All the gradients in a custom DDPG agent class are zero!

Hi!

I'm creating a custom DDPG agent inside a class. The train function of the Reinforcement Learning Toolbox run. This is the class (the core of the code is in the function learnImpl):

classdef CustomDDPGAgent < rl.agent.CustomAgent

    %% Properties (set properties attributes accordingly)
    properties
        Actor
        Critic
        Options
        ActorOptimizer
        CriticOptimizer
        NumObservation
        NumAction
        TargetActor
        TargetCritic
        ReplayBuffer
        ExplorationNoise
        ObservationBuffer
        ActionBuffer
        RewardBuffer
        NextObservationBuffer
        pippo 
        Counter
        BufferLength % New property to define buffer length
        % criticLoss
        % criticParams
        criticGradients
        actionsFromActor
        qValuesFromCritic
        actorGradients
        actorParams
        actnet
        criticnet
        experiences
    end

    properties (Access = private)
        NumUnseenExperiences % New property to track the number of unseen experiences
    end
    properties (Access = private,Transient)
        % Accelerated gradient function, not saved with the agent
        AccelGradFcnCrt = []
        AccelGradFcnAct = []
    end
    

    %% Necessary Functions
    %======================================================================
    % Implementation of public function
    %======================================================================

    methods
        function obj = CustomDDPGAgent(Actor, Critic, Options, SampleTime, TargetActor, TargetCritic, actnet, criticnet)
            % Call the abstract class constructor.
            obj = [e-mail]();
            obj.ObservationInfo = Actor.ObservationInfo;
            obj.ActionInfo = Actor.ActionInfo;

            % Register sample time. For MATLAB environment, use -1.
            obj.SampleTime = SampleTime;

            % Register actor, critic, and agent options.
            obj.pippo = 0;
            obj.Actor = Actor;
            obj.Critic = Critic;
            obj.Options = Options;
            obj.actnet = actnet;
            obj.criticnet = criticnet;

            obj.ActorOptimizer = rlOptimizer(Options.OptimizerOptionsActor);
            obj.CriticOptimizer = rlOptimizer(Options.OptimizerOptionsCritic);
            obj.ExplorationNoise = Options.ExplorationNoise;
            % obj.criticLoss = [];
            % obj.criticParams = [];
            obj.criticGradients = [];
            obj.actionsFromActor = [];
            obj.qValuesFromCritic = [];
            obj.actorGradients = [];
            obj.actorParams = [];
            obj.experiences = [];
            obj.NextObservationBuffer = [];
           

            % Cache the number of observations and actions.
            obj.NumObservation = prod(obj.Actor.ObservationInfo.Dimension);
            obj.NumAction = prod(obj.Actor.ActionInfo.Dimension);

            % Initialize target networks and replay buffer.
            obj.TargetActor = TargetActor;
            obj.TargetCritic = TargetCritic;

            obj.BufferLength = Options.ExperienceBufferLength; % Set buffer length
            obj.NumUnseenExperiences = 0; % Initialize number of unseen experiences

            % Initialize buffer and counter.
            resetImpl(obj);
        end
    end    

    %======================================================================
    % Implementation of abstract function
    %======================================================================

    methods (Access = protected)
        function Action = getActionImpl(obj, Observation)
            % Compute an action using the policy given the current
            % observation.
            
            Action = getAction(obj.Actor, Observation);
        end

        function Action = getActionWithExplorationImpl(obj, Observation)
            % Compute an action using the exploration policy given the
            % current observation.

            % DDPG: Deterministic actors explore by adding noise to the action
            Action = getAction(obj.Actor, Observation);
            if iscell(Action)
                Action = cell2mat(Action);
            end
            % Generate Ornstein-Uhlenbeck noise.
            Noise = obj.ExplorationNoise.XPrev + obj.ExplorationNoise.MeanAttractionConstant * (obj.ExplorationNoise.Mu - obj.ExplorationNoise.XPrev) * obj.ExplorationNoise.Dt + obj.ExplorationNoise.CurrentStandardDeviation * randn(obj.NumAction, 1) * sqrt(obj.ExplorationNoise.Dt);
            Action = Action + Noise;
            % Store the noise for the next time step.
            obj.ExplorationNoise.XPrev = Noise;
        end

        function Action = learnImpl(obj, Experience)
            % Extract data from experience
            Obs = Experience{1};
            Action = Experience{2};
            Reward = Experience{3};
            NextObs = Experience{4};
            IsDone = Experience{5};
            % Save experience to replay buffer
            obj.NumUnseenExperiences = obj.NumUnseenExperiences + 1;
            if obj.NumUnseenExperiences <= obj.BufferLength
                obj.ObservationBuffer(:,:,obj.NumUnseenExperiences) = Obs{1};
                obj.ActionBuffer(:,:,obj.NumUnseenExperiences) = Action{1};
                obj.RewardBuffer(:,obj.NumUnseenExperiences) = Reward;
                obj.NextObservationBuffer(:,:,obj.NumUnseenExperiences) = NextObs{1};
            else
                % Buffer is full, overwrite oldest experience
                obj.ObservationBuffer(:,:,(mod(obj.NumUnseenExperiences - 1, obj.BufferLength) + 1)) = Obs{1};
                obj.ActionBuffer(:,:,(mod(obj.NumUnseenExperiences - 1, obj.BufferLength) + 1)) = Action{1};
                obj.RewardBuffer(:,(mod(obj.NumUnseenExperiences - 1, obj.BufferLength) + 1)) = Reward;
                obj.NextObservationBuffer(:,:,(mod(obj.NumUnseenExperiences - 1, obj.BufferLength) + 1)) = NextObs{1}; 
            end
            % Choose an action for the next state
            Action = getActionWithExplorationImpl(obj, NextObs);
            obj.Counter = obj.Counter + 1;
            % Update the standard deviation
            decayedStandardDeviation = obj.ExplorationNoise.CurrentStandardDeviation * (1 - obj.ExplorationNoise.StandardDeviationDecayRate);
            obj.ExplorationNoise.CurrentStandardDeviation = max(decayedStandardDeviation, obj.ExplorationNoise.MinimumStandardDeviation);
            BatchSize = obj.Options.MinibatchSize;
            % Learn from episodic data if enough samples are available
            if obj.NumUnseenExperiences >= BatchSize
                % Sample experiences from replay buffer
                obj.pippo = obj.pippo + 1;
                obj.criticGradients = dlfeval(@trainCritic,obj.criticnet,{obj.NextObservationBuffer},{obj.ObservationBuffer},{obj.ActionBuffer},obj.RewardBuffer,IsDone,obj.TargetActor,obj.TargetCritic,obj.Critic,obj.Options.DiscountFactor);
                obj.Counter = 1;
                obj.NumUnseenExperiences = 0;
            end
        end
    end

    %% Optional Functions
    %======================================================================
    % Implementation of optional function
    %======================================================================
    methods (Access = protected)

        function resetImpl(obj)
            % (Optional) Define how the agent is reset before training.

            resetBuffer(obj);
            obj.NumUnseenExperiences = 0; 
            obj.Counter = 1;
            obj.ExplorationNoise.XPrev = zeros(size(obj.ExplorationNoise.XPrev));
        end
    end

    methods (Access = private)

        function resetBuffer(obj)
             obj.ObservationBuffer = dlarray(zeros(obj.NumObservation,1,obj.Options.MinibatchSize));
             obj.ActionBuffer = dlarray(zeros(obj.NumAction,1,obj.Options.MinibatchSize));
             obj.RewardBuffer = dlarray(zeros(1,obj.Options.MinibatchSize));
             obj.NextObservationBuffer = dlarray(zeros(obj.NumObservation,1,obj.Options.MinibatchSize));
        end
    end
end

function out = trainCritic(criticnet,nextStates,states,actions,rewards,isDone,TargetActor,TargetCritic,Critic,DiscountFactor)
    targetActions = getAction(TargetActor, nextStates);
    targetQ = getValue(TargetCritic, nextStates, targetActions);
    currentQ = getValue(Critic, states, actions);
    targets = rewards + DiscountFactor * targetQ .* ~isDone;
    criticLoss = 0.5 * mean((targets - currentQ).^2);
    disp(criticLoss)
    out = dlgradient(criticLoss,criticnet.Learnables);
end

For now I want to see if it correctly calculates the gradient of the critic parameters, so the class is not complete. But by running some training episodes, the gradient of all parameters always remains equal to zero.

The error is not related to the hyperparameters, but to the fact of how the gradient is calculated.

What is the error?

Thank you all in advance!

Answer 1

[Avadhoot]

Hi @Petre Ricioppo,

As you mentioned the error might be related to how the gradients are calculated. Debugging gradient calculation is not straightforward. There are a few things you should check to ensure that the gradients are computed correctly.

1) Data types:

Ensure that all inputs to "dlgradient" (i.e., the inputs to "trainCritic" that are involved in computing "criticLoss") are "dlarray" objects with the proper data type (e.g., single or double) and have the "Gradient" attribute enabled. If the inputs are not dlarrays or the "Gradient" attribute is not enabled, "dlgradient" will not compute gradients.

2) Loss function:

Check if all the operations performed inside the loss function are differentiable. In your case they are so the issue may not stem from here,

3) Debug gradient calculations:

Before calling dlgradient, ensure that the "Learnables" of "criticnet" are dlarray objects with the "Gradient" attribute enabled. You can check this by inspecting the properties of "criticnet.Learnables" before the "dlgradient" call. You can also check within the loss function if the gradients are still zero at that stage.

I am looking forward to what you find. I hope this helps.