Message-Dropout: An Efficient Training Method for Multi-Agent DeepReinforcement Learning

Woojun Kim and Myungsik Cho and Youngchul SungSchool of Electrical Engineering, KAIST, Koreawoojun.kim, ms.cho, ycsung@kaist.ac.kr

Abstract

In this paper, we propose a new learning technique namedmessage-dropout to improve the performance for multi-agentdeep reinforcement learning under two application scenar-ios: 1) classical multi-agent reinforcement learning with di-rect message communication among agents and 2) central-ized training with decentralized execution. In the first appli-cation scenario of multi-agent systems in which direct mes-sage communication among agents is allowed, the message-dropout technique drops out the received messages fromother agents in a block-wise manner with a certain proba-bility in the training phase and compensates for this effectby multiplying the weights of the dropped-out block unitswith a correction probability. The applied message-dropouttechnique effectively handles the increased input dimensionin multi-agent reinforcement learning with communicationand makes learning robust against communication errors inthe execution phase. In the second application scenario ofcentralized training with decentralized execution, we par-ticularly consider the application of the proposed message-dropout to Multi-Agent Deep Deterministic Policy Gradient(MADDPG), which uses a centralized critic to train a de-centralized actor for each agent. We evaluate the proposedmessage-dropout technique for several games, and numeri-cal results show that the proposed message-dropout techniquewith proper dropout rate improves the reinforcement learningperformance significantly in terms of the training speed andthe steady-state performance in the execution phase.

1 IntroductionMulti-Agent Deep Reinforcement Learning (MADRL) isgaining increasing attention from the research communitywith the recent success of deep learning because many ofpractical decision-making problems such as connected self-driving cars and collaborative drone navigation are modeledas multi-agent systems requiring action control. There aremainly two approaches in MADRL: one is centralized con-trol and the other is decentralized control. The centralizedcontrol approach assumes that there exists a central con-troller which determines the actions of all agents based on allthe observations of all agents. That is, the central controllerhas a policy which maps the joint observation to a joint ac-tion. Since the action is based on the joint observation, this

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approach eases the problem of lack of full observability ofthe global state in partially observable environments (Gold-man and Zilberstein 2004). However, this approach has theproblem of the curse of dimensionality because the state-action space grows exponentially as the number of agentsincreases (Busoniu, Babuska, and De Schutter 2010). More-over, exploration, which is essential in RL, becomes moredifficult than the single-agent RL case due to the huge state-action space. Hence, to simplify the problem, the decentral-ized control approach was considered. In fully decentralizedmulti-agent control, each agent decides its action based onlyon own observation, while treating other agents as a partof the environment, to reduce the curse of dimensionality.However, this approach eliminates the cooperative benefitfrom the presence of other agents and suffers from perfor-mance degradation.

In order to improve the performance of the decentral-ized control, several methods have been studied. First, multi-agent systems with decentralized control with communica-tion (DCC) was studied (Goldman and Zilberstein 2004). Inthe framework of MADRL with DCC, the agents communi-cate with each other by sending messages both in the train-ing and execution phases, and the policy of each agent pa-rameterized by a deep neural network determines the actionof the agent based on own observation and the received mes-sages from other agents. To incorporate the messages fromother agents, the size of the deep neural network of eachagent should be increased as the number of message-passingagents increases. However, if the network size becomes toolarge, training becomes difficult and may fail. Another ap-proach is centralized learning with decentralized execution,allowing each agent to use the information of other agentsonly in the training phase. In particular, recently-introducedMADDPG (Lowe and Mordatch 2017), which uses a cen-tralized critic to train a decentralized policy for each agent,belongs to this second category. In MADDPG, the central-ized critic takes all of other agents’ observations and actionsas input and hence the input space of each critic exponen-tially grows with the number of agents. In both approaches,as the number of agents in the system increases, the inputdimension increases, learning becomes difficult, and muchdata is required for training. Hence, it is an important prob-lem to properly handle the increased input dimension anddevise an efficient learning algorithm for such MADRL with

information exchange.In this paper, motivated from dropout (Srivastava et al.

2014), we propose a new training method, named message-dropout, yielding efficient learning for MADRL with infor-mation exchange with large input dimensions. The proposedmethod improves learning performance when it is appliedto MADRL with information exchange. Furthermore, whenit is applied to the scenario of DCC, the proposed methodmakes learning robust against communication errors in theexecution phase.

2 Background2.1 A Partially Observable Stochastic GameIn MADRL, multiple agents learn how to act to maximizetheir future rewards while sequentially interacting with theenvironment. The procedure can be described by a PartiallyObservable Stochastic Game (POSG) defined by the tuple< I,S, Ai, Ωi, T ,O, Ri >, where I is the set ofagents 1, · · · , N, Ai is the action space of agent i, andΩi is the observation space of agent i. At each time step t,the environment has a global state st ∈ S and agent i ob-serves its local observation oit ∈ Ωi, which is determinedby the observation probability O : S ×

−→A ×

−→Ω → [0, 1],

where−→A =

∏Ni=1Ai and

−→Ω =

∏Ni=1 Ωi are the joint

action space and the joint observation space, respectively.Agent i executes action ait ∈ Ai, which yields the nextglobal state st+1 with the state transition probability T :

S ×−→A × S → [0, 1], receives the reward rit according to

the reward function Ri : S ×−→A → R, and obtains the next

observation oit+1. The discounted return for agent i is de-fined as Rit =

∑∞t′=t γ

t′rit′ where γ ∈ [0, 1] is the discount-ing factor. In POSG, the Q-function Qi(s, ai) of agent i canbe approximated by Qi(τ i, ai), where τ i is the joint action-observation history of agent i. However, learning the action-value function based on the action-observation history is dif-ficult. In this paper, we simply approximate Qi(s, ai) withQi(oi, ai) = E[Rit|oit = oi, ait = ai]. Note that a recur-rent neural network can be used to minimize the approxi-mation error between Qi(s, ai) and Qi(oi, ai) (Hausknechtand Stone 2015). The goal of each agent is to maximize itsexpected return, which is equivalent to maximizing its Q-function. (Note that in this paragraph, we explained the fullydecentralized case.)

2.2 Independent Q-LearningIndependent Q-Learning (IQL), which is one of the populardecentralized multi-agent RL algorithms in the fully observ-able case (Tan 1993), is a simple extension of Q-learningto multi-agent setting. Each agent estimates its own opti-mal Q-function, Q∗(s, a) = argmaxπQ

π(s, a), which sat-isfies the Bellman optimality equation Q∗(s, a) = E[r +γmaxa′Q∗(s′, a′)|s, a]. Under the assumption of full ob-servability at each agent and fully decentralized control,Tampuu et al. combined IQL with deep Q-network (DQN),and proposed that each agent trains its Q-function parame-terized by a neural network θi by minimizing the loss func-

tion (Tampuu et al. 2017)

L(θi) = E(s,ai,r,s′)∼Di[(yi −Qi(s, ai; θi))2

](1)

where Di is the replay memory and yi = r +γmaxaiQ(s′, ai; θi−) is the target Q-value for agent i. Here,θi− is the target network for agent i.

In the case of POSG with fully decentralized control, theabove loss function can be modified to

L(θi) = E(oi,ai,ri,(oi)′)∼Di[(yi −Qi(oi, ai; θi))2

](2)

where yi = ri+γmaxaiQ(oi, ai; θi−). Here,Qi(s, ai; θi) inthe fully observable case is approximated withQi(oi, ai; θi)with the local partial observation oi, as described in Section2.1.

2.3 Multi-Agent DDPGAs an extension of DDPG to multi-agent setting, MADDPGwas proposed to use a decentralized policy with a central-ized critic for each agent (Lowe and Mordatch 2017). Thecentralized critic uses additional information about the poli-cies of other agents, and this helps learn the policy effec-tively in the training phase. The centralized critic for agenti is represented by Qiµ(x,−→a ; θiQ) parameterized by a neu-ral network θiQ, where µ = µ1, ..., µN is the collection ofall agents’ deterministic policies, x = (o1, · · · , oN ), and−→a = (a1, · · · , aN ). Each agent trains its Q-function byminimizing the loss function

L(θiQ) = Ex,a,x′∼D[(yi −Qiµ(x,−→a ; θiQ))2

]where yi = ri + γQiµ′ (x′,

−→a′ ; θi−Q )|a′j=µ′j(o′j), x′ =

(o′1, · · · , o′N ), and D is the replay memory. Here, µ′ =

µ′1, · · · , µ′N is the set of target policies and θi−Q is theparameter of the target Q network for agent i.

Then, the policy for agent i, which is parameterized by θiµ,is trained by deterministic policy gradient to maximize theobjective J(θiµ) = E [Ri], and the gradient of the objectiveis given by

∇θiµJ(µi) = Ex,a∼D

[∇θiµµ

i(oi)∇aiQiµ(x,−→a ))|ai=µi(oi)]

2.4 DropoutDropout is a successful neural network technique. For agiven neural network, constituting units (or nodes) in theneural network are randomly dropped out with probabilityp independently in the training phase to avoid co-adaptationamong the units in the neural network (Srivastava et al.2014). In the test phase, on the other hand, all the units areincluded but the outgoing weights of those units affected bydropout are multiplied by 1 − p. There is a good interpre-tation of dropout: efficient model averaging over randomlygenerated neural networks in training. For a neural networkwith N units, dropout samples the network to be trainedfrom 2N differently-thinned networks which share the pa-rameter in the training phase due to independent droppingout of the N units. Scaling the weights of the units at thetest phase can be regarded as averaging over the ensemble of

(a) (b)

(c) (d)

Figure 1: N = 3: All candidates for agent 1’s Q-network

subnetworks. It is shown that in the particular case of a sin-gle logistic function, dropout corresponds to geometric aver-aging (Baldi and Sadowski 2013). Thus, dropout efficientlycombines the exponentially-many different neural networksas one network and can also be regarded as a kind of ensem-ble learning (Hara, Saitoh, and Shouno 2016).

3 Methodology3.1 Decentralized Control with CommunicationIn this subsection, we consider the direct communicationframework in (Goldman and Zilberstein 2004) for DCC.This framework can be modeled by adding messages toPOSG. Each agent exchanges messages before action, andchooses action based on its own observation and the receivedmessages. Hence, the policy of agent i is given by πi : Ωi ×Mi × Ai → [0, 1], whereMi =

∏j 6=iM

ji andMj

i is thespace of received messages from agent j to agent i. We de-note agent i’s received message from agent j asmi,j ∈Mj

i ,and mi = (mi,1, · · · ,mi,i−1,mi,i+1, · · · ,mi,N ) is the col-lection of the received messages at agent i. For the learningengine at each agent, we use the Double Deep Q-Network(DDQN) which alleviates the overestimation problem of Q-learning (Van Hasselt, Guez, and Silver 2016). In this case,the Q-function of agent i parameterized by the neural net-work θi is given byQi(oi,mi, ai; θi) = E[Ri|oi,mi, ai; θi].Then, the Q-function of agent i is updated by minimizing theloss function

L(θi) = E(oi,mi,ai,ri,o′i,m′i)∼Di[(yi −Qi(oi,mi, ai; θi))2

](3)

where yi = ri + γQi(o′i,m′

i, argmaxa′Q

i(o′i,m′

i, a′; θi); θi−).

We will refer to this scheme as simple DCC.The problem of simple DCC is that the input dimension of

theQ neural network at each agent linearly increases and thesize of the input state space increases exponentially, as the

number of agents increases. Thus, the number of requiredsamples for learning increases, and this decreases the speedof learning. Another issue of simple DCC is that the portionof each agent’s own observation space in the input space ofthe Q-function decreases as the number of agents increases.Typically, the own observation of each agent is most impor-tant. Hence, the importance of each agent’s own observationis not properly weighted in simple DCC.

3.2 Message-DropoutTo address the aforementioned issues of simple DCC, wepropose a new neural network technique, named message-dropout, which can be applied to decentralized control withcommunication of messages.

Message-dropout drops out the received other agents’messages at the input of the Q-network at each agent in-dependently in the training phase. That is, all units cor-responding to the message from one dropped other agentare dropped out simultaneously in a blockwise fashion withprobability p and this blockwise dropout is independentlyperformed across all input unit blocks corresponding to allother agents’ messages. On the other hand, the outgoingweights of those input units on which dropout is applied aremultiplied by 1 − p, when the policy generates actual ac-tion. Note that dropout is not applied to the input units cor-responding to each agent’s own observation. To illustrate,let us consider agent 1 in an environment with total threeagents, as shown in Fig. 1. The Q-network of agent 1 hasthree input blocks: one for own observation and two for themessages from two other agents. By applying the proposedmessage dropout, as shown in Fig. 1, we have the four pos-sible configurations for the input of the Q-network of agent1:

(o1,m1,2,m1,3), (o1,m1,2,−→03), (o1,

−→02,m

1,3), (o1,−→02,−→03)

where−→0j ∈ R|M

ji | with all zero elements represents the

dropped-out input units.Now, we explain the Q-learning process for DCC with

message-dropout (DCC-MD). Consider the learning at agenti. Agent i stores the transition (oi,mi, ai, ri, o′

i,m′

i) into

its replay memory Di. To train the Q-function, agent i sam-ples a random mini-batch of transitions from Di, denoted(oij ,mi

j , aij , r

ij , o

ij+1,m

ij+1), j ∈ J i. Message-dropout is

performed independently for each j ∈ J i and the message-dropout performed observation and its transition are givenby

mji = (bj,1m

i,1j , · · · , bj,i−1mi,i−1

j , bj,i+1mi,i+1j , · · · , bj,Nmi,N

j )

mij+1 = (bj,1m

i,1j+1, · · · , bj,i−1m

i,i−1j+1 , bj,i+1m

i,i+1j+1 , · · · , bj,Nm

i,Nj+1)

where the scalar value bj,k ∼ Bernoulli(p). Note that thesame binary mask is used to define mi

j and mij+1. Then,

the Q-function is updated by minimizing the loss

L(θi) = E(oij ,mij ,a

ij ,r

ij ,o

ij+1,m

ij+1)∼Di

[(yij −Qi(oij , mj

−i, aij ; θi))2](4)

where yij = rij + γQi(oij+1, ˜mj+1i, argmaxaiQ

iexec(o

ij+1,m

ij+1, a

i; θi); θi−)

Here, Qiexec(oi,mi, ai; θi) is the Q-function parameterized

by the neural network whose outgoing weights of mi are

Algorithm 1 DCC with Message-Dropout (DCC-MD)

Initialize θ1, · · · , θN and θ−1, · · · , θ−N .

for episode = 1, 2, · · · doInitialize state s1.for t < T and st 6= terminal do

Each agent i observes oit and sends mit

for each agent i = 1, 2, · · · , N doReceive messages mi

t.With probability ε select a random action aitotherwise select the action ait from (5).

end forExecute −→a and each agent i receives rit and oit+1.Each agent sends the message to other agents.for each agent i = 1, 2, · · · , N do

Store transition (oit,mit, a

it, r

it, o

it+1,m

it+1) in Di

Sample (oij ,mij , a

ij , r

ij , o

ij+1,m

ij+1), j ∈ J i

from DiGenerate a binary mask and obtain mi

j and mij+1

Update θi by minimizing the loss function (4)end forUpdate the target network

end forend for

multiplied by 1 − p. Note that we use Qiexec to predict thenext action when evaluating the target Q-value yij . Finally,the policy πi is given by

πi = argmaxaiQiexec(o

i,mi, ai; θi). (5)

Interpretation In the training phase with message-dropout, agent i drops out some of other agents’ messagesin mi, while keeping own observation. As a result, the in-put space of the Q-network of agent i is projected onto adifferent subspace (of the original full input space) that al-ways includes own observation space at each training timesince the dropout masks change at each training time. Theinput spaces of the four Q-networks in the example of Fig.1 are Ω1 × M2

1 × M31, Ω1 × M2

1, Ω1 × M31, Ω1 (all

include agent’s own observation Ωi). In the general caseof N agents, message-dropout samples the network to betrained from 2N−1 differently-thinned networks which al-ways include the agent’s own observation. Note that the Q-network whose all received messages are retained is the Q-network of simple DCC and the Q-network whose all obser-vations from other agents are dropped is the Q-network forfully decentralized DDQN without communication. Thus,2N−1 differently-thinned networks include the Q-networksof simple DCC and fully decentralized DDQN. Message-dropping out in the training phase and multiplying the out-going weights of mi by 1 − p in the test phase yields anapproximate averaging over the ensemble of those networks.Note that simple DCC and fully decentralized DDQN are thespecial cases of the dropout rate p = 0 and p = 1, respec-tively. Thus, for 0 < p < 1, the proposed scheme realizessome network between these two extremes.

3.3 MADDPG-MDMessage-dropout can also be applied to the framework ofcentralized training with decentralized execution, particu-larly to the setting in which each agent uses additional infor-mation from other agents during training as the input of thenetwork. For example, in MADDPG, the centralized criticQiµ(x,−→a ) takes all agents’ observations and actions as in-put, and hence the input space of the centralized critic foreach agent increases exponentially as the number of agentsincreases. The proposed technique, message-dropout, is ap-plied to the training phase of the centralized critic to ad-dress the aforementioned problem. The centralized criticwith message-dropout applied is trained to minimize the lossfunction:

L(θiQ) = Ex,a,x′∼D[(yi −Qiµ(x,−→a ; θiQ))2

], (6)

where yi = ri + γQiµ′ (x′,−→a′ ; θi−Q )|a′j=µ′j(o′j)

and x = (oi, o−i). Note that o−i =(b1o

1, · · · , bi−1oi−1, bi+1oi+1, · · · , bNoN ), where bi

∼ Bernoulli(p), and the same binary mask is used to obtainx and x′ as in DCC-MD. Then, the policy for agent i istrained by maximizing the objective J(θiµ) = E [Ri], andthe gradient of the objective can be written as

∇θiµJ(µi) = Ex,a∼D

[∇θiµµ

i(oi)∇aiQiµ(x,−→a ))|ai=µi(oi)].

We refer to MADDPG with message-dropout applied asMADDPG-MD.

4 ExperimentIn this section, we provide some numerical results to evalu-ate the proposed algorithm in the aforementioned two sce-narios for MADRL with information exchange. First, wecompare DCC-MD with simple DCC and Fully Decentral-ized Control (FDC) in two environments of pursuit and co-operative navigation. Second, we compare MADDPG-MDwith MADDPG and independent DDPG (simply DDPG) inthe environment of waterworld. Then, we provide in-depthablation studies to understand the behavior of message-dropout depending on various parameters. Finally, we in-vestigate DCC-MD in unstable environments in which somelinks of communication between agents can be broken in theexecution phase

Although some compression may be applied, for simplic-ity we here assume that each agent’s message is its observa-tion itself, which is shown to be optimal when the commu-nication cost is ignored in the framework of DCC (Goldmanand Zilberstein 2004). Hence, Mj

i = Ωj for all agent i,and the policy function becomes πi :

−→Ω × Ai → [0, 1],

where−→Ω = (Ω1, · · · ,ΩN ). (A brief study on message-

dropout with message generation based on auto-encoder ap-plied to raw observation is given in the supplementary file ofthis paper. It is seen that similar performance improvementis achieved by message-dropout in the case of compressedmessage. Hence, message-dropout can be applied on top ofmessage compression for MADRL with message communi-cation.)

(a) (b) (c)

Figure 2: Considered environments: (a) Pursuit, (b) Cooper-ative navigation, and (c) Waterworld

4.1 EnvironmentPursuit The pursuit game is a standard task for multi-agent systems (Vidal et al. 2002). The environment is madeup of a two-dimensional grid and consists ofN pursuers andM evaders. The goal of the game is to capture all evadersas fast as possible by training the agents (i.e., pursuers). Ini-tially, all the evaders are at the center of the two-dimensionalgrid, and each evader randomly and independently choosesone of five actions at each time step: move North, East,West, South, or Stay. (Each evader stays if there exists a pur-suer or a map boundary at the position where it is going tomove.) Each pursuer is initially located at a random posi-tion of the map and has five possible actions: move North,East, West, South or Stay. When the four sides of an evaderare surrounded by pursuers or map boundaries, the evaderis removed and the pursuers who capture the evader receiveR+ reward. All pursuers receive −R−1 reward for each timestep and −R−2 reward if the pursuer hits the map bound-ary (the latter negative reward is to promote exploration).An episode ends when all the evaders are captured or Ttime steps elapse. As in (Gupta, Egorov, and Kochenderfer2017), each pursuer observes its surrounding which consistsof map boundary, evader(s), or other pursuer(s). We assumethat each pursuer can observe up to D distances in four di-rections. Then, the observed information of each pursuercan be represent by a 3 × (2D + 1) × (2D + 1) obser-vation window (which is the observation of each agent): a(2D + 1) × (2D + 1) window detecting other pursuer(s),a (2D + 1) × (2D + 1) window detecting evader(s), anda (2D + 1) × (2D + 1) window detecting the map bound-ary. For the game of pursuit, we set R+ = 5, R−1 = 0.05,R−2 = 0.5, T = 500, M = 2 and D = 3 and simulate twocases: N = 6 and N = 8. The map size of the two cases are15× 15 and 17× 17 respectively.

Cooperative navigation Cooperative navigation, whichwas introduced in (Mordatch and Abbeel 2017), consists ofN agents and L landmarks. The goal of this environmentis to occupy all of the landmarks while avoiding collisionsamong agents. The observation of each agent is the con-catenation of its position and velocity, the locations of land-marks, and the locations of other agents. Since we considerpartially observable setting, we assume that each agent ob-serves the locations of other agents only if the distance isless than D. Each agent receives a negative reward −R−1 asthe negative of the minimun of the distances from the agentto the L landmarks and receives a negative reward −R−2 if

the collision among agents occurs. In this environment, weset R−2 = 2, and simulate two cases: N = 8, L = 8 andN = 10, L = 10.

Waterworld Waterworld is an extended environment ofpursuit to a continuous domain (Gupta, Egorov, and Kochen-derfer 2017). The environment is made up of a two-dimensional space and consists of N pursuers and M foodtargets, L poison targets, and one obstacle. The goal of theenvironment is to capture as many food targets as possiblewithin a given episode of T time steps while avoiding poi-son targets. In order to make the game more cooperative,at least K agents need to cooperate to catch a food target.Each agent takes two-dimensional physical actions to the en-vironment and has observation which consists of its positionand information from 25 range-limited sensors of the agent.The sensors of each agent are used to offer the distancesand velocities of other agents, food targets, and poison tar-gets. The agents receive a reward R+

1 when they capture afood target and are penalized by getting reward −R−1 whenthey encounter a poison target. To promote exploration, areward R+

2 is given to an agent if the agent touches a foodtarget. They also receive an action penalty reward definedas the square norm of the action. In this environment, weset R+

1 = 10, R+2 = 0.01, R−1 = 0.1, and T = 500, and

simulate two cases: N = 8,K = 4 and N = 10,K = 5.The three environments are briefly illustrated in Fig. 2.

4.2 Model ArchitectureInstead of using the concatenation of the agent’s own ob-servation and the received messages as the input of the Q-function, we use an architecture for the Q-function that em-phasizes the agent’s own observation. The proposed neuralnetwork architecture for the Q-function for agent i is repre-sented by

Qi(oi, ai,mi; θi) = h(f(oi, ai), g(mi)) (7)

where f and g are the neural networks that extracts fea-tures of own observation and the received messages, re-spectively, and h is the neural network that produces theexpected return by using the output of f and g. In MAD-DPG, we replace mi with the concatenation of o−i anda−i, where o−i = (o1, · · · , oi−1, oi+1, · · · , oN ) and a−i =(a1, · · · , ai−1, ai+1, · · · , aN ). Note that f , g and h are de-pendent on the task since the input and action dimensions ofeach task are different. The detailed structures of f , g and hare explained in supplementary material in a separate file.

4.3 ResultPursuit For the pursuit game, we compared DCC-MDwith simple DCC and FDC by varying the dropout rate asp = [0, 0.1, 0.2, 0.3, 0.4, 0.5, 1.0]. Note that DCC-MD withp = 0 corresponds to simple DCC, whereas DCC-MD withp = 1 corresponds to FDC. The performance of each algo-rithm was measured by the number of evader catches in 500time steps after training. Fig. 3a and Fig. 3b show the num-ber of evader catches (in 500 time steps after training) aver-aged over 1000 episodes and 7 random seeds, with respectto the drop rate. It is seen that the performance improves as

(a) N = 6 (b) N = 8

0 2000 4000 6000 8000 10000time step

10

9

8

7

6

5

sum

of r

ewar

ds

DCC(=MD-0)MD-0.2MD-0.5FDC(=MD-1)

(c) N = 8

0 2000 4000 6000 8000 10000time step

13

12

11

10

9

8

7

sum

of r

ewar

ds

DCC(=MD-0)MD-0.2MD-0.5FDC(=MD-1)

(d) N = 10

0 50 100 150 200 250 300 350Episode(1e2)

0

20

40

60

80

100

120

140

160

The

num

ber o

f foo

d ta

rget

cat

ches

Vanilla MADDPGMADDPGDDPGMADDPG-MD0.5MADDPG-MD0.2

(e) N = 8

0 50 100 150 200Episode(1e2)

0

20

40

60

80

100

120

140

The

num

ber o

f foo

d ta

rget

cat

ches

Vanilla MADDPGMADDPGDDPGMADDPG-MD0.5MADDPG-MD0.2

(f) N = 10

Figure 3: Results - (a)/(b): Pursuit - performance after training as a function of the dropout rate, (c)/(d): cooperative navigation- learning curve, and (e)/(f): waterworld - learning curve)

the dropout rate increases initially and then the performancedeteriorates as the dropout rate further increases after a cer-tain point. In the considered tasks, the best dropout rate isaround [0.2, 0.3]. It is seen that DCC-MD with proper droprate significantly outperforms both simple DCC and FDC.Note that in the case that the number of agents is N = 8,simple DCC has even worse performance than FDC. This isbecause simple DCC does not learn properly due to the largestate space for large N .

Cooperative navigation In this environment, we com-pared DCC-MD with the dropout rate 0.2 and 0.5 with sim-ple DCC and FDC. Figs. 3c and 3d show the learning curvesfor two cases (N = 8, L = 8) and (N = 10, L = 10),respectively. The y-axis is the sum of all agents’ rewards av-eraged over 7 random seeds, and the x-axis is time step. Itis seen that DCC-MD with the dropout rate 0.2 and 0.5 out-performs simple DCC and FDC.

Waterworld In the waterworld environment, we now con-sidered (independent) DDPG, vanilla MADDPG, MAD-DPG, and MADDPG-MD with the drop rate 0.2 and 0.5, andcompared their performances. Here, MADDPG is the modi-fied version of vanilla MADDPG to which the proposed net-work architecture described in Section 4.2 is applied. Figs.3e and 3f show the learning curves of the four algorithmsfor two cases (N = 8,K = 4) and (N = 10,K = 5), re-spectively. The y-axis is the number of food target catchesaveraged over random seeds, and the x-axis is time step. Itis seen that MADDPG-MD outperforms both (independent)DDPG and MADDPG. Note that fully decentralized (inde-pendent) DDPG has the fastest learning speed at the initialstage due to its small input dimension, but its performancedegrades as time step goes because of no cooperation. It isnoteworthy that MADDPG-MD almost achieves the initiallearning speed of (independent) DDPG while it yields farbetter performance at the steady state.

4.4 Ablation Studies

With the verification of the performance gain of themessage-dropout technique, we performed in-depth ablationstudies regarding the technique with respect to the four keyaspects of the technique: 1) the dropout rate, 2) block-wisedropout versus element-wise dropout, 3) retaining agent’sown observation without dropout, and 4) the model archi-tecture.

Dropout rate As mentioned in Section 2, we can viewthat message-dropout generates an ensemble of 2N−1

differently-thinned networks and averages these thinned net-works. From this perspective, the dropout rate determinesthe distribution of the thinned networks. For example, allthe 2N−1 networks are uniformly used to train the ensem-ble Q-network if the dropout rate is 0.5. As the dropout rateincreases, the overall input space shrinks in effect and theportion of the own observation space becomes large in theoverall input space, since we apply message-dropout only tothe message inputs from other agents. Hence, it is expectedthat the learning speed increases especially for large N asthe dropout rate increases. Figs. 4a and 4b show the learn-ing performance of the algorithms in the training phase. Thex-axis is the current time step, and the y-axis is the num-ber of evader catches in 500 time steps. As expected, it isseen that the learning speed increases as the dropout rate in-creases. This behavior is clearly seen in Fig. 4b, where thenumber of agents isN = 8. Note that message-dropout withproper drop rate achieves gain in both the learning speedand the steady-state performance. Figs. 3a and 3b show thecorresponding performance in the execution phase after thetraining. It seems that the drop rate of 0.2 to 0.5 yields sim-ilar performance and the performance is not so sensitive tothe drop rate within this range.

Block-wise dropout versus element-wise dropout Wecompared message-dropout with standard-dropout whichdrops the messages of other agents out in an element-wise manner while retaining each agent’s own observation.Fig. 5a shows that message-dropout yields better perfor-mance than the standard element-wise dropout. The differ-ence between message-dropout and standard-dropout is theprojected subspaces of the input space. Message-dropoutprojects the input space onto 2N−1 subspaces which alwaysinclude own observation space, whereas standard-dropoutprojects the input space onto 2(N−1)|M

ji | subspaces which

contain the projected subspaces by message-dropout.

Retaining agent’s own observation without dropoutWe compared message-dropout with full message-dropoutwhich applies message-dropout to each agent’s own ob-servation as well as the messages of other agents. Fig. 5ashows that the full message-dropout increases the trainingtime, similarly to the known fact that in general dropout in-creases the training time (Srivastava et al. 2014). Whereasfull message-dropout yields slower training than simple

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DCC, message-dropout makes training faster than simpleDCC. Note that standard elementwise dropout without drop-ping agent’s own observation also yields faster training thansimple DCC, but full standard elementwise dropout fails totrain. Hence, it is important to retain each agent’s own ob-servation without dropping out when dropout is applied toMADRL with information exchange.

Model architecture We used the neural network architec-ture of Q-function that is described in Section 4.2 for all en-vironments. In pursuit withN = 8 and waterworld, learningfailed with the simple model architecture that uses the con-catenation of each agent’s own observation and the receivedmessages as input. Hence, the proposed model architectureis advantageous when the input space of Q-function is large.Note that the proposed model architecture has more layersfor agent’s own observation than those for the received mes-sages from other agents as shown in Fig. 5b, and hence thefeature for more important agent’s own observation is wellextracted. An interested reader is referred to the supplemen-tary file.

4.5 Test in The Unstable EnvironmentUp to now, we have assumed that communication amongagents is stable without errors in both training and executionphases. Now, we consider the situation in which the com-munication is stable in the training phase but unstable in theactual execution phase. Such situations occur when the train-ing is done in a controlled environment but the execution is

Figure 6: (Half) : the performance of each method in the casewhere the half of connection between agents are broken inpursuit, (All) : The performance of each method in the casewhere all connection between agents are broken in pursuit

performed in an uncontrolled environment with real deploy-ment. We considered two communication-unstable cases:case 1 is the case that randomly chosen half of the connec-tions among agents are broken, and case 2 is the case thatall connections among agents is broken. When the commu-nication between two agents is broken, we use a zero vectorinstead of the message received from each other. Note thatthe performance of FDC does not change since it requiresno communication.

Fig. 6 shows the average number of evader catches in thetwo considered cases in pursuit. It is seen that DCC-MD out-performs both simple DCC and FDC when the communi-cation link is broken but not all links are broken. It meansthat message-dropout in the training phase makes the learn-ing robust against communication errors and can still out-perform FDC even with other agents’ messages coming lessfrequently. On the other hand, when the communication linkis too unstable, DCC-MD cannot recover this communica-tion loss (but still better than simple DCC), but FDC is bet-ter in this case. Hence, the message-dropout in the trainingphase can be useful in the situation in which communicationamong agents is erroneous with a certain probability in thereal execution phase.

5 Related WorkRecent works in MADRL focus on how to improve theperformance compared to FDC composed of independentlylearning agents. To harness the benefit from other agents,(Foerster et al. 2016) proposed DIAL, which learns the com-munication protocol between agents by passing gradientsfrom agent to agent. (Foerster et al. 2018) proposed themulti-agent actor-critic method called COMA, which uses acentralized critic to train decentralized actors and a counter-factual baseline to address the multi-agent credit assignmentproblem.

In most MADRL algorithms such as those mentionedabove, the input space of the network (policy, critic, etc.)grows exponentially with the number of agents. Thus, weexpect that message-dropout can be combined with thosealgorithms to yield better performance. To handle the in-

creased dimension in MADRL, (Yang et al. 2018) proposedmean field reinforcement learning in which the Q-function isfactorized by using the local action interaction and approx-imated by using the mean field theory. Whereas mean fieldreinforcement learning handles the action space within theinput space consisting of action and observation, message-dropout can handle not only the action space but also theobservation space.

6 ConclusionIn this paper, we have proposed the message-dropout tech-nique for MADRL. The proposed message-dropout tech-nique applied to DQN effectively handles the increasedinput dimension in MADRL with information exchange,where each agent uses the information of other agents totrain the policy. We have provided ablation studies on theperformance of message-dropout with respect to variousaspects of the technique. The studies show that message-dropout with proper dropout rates significantly improvesperformance in terms of the training speed and the steady-state performance. Furthermore, in the scenario of decentral-ized control with communication, message-dropout makeslearning robust against communication errors in the execu-tion phase. Although we assume that the communication be-tween agents is fully allowed, message-dropout can be ap-plied to the scenario in which communication between lim-ited pairs of agents is available too.

7 AcknowledgmentsThis work was supported in part by the ICT R&D pro-gram of MSIP/IITP (2016-0-00563, Research on AdaptiveMachine Learning Technology Development for IntelligentAutonomous Digital Companion) and in part by the Na-tional Research Foundation of Korea(NRF) grant funded bythe Korea government(Ministry of Science and ICT) (NRF-2017R1E1A1A03070788).

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