposed in the literature to tackle the problem of load balancing in a general context of distributed systems.
+In this section, the relevant techniques proposed in the literature to tackle the problem of load balancing in a general context of distributed systems are reviewed.
-As pointed above, the most interesting approach to this issue has been proposed by Bertsekas and Tsitsiklis~\cite{bertsekas+tsitsiklis.1997.parallel}. This algorithm which is outlined in Section~\ref{sec.bt-algo} for the sake of comparison, has been borrowed and adapted in many works. For instance, in~\cite{CortesRCSL02} a static load balancing (called DASUD) for non negative integer number of divisible loads in arbitrary networks topologies is investigated. The term {\it "static"} stems from the fact that no loads are added or consumed during the load balancing process. The theoretical correctness proofs of the convergence property are given. Some generalizations of the same authors' own work for partially asynchronous discrete load balancing model are presented in~\cite{cedo+cortes+ripoll+al.2007.convergence}. The authors prove that the algorithm's convergence is finite and bounded by the straightforward network's diameter of the global equilibrium threshold in the network. In~\cite{bahi+giersch+makhoul.2008.scalable}, a fault tolerant communication version is addressed to deal with average consensus in wireless sensor networks. The objective is to have all nodes converged to the average of their initial measurements based only on nodes' local information. A slight adaptation is also considered in~\cite{BahiCG10} for dynamic networks with bounded delays asynchronous diffusion. The dynamical aspect stands at the communication level as links between the network's resources may be intermittent.
+As pointed above, the most interesting approach to this issue has been proposed by Bertsekas and Tsitsiklis~\cite{bertsekas+tsitsiklis.1997.parallel}. This algorithm which is outlined in Section~\ref{sec.bt-algo} for the sake of comparison, has been borrowed and adapted in many works. For instance, in~\cite{CortesRCSL02} a static load balancing (called DASUD) for non negative integer number of divisible loads in arbitrary networks topologies is investigated. The term {\it "static"} stems from the fact that no loads are added or consumed during the load balancing process. The theoretical correctness proofs of the convergence property are given. Some generalizations of the same authors' own work for partially asynchronous discrete load balancing model are presented in~\cite{cedo+cortes+ripoll+al.2007.convergence}. The authors prove that the algorithm's convergence is finite and bounded by the straightforward network's diameter of the global equilibrium threshold in the network. In~\cite{bahi+giersch+makhoul.2008.scalable}, a fault tolerant communication version is addressed to deal with average consensus in wireless sensor networks. The objective is to have all nodes converging to the average of their initial measurements based only on nodes' local information. A slight adaptation is also considered in~\cite{BahiCG10} for dynamic networks with bounded delays asynchronous diffusion. The dynamical aspect stands at the communication level as links between the network's resources may be intermittent.
-Cybenko~\cite{Cybenko89} propose a {\it diffusion} approach for hypercube multiprocessor networks.
+Cybenko~\cite{Cybenko89} proposes a {\it diffusion} approach for hypercube multiprocessor networks.
The author targets both static and dynamic random models of work distribution.
The convergence proof is derived based on the {\it eigenstructure} of the
iteration matrices that arise in load balancing of equal amount of
-computational works. A static load balancing for both synchronous and asynchronous ring networks is addressed in~\cite{GehrkePR99}. The authors assume that at any time step, at most one token (units of load) can be transmitted along any edge of the ring and no tokens are created during the balancing phase. They show that for every initial token distribution, the proposed algorithm converges to the stable equilibrium with tighter linear bounds of time step-complexity.
+computational works. A static load balancing for both synchronous and asynchronous ring networks is addressed in~\cite{GehrkePR99}. The authors assume that at any time step, one token at the most (units of load) can be transmitted along any edge of the ring and no tokens are created during the balancing phase. They show that for every initial token distribution, the proposed algorithm converges to the stable equilibrium with tighter linear bounds of time step-complexity.
In order to achieve the load balancing of cloud data centers, a LB technique based on Bayes theorem and Clustering is proposed in~\cite{zhao2016heuristic}. The main idea of this approach is that, the Bayes theorem is combined with the clustering process to obtain the optimal clustering set of physical target hosts leading to the overall load balancing equilibrium. Bidding is a market-technique for task scheduling and load balancing in distributed systems
-that characterize a set of negotiation rules for users' jobs. For instance, Izakian et al~\cite{IzakianAL10} formulate a double auction mechanism for tasks-resources matching in grid computing environments where resources are considered as provider agents and users as consumer ones. Each entity participates in the network independently and makes autonomous decisions. A provider agent determines its bid price based on its current workload, and each consumer agent defines its bid value based on two main parameters: average remaining time and remaining resources for bidding. Based on JADE simulator, the proposed algorithm exhibits better performances in terms of successful execution rates, resource utilization rates and fair profit allocation.
+that characterize a set of negotiation rules for users' jobs. For instance, Izakian et al~\cite{IzakianAL10} formulate a double auction mechanism for tasks-resources matching in grid computing environments where resources are considered as provider agents and users as consumer ones. Each entity participates in the network independently and makes autonomous decisions. A provider agent determines its bid price based on its current workload and each consumer agent defines its bid value based on two main parameters: average remaining time and remaining resources for bidding. Based on JADE simulator, the proposed algorithm exhibits better performances in terms of successful execution rates, resource utilization rates and fair profit allocation.
Choi et al.~\cite{ChoiBH09} address the problem of robust task allocation in arbitrary networks. The proposed
-approaches combine bidding approach for task selection and consensus procedure scheme for
+approaches combine a bidding approach for task selection and a consensus procedure scheme for
decentralized conflict resolution. The developed algorithms are proven to converge to a conflict-free assignment in
-both single and multiple task assignment problem. An online stochastic dual gradient LB algorithm which is called DGLB is proposed in~\cite{chen2017dglb}. The authors deal with both workload and energy management for cloud networks consisting of multiple geo-distributed mapping nodes and data Centers. To enable online distributed implementation, tasks are decomposed both across time and space by leveraging a dual decomposition approach. Experimental results corroborate the merits of the proposed algorithm.
+both single and multiple task assignment problems. An online stochastic dual gradient LB algorithm, which is called DGLB, is proposed in~\cite{chen2017dglb}. The authors deal with both workload and energy management for cloud networks consisting of multiple geo-distributed mapping nodes and data Centers. To enable online distributed implementation, tasks are decomposed both across time and space by leveraging a dual decomposition approach. Experimental results corroborate the merits of the proposed algorithm.
-In~\cite{tripathi2017non} a LB algorithm based on game theory is proposed for distributed data centers. The authors formulate the LB problem as a non-cooperative game among front-end proxy servers and characterize the structure of Nash equilibrium. Based on the obtained Nash equilibrium structure, they derive a LB algorithm for computing the Nash equilibrium. They show through simulations that the proposed algorithm ensures fairness among the users and good average latency across all client regions. A hybrid task scheduling and load balancing dependent and independent tasks for master-slaves platforms is addressed in~In~\cite{liu2017dems}. To minimize the response time of the submitted jobs, the proposed algorithm which is called DeMS is splitted in three stages: i) communication overhead reduction between masters and slaves, ii) task migration to keep the workload balanced iii) and precedence task graphs partitioning.
+In~\cite{tripathi2017non} a LB algorithm based on game theory is proposed for distributed data centers. The authors formulate the LB problem as a non-cooperative game among front-end proxy servers and characterize the structure of Nash equilibrium. Based on the obtained Nash equilibrium structure, they derive a LB algorithm to compute the Nash equilibrium. They show through simulations that the proposed algorithm ensures fairness among the users and a good average latency across all client regions. A hybrid task scheduling and load balancing dependent and independent tasks for master-slaves platforms are addressed in~In~\cite{liu2017dems}. To minimize the response time of the submitted jobs, the proposed algorithm which is called DeMS is split into three stages: i) communication overhead reduction between masters and slaves, ii) task migration to keep the workload balanced iii) and precedence task graphs partitioning.
-Several LB techniques based on artificial intelligence have been also proposed in the literature: genetic algorithm (GA) \cite{subrata2007artificial}, honey bee behavior \cite{krishna2013honey, kwok2004new}, tabu search \cite{subrata2007artificial} and fuzzy logic \cite{salimi2014task}. The main strength of these techniques comes from their ability to seek in large search spaces, which arises in many combinatorial optimization problems. For instance, the works in~\cite{cao2005grid, shen2014achieving} have been proposed to tackle the load balancing problem using the multiagent approach where each agent is responsible for load balancing for a subset of nodes in the network. The agent objective is to minimize jobs' response time and host idle time dynamically. In~\cite{GrosuC05}, the authors formulate the load balancing problem as a non-cooperative game among users. They use the Nash equilibrium as the solution of this game to optimize the response time of all jobs in the entire system. The proposed scheme guarantees the optimal task allocation for each user with low time complexity. A game theoretic approach to tackle the static load balancing problem is also investigated in~\cite{PenmatsaC11}. To provide fairness to all users in the system, the load balancing problem is formulated as a non-cooperative game among the users to minimize the response time of the submitted users' jobs. As in~\cite{GrosuC05}, the authors use the concept of Nash equilibrium as the solution of a non-cooperative game. Simulations results show that the proposed scheme perform near optimal solutions compared to other existing techniques in terms of fairness.
+Several LB techniques, based on artificial intelligence, have also been proposed in the literature: genetic algorithm (GA) \cite{subrata2007artificial}, honey bee behavior \cite{krishna2013honey, kwok2004new}, tabu search \cite{subrata2007artificial} and fuzzy logic \cite{salimi2014task}. The main strength of these techniques comes from their ability to seek in large search spaces, which arises in many combinatorial optimization problems. For instance, the works in~\cite{cao2005grid, shen2014achieving} have been proposed to tackle the load balancing problem using the multiagent approach where each agent is responsible for load balancing for a subset of nodes in the network. The agent objective is to minimize jobs' response time and host idle time dynamically. In~\cite{GrosuC05}, the authors formulate the load balancing problem as a non-cooperative game among users. They use the Nash equilibrium as the solution of this game to optimize the response time of all jobs in the entire system. The proposed scheme guarantees the optimal task allocation for each user with low time complexity. A game theoretic approach to tackle the static load balancing problem is also investigated in~\cite{PenmatsaC11}. To provide fairness to all users in the system, the load balancing problem is formulated as a non-cooperative game among the users to minimize the response time of the submitted users' jobs. As in~\cite{GrosuC05}, the authors use the concept of Nash equilibrium as the solution of a non-cooperative game. Simulation results show that the proposed scheme offers near optimal solutions compared to other existing techniques in terms of fairness.
\label{sec.bt-algo}
In this section, we present a brief description of Bertsekas and Tsitsiklis' algorithm~\cite{bertsekas+tsitsiklis.1997.parallel} using its original notations.
-A network is modeled as a connected undirected graph $G=(N,A)$, where $N$ is set
+A network is modeled as a connected undirected graph $G=(N,A)$, where $N$ is a set
of processors and $A$ is a set of communication links. The processors are
labeled $i = 1,...,n$, and a link between processors $i$ and
-$j$ is denoted by $(i, j)\in A$. The set of neighbors of processor $i$ is denoted by $V(i)$.
+$j$ is denoted by $(i, j)\in A$. The set of processor $i$'s neighbors is denoted by $V(i)$.
%In this work, we consider that
%Processors are considered to be homogeneous for the sake of simplicity. It is easily extendable to the case of heterogeneous platforms by scaling the processor's load by its computing power~\cite{ElsMonPre02}.
%In order prove the convergence of asynchronous iterative load balancing
%the processor's load is represented by a continuous variable for notational
%convenience.
\medskip
-When a processor sends a part of its load to one or some of its neighbors, the
+When a processor sends a part of its load to one or to some of its neighbors, the
transfer takes time to be completed. Let $s_{ij}(t)$ be the amount of load that
processor $i$ has transferred to processor $j$ at time $t$ and let $r_{ij}(t)$ be the
-amount of load received by $j$ from $i$ at time $t$. Then
+amount of loads received by $j$ from $i$ at time $t$. Then
the amount of load of processor $i$ at time $t+1$ is given by:
\begin{equation}
%less loaded after that.
\medskip
-This condition prohibits the possibility that two nodes keep sending load to each
+This condition prohibits the possibility that two nodes keep sending loads to each
other back and forth, without reaching equilibrium.
\medskip
Nevertheless, we think that this condition may lead to deadlocks in some
-cases. For example, consider a linear chain graph network of only three processors and that processor $1$
-is linked to processor $2$ which is also linked to processor $3$, but processors $1$ and $3$ are not neighbors.
+cases. For example, consider a linear chain graph network of only three processors in which processor $1$
+is linked to processor $2$ which is also linked to processor $3$, but in which processors $1$ and $3$ are not neighbors.
%(i.e. a simple chain which 3 processors).
\noindent Now consider that we have the following load values at time~$t$:
\end{align*}
%{\bf RAPH, pourquoi il y a $x_3^2$?. Sinon il faudra reformuler la suite, c'est mal dit}
-Owing to the algorithm's specification, processor $2$ can either send
-load to processor $1$ or processor
-$3$. If it sends load to processor $1$, it will not satisfy condition
+Owing to the algorithm's specifications, processor $2$ can either send
+loads to processor $1$ or processor
+$3$. If it sends loads to processor $1$, it will not satisfy condition
\eqref{eq.ping-pong} because after that sending it will be less loaded than
$x_3^2(t)$. So we consider that the \emph{ping-pong} condition is probably too
strong. %Currently, we did not try to make another convergence proof without this condition or with a weaker condition.
questions arise when dealing with realistic models of
computation and communication. As reported above, the
algorithm's description is too succinct and no details are
-given on what is really sent and how the load balancing decision
+given on what is really sent and how the load balancing decisions
are taken. To our knowledge, the only first attempt for a possible
implementation of this algorithm is investigated in~\cite{bahi+giersch+makhoul.2008.scalable} under the same conditions. Thus, in order to assess the performances
-of the new \besteffort{}, we naturally chose to compare it to this anterior
+of the new \besteffort{}, we naturally chose to compare it to this previous
work. More precisely, we will use the algorithm~2 from
-\cite{bahi+giersch+makhoul.2008.scalable} and, through out the paper, we will
+\cite{bahi+giersch+makhoul.2008.scalable} and, throughout the paper, we will
reference it under the original name {\it Bertsekas and Tsitsiklis} for the sake of convenience and readability.
\smallskip
\subsection{Basic strategy}
The description of our algorithm will be given from the point of view a processor~$i$.
The principle of the \besteffort{} strategy is that each processor
-that detects it has more load than some of its neighbors, sends some load to the
-most of its less loaded neighbors, doing its best to reach the equilibrium
+detecting itself to be more loaded than some of its neighbors, sends some load to its less loaded neighbors, doing its best to reach the equilibrium
between the involved neighbors and itself.
More precisely, %when a processor $i$ is in its load-balancing phase,
With the aforementioned basic strategy, each node does its best to reach the
equilibrium with its neighbors. However, one question should be outlined here:
-How can we handle the case where two (or more) node initiators that may send
-concurrently some amount of load to the the same less loaded neighbor? Indeed,
+how can we handle the case where two (or more) node initiators might concurrently send
+ some loads to the same least loaded neighbor? Indeed,
%since each node may take the same kind of decision at the same time,
-there is a risk that a node will receive load from
-several of its neighbors, and then is temporary going off the equilibrium state.
+there is a risk that a node will receive loads from
+several of its neighbors, and then might temporary go off the equilibrium state.
This is particularly true with strongly connected applications.
-In order to reduce this effect, we add the ability to level the amount of load to send.
+In order to reduce this effect, we add the ability to level the amount of loads to send.
The idea, here, is to make as few steps as possible toward the equilibrium, such that a
potentially unsuitable decision pointed above has a lower impact on the local equilibrium.
Roughly speaking, once $s_{ij}$ is estimated as previously explained, it is simply weighted by
\subsubsection{Load-balancing thread}
The load-balancing thread is in charge of running the load-balancing algorithm,
-and exchange the control messages. As shown in Algorithm~\ref{algo.lb}, it
+and exchanging the control messages. As shown in Algorithm~\ref{algo.lb}, it
iteratively runs the following operations:
\begin{itemize}
\item get the control messages that were received from the neighbors;
\item run the load-balancing algorithm;
\item send control messages to the neighbors, to inform them about the
processor's current load, and possibly the future virtual load transfers;
-\item wait a minimum (configurable) amount of time, to avoid to
- iterate too fast.
+\item wait a minimum (configurable) amount of time, to avoid iterating too fast.
\end{itemize}
\begin{algorithm}
\begin{description}
\item[\textbf{strategies:}] \makhoul{}, or \besteffort{} with $k\in
\{1,2,4\}$
-\item[\textbf{variants:}] with, or without virtual load
+\item[\textbf{variants:}] with, or without virtual loads
%\item[\textbf{domain:}] real load, or integer load
\end{description}
%
The simulations were run until reaching the global equilibrium threshold.
%the load was nearly balanced among the participating nodes.
-More precisely the simulation stops when each node holds
-an amount of load at least less than 1\% of the load average.
+More precisely, the simulation stops when each node holds
+an amount of load at least inferior to 1\% of the load average.
%, during an arbitrary
%number of computing iterations (2000 in our case).
%of 125~MB/s, with a latency of 50~$\mu$s. The backbone has a network bandwidth
%of 2.25~GB/s, with a latency of 500~$\mu$s.
-In order to make our experiments, an heterogeneous grid platform descriptions were created by taking a subset of the
+In order to make our experiments, an heterogeneous grid platform description were created by taking a subset of the
Grid'5000 infrastructure\footnote{Grid'5000 is a French large scale experimental
Grid (see \url{https://www.grid5000.fr/}).}, as described in the platform file
\texttt{g5k.xml} distributed with SimGrid. Note that the heterogeneity of the
platform here only comes from the network topology. Indeed,
processors are considered to be homogeneous for the sake of simplicity.
-It is easily extendable to the case of heterogeneous platforms
+However, this situation is easily extendable to the case of heterogeneous platforms
by scaling the processor's load by its computing power~\cite{ElsMonPre02}.
%since our
%algorithms currently do not handle heterogeneous computing resources,
The
-processor speeds were normalized, and we arbitrarily chose to fix them to
+processor speeds were normalized, and we arbitrarily chose to fix them at
1~GFlop/s. Each type of platform with four different numbers of computing
nodes: 16, 64, 256, and 1024 nodes is built in a similar way.
\subsubsection{Configurations}
The distributed processes of the application were then logically organized along
-three possible typologies: a line, a torus or an hypercube. Tests were divided into two groups on the basis of the initial distribution of the global load: i)
-tests were performed with the total load initially on only one node%(at one end for the line topology)
-, ii) and other tests for which the load was initially randomly distributed across all the
-participating nodes of the platform. The total amount of load was fixed to a number of load
-units equal to 1000 times the number of node. The average load is then of 1000
+three possible typologies: a line, a torus or an hypercube. Tests were divided into two groups on the basis of the initial distribution of the global load: i) some tests were performed with the total load initially on only one node, ii) and other tests were performed for which the load was initially randomly distributed across all the
+participating nodes of the platform. The total amount of loads was fixed to a number of load
+units equal to 1,000 times the number of node. The average load is then of 1,000
load units.
For all the previous configurations, the
-computation and communication costs of a load unit are defined. We chose them, such as to
+computation and communication costs of a load unit are defined. They were chosen so as to
have two different computation to communication ratios (CCR), and hence characterize
two different types of applications:
\begin{itemize}
In order to evaluate and compare the different load balancing strategies, several metrics were considered. Our goal, when choosing these metrics, is to have
something tending to a constant value, i.e. to have a measure which is not
-changing anymore once the convergence state is reached. Moreover, we want to
-have some normalized value, in order to be able to compare them across different
-settings. With these constraints in mind, we define the following metrics:
+changing anymore once the convergence state is reached. Moreover, the goal is to
+have some normalized values, in order to be able to compare them across different
+settings. With these constraints in mind, the following metrics are defined:
%
\begin{description}
\item[\it{average idle time:}] that is the total time spent, when the nodes
column, and $1/10$ on the right column. %With a computation/communication cost
%ratio of $1/1$ the results are just between these two extrema, and definitely
%don not give additional information, so we chose not to show them here.
-On each of Figures~\ref{fig.results1} and~\ref{fig.resultsN}, the results
+On each Figure, ~\ref{fig.results1} and~\ref{fig.resultsN}, the results
are given for the process topology being, from top to bottom, a line, a torus or
an hypercube.
We can see that the relative performance of these strategies is mainly
influenced by the application topology structure. It is for the line topology that the
-difference is the more important. In this case, the \besteffort{} strategy is
-readily faster than the \makhoul{} strategy. This can be explained by the
+difference is the most important. In this case, the \besteffort{} strategy is
+really faster than the \makhoul{} strategy. This can be explained by the
fact that the \besteffort{} strategy tries to distribute the load fairly between
-all the nodes and are in a good agreement with the line topology since it is easy
+all the nodes and is in a good agreement with the line topology since it is easy
to load balance the load efficiently.
In contrast, for the hypercube topology, the \besteffort{}' performances are lower than
-the \makhoul{} strategy. In this case, the \makhoul{} strategy which
-tries to give more load to few neighbors reaches the equilibrium faster.
+the \makhoul{} strategy. In this case, the \makhoul{} strategy, which
+tries to give more load to few neighbors, reaches the equilibrium faster.
For the torus topology, for which the number of links is between the line and
the hypercube, the \makhoul{} strategy is slightly better but the difference is
more nuanced when the initial load is only on one node. The only case where the
\makhoul{} strategy is really faster than the \besteffort{} strategy is with the
-random initial distribution when the communication are slow.
+random initial distribution when communications are slow.
Generally speaking, the number of interconnection is very important. Indeed, the more
-the interconnection links are, the faster the \makhoul{} strategy is because
-it distributes quickly significant amount of load, even if this is unfair, between
-all the neighbors. However, the \besteffort{} strategy distributes the
-load fairly when needed and is is better for sparse connected applications.
+numerous the interconnection links are, the faster the \makhoul{} strategy is because
+it distributes quickly significant amount of loads, even if the distribution may be unfair, between
+all neighbors. However, the \besteffort{} strategy distributes the
+load fairly when needed and is better for sparse connected applications.
\smallskip
When the load to balance is initially randomly distributed over all nodes, we can see from Figure \ref{fig.resultsN} that the effect of virtual load is not significant for the line topology structure. However, for both torus and hypercube structures with CCR = 1/10 (on the left of the figure), the performance of virtual load transfers is significantly better. This is explained by the fact
-that for small CCR values, high communication costs plays quite a significant role. Moreover, the impact of
-communication becomes less important as the CCR values increases, since larger CCR values result in smaller communication times. We also tested the impact of CCR values on the performance of each algorithm in terms of idle times. From Figures~\ref{fig.results1} and ~\ref{fig.resultsN} we can find that our virtual load scheme achieves
-a really good average idle times, which is quite close to both its own simple version and its direct competitor {\it Bertsekas and Tsitsiklis} algorithm. As expected, for coarse grain applications (CCR =10/1), idle times are close to 0 since processors are inactive the most of times compared to fine grain applications.
+that for small CCR values, high communication costs play quite a significant role. Moreover, the impact of
+communication becomes less important as the CCR values increase, since larger CCR values result in smaller communication times. The impact of CCR values were also tested on the performance of each algorithm in terms of idle times. From Figures~\ref{fig.results1} and ~\ref{fig.resultsN} virtual load scheme can be seen to achieve really good average idle times, which is quite close to both its own simple version and its direct competitor {\it Bertsekas and Tsitsiklis} algorithm. As expected, for coarse grain applications (CCR =10/1), idle times are close to 0 since processors are inactive most of the time compared to fine grain applications.
\smallskip
Taken as a whole, the results illustrated in Figures~\ref{fig.results1} and ~\ref{fig.resultsN} clearly show that our proposal outperforms the Betsekas and Tsistlikis algorithm.
-These results indicate that local load balancing decisions has a significant impact on the global
+These results indicate that local load balancing decisions have a significant impact on the global
convergence time achieved by the compared strategies. This is because, upon load imbalance detection, assigning an amount of load in an unfair way between neighbors will severely increase the total number of iterations required by the algorithm before reaching the final stable distributions. The reason of the poorer performance of {\it Bertsekas and tsistsilikis} algorithm can be explained by the inconvenience of the iterative load balance policy adopted for load distribution between neighbors. Neighbors are selected in such a way that the {\it ping-pong} condition holds. Doing so, loads are not really assigned to processor neighbors which would allow them to be fairly balanced.
\smallskip
-Unlike {\it Betsekas and Tsistlikis} algorithm, our approach is not really sensitive when
-we deal with realistic models of computation and communication. This is due to two main features: i) the use of "virtual load" transfers which allows nodes to predict the load they receive in the subsequent iterations steps, ii) and the greedy neighbors selection adopted by our algorithm at each time step in the load balancing process. The involved neighbors are selected in such a way that load difference between the computational resources is minimized as low as possible.
+Unlike the {\it Betsekas and Tsistlikis} algorithm, our approach is not really sensitive when dealing with realistic models of computation and communication. This is due to two main features: i) the use of "virtual load" transfers which allows nodes to predict the load they receive in the subsequent iterations steps, ii) and the greedy neighbors selection adopted by our algorithm at each time step in the load balancing process. The involved neighbors are selected in such a way that the load difference between the computational resources is minimized as much as possible.
\smallskip
-Comparing the results of the extended version (with virtual load) to the results of the simple one, we observe in Figs.~\ref{fig.results1} and ~\ref{fig.resultsN} that the improved version gives the best performances. It always improves both convergence and idle times significantly in all figures. This is because, with virtual load transfers, the algorithm seeks greedily to ensure a certain degree of load balancing for processors by taking into account the information about the predictive loads not received yet. Consequently, this leads to optimize the final convergence time of the load balancing process. Similarly, the extended version achieves much better results than the simple one when considering larger platforms, as shown in Figs.~\ref{fig.results1} and ~\ref{fig.resultsN}.
+Comparing the results of the extended version (with virtual load) to the results of the simple one, it can be observed in Figs.~\ref{fig.results1} and ~\ref{fig.resultsN} that the improved version gives the best performances. It always improves both convergence and idle times significantly in all figures. This is because, with virtual load transfers, the algorithm seeks greedily to ensure a certain degree of load balancing for processors by taking into account the information about the predictive loads not received yet. Consequently, this leads to optimizing the final convergence time of the load balancing process. Similarly, the extended version achieves much better results than the simple one when considering larger platforms, as shown in Figs.~\ref{fig.results1} and ~\ref{fig.resultsN}.
\smallskip
-We also find in Figs.~\ref{fig.results1} and ~\ref{fig.resultsN} that the performance difference between the improved version of our proposal and its simple version (without virtual load) increases when the CCR increases. This interesting result comes from the fact that larger CCR values reveals that we are dealing with intensive computations applications in grid platforms. Thus, in order to reduce the convergence time of the load balancing for such applications, it is important to take suitable decisions upon local load imbalance detection. That is why we added {\it virtual load} transfers scheme to the {\it best effort} strategy to perfectly balance the load of processors at each step of the load balancing process.
+We also find in Figs.~\ref{fig.results1} and ~\ref{fig.resultsN} that the performance difference between the improved version of our proposal and its simple version (without virtual load) increases when the CCR increases. This interesting result comes from the fact that larger CCR values reveal that we are dealing with intensive computations applications in grid platforms. Thus, in order to reduce the convergence time of the load balancing for such applications, it is important to take suitable decisions upon local load imbalance detection. That is why we added {\it virtual load} transfers scheme to the {\it best effort} strategy to perfectly balance the load of processors at each step of the load balancing process.
\smallskip
-Finally, it is worthwhile noting from Figures~\ref{fig.results1} and ~\ref{fig.resultsN}, that the algorithm's convergence time increases together with network's size. We also see that the idle time increases together with the size of the network when a load is initially on a single node (Figure~\ref{fig.results1}),
-as expected. In addition, it is interesting to note that when the number of nodes increases, there is not substantial difference in the increase of the convergence time, compared to the simple version without virtual load. This is explained by the fact that the increase in the convergence time is already absorbed by the virtual load transfers between processors being in line with the network's size.
-
-%For the hypercube, in any case, the effect of the virtual load is visible. It is more visible when communications have a more important role (i.e. with the mainly communicating case).
-
-
-%Dans ce cas légère amélioration de la cvg. max. Temps moyen de cvg. amélioré,
-%mais plus de temps passé en idle, surtout quand les comms coutent cher.
-
-%\subsubsection{The \besteffort{} strategy with an initial random load
-% distribution, and larger platforms}
+Finally, it is worthwhile noting from Figures~\ref{fig.results1} and ~\ref{fig.resultsN}, that the algorithm's convergence time increases together with the size of the network. We also see that the idle time increases together with the size of the network when a load is initially on a single node (Figure~\ref{fig.results1}),
+as expected. In addition, it is interesting to note that when the number of nodes increases, there is no substantial difference in the increase of the convergence time, compared to the simple version without virtual load. This is explained by the fact that the increase in the convergence time is already absorbed by the virtual load transfers between processors being in line with the network's size.
-%In
-%Mêmes conclusions pour line et hcube.
-%Sur tore, BE se fait exploser quand les comms coutent cher.
-%\FIXME{virer les 1024 ?}
-
-%\subsubsection{With the virtual load extension with an initial random load
-% distribution}
-
-%Soit c'est équivalent, soit on gagne -> surtout quand les comms coutent cher et
-%qu'il y a beaucoup de voisins.
\subsubsection{The $k$ parameter}
\label{results-k}
fairly and consequently a significant amount of the load is transferred between
processors. In this case, it is possible to reduce the convergence time by
using the leveler parameter (parameter $k$). The advantage of using this
-solution is particularly efficient when the initial load is randomly distributed
+solution is particularly true when the initial load is randomly distributed
on the nodes with torus and hypercube topologies and slow communication. When
-virtual load scheme is used, the effect of this parameter is also perceptible
-with the same conditions. %However, sometimes this parameter may have a negative effect on the convergence time.
+a virtual load scheme is used, the effect of this parameter is also perceptible
+in the same conditions.
\subsubsection{With non negative integer load values}
-In addition to the first tests devoted to the case of non negative real load values, we also carried out further experiments with integer load values to assess the performance of our proposal.
-%We also performed some experiments with integer load instead of load with real
-%number values.
-As expected,
-%In this case,
-the obtained results have globally the same behavior, that is why we decided not to show similar figures. The most
+In addition to the first tests devoted to the case of non negative real load values, further experiments were also carried with integer load values to assess the performance of our proposal.
+As expected, the obtained results globally have the same behavior, that is why we decided not to show similar figures. The most
interesting result, from our point of view, is that the virtual mode allows
processors in a line topology to converge to the uniform load balancing state. Without
-the virtual load, most of the time, processors converge to what we call the
+the virtual load, most of the time, processors converge to what is called the
``stairway effect'', that is to say that there is only a difference of at most one unit load between any pairs of neighbor nodes, i.e. the load difference between each processor and its neighbors is within one unit load (for example with 10 processors, we
obtain 10 9 8 7 6 6 7 8 9 10 instead of 8 8 8 8 8 8 8 8 8 8).
\smallskip
-To summarize and broadly speaking, the simulation results led us to show that, with few exceptions (without virtual load), our proposal is superior to the {\it Bertsekas and Tsiltsikis} algorithm in all the tested scenarios. The illustrated results indicate that network size, CCR values and initial load distribution have a significant impact on the algorithm's performances. Thus, this experimental study corroborate the usefulness of our algorithm, and confirms that when dealing with realistic model platforms, both {\it best effort} strategy and {\it virtual load} transfers play an important role on the achieved idle and convergence times.
-
-
-%Cas normal, ligne -> converge pas (effet d'escalier).
-%Avec vload, ça converge.
-
-%Dans les autres cas, résultats similaires au cas réel: redire que vload est
-%intéressant.
-
-%\FIXME{ajouter une courbe avec l'équilibrage en entier}
-
-%\FIXME{virer la metrique volume de comms}
-
-%\FIXME{ajouter une courbe ou on voit l'évolution de la charge en fonction du temps : avec et sans vload}
-
-% \begin{itemize}
-% \item cluster ou grid, entier ou réel, ne font pas de grosses différences
-% \item bookkeeping? améliore souvent les choses, parfois au prix d'un retard au démarrage
-% \item makhoul? se fait battre sur les grosses plateformes
-% \item taille de plateforme?
-% \item ratio comp/comm?
-% \item option $k$? peut-être intéressant sur des plateformes fortement interconnectées (hypercube)
-% \item volume de comm? souvent, besteffort/plain en fait plus. pourquoi?
-% \item répartition initiale de la charge ?
-% \item integer mode sur topo. line n'a jamais fini en plain? vérifier si ce n'est
-% pas à cause de l'effet d'escalier que bk est capable de gommer.
-% \end{itemize}}
-
-% On veut montrer quoi ? :
-
-% 1) best plus rapide que les autres (simple, makhoul)
-% 2) avantage virtual load
-
-% Est ce qu'on peut trouver des contre exemple?
-% Topologies variées
-
-
-% Simulation avec temps définies assez long et on mesure la qualité avec : volume de calcul effectué, volume de données échangées
-% Mais aussi simulation avec temps court qui montre que seul best converge
-
-% Expés avec ratio calcul/comm rapide et lent
-
-% Quelques expés avec charge initiale aléatoire plutot que sur le premier proc
-
-% Cadre processeurs homogènes
-
-% Topologies statiques
-
-% On ne tient pas compte de la vitesse des liens donc on la considère homogène
+To summarize the simulation results led us to show that, with a few exceptions (without virtual load), our proposal is superior to the {\it Bertsekas and Tsiltsikis} algorithm in all the tested scenarios. The illustrated results indicate that network size, CCR values and initial load distribution have a significant impact on the algorithm's performances. Thus, this experimental study corroborates the usefulness of our algorithm, and confirms that when dealing with realistic model platforms, both {\it best effort} strategy and {\it virtual load} transfers play an important role on the achieved idle and convergence times.
-% Prendre un réseau hétérogène et rendre processeur homogène
-% Taille : 10 100 très gros
\section{Conclusion}
\label{conclusions-remarks}
-In this paper, we have presented a new asynchronous load balancing algorithm for non negative real numbers
-of divisible loads in distributed systems. The proposed algorithm which is called {\it best effort strategy}
+In this paper, a new asynchronous load balancing algorithm for non negative real numbers
+of divisible loads in distributed systems was presented. The proposed algorithm which is called {\it best effort strategy}
seeks greedily for loads imbalance detection and tries to achieve efficient local load equilibrium
between neighbors. Our proposal is based on {\it a clairvoyant virtual loads' transfer} scheme which allows nodes to predict the future loads they will receive in the subsequent iterations.
This leads to a noticeable speedup of the global convergence time of the load balancing process.
-Based on SimGrid simulator, we have demonstrated that, when we deal with realistic models of computation and communication, our algorithm exhibits better performances than its direct competitor from the literature. This makes it a viable choice for load balancing of both non negative real and integer divisible loads in distributed computing systems. % un peu gonflé peut être pour la dernière phrase.
+Based on SimGrid simulator, we have demonstrated that, when dealing with realistic models of computation and communication, our algorithm exhibits better performances than its direct competitor from the literature. This makes it a viable choice for load balancing of both non negative real and integer divisible loads in distributed computing systems. % un peu gonflé peut être pour la dernière phrase.
\section*{Acknowledgments}
