Agents.jl: Differentiable ABM?

[kavir1698]

I have tried to use Zygote.jl to differentiate some random and small ABMs. But I get many different errors. Are there data structures that are not supported by Zygote yet? A differentiable ABM would be amazing and allowing to solve many problems.

[Datseris]

I know nothing about zygote, nor the data structures it needs. I can't add anything to the discussion here. :(

p.s.: This issue is not well described. You should consider adding more information about the errors you get, what you are trying to do, a MWE, and more.

[kavir1698]

Agent-based model are often very complex and one needs to run many simulations and replicates to explore parameter space and understand the behavior of a model. If we were able to differentiate AMBs with Automatic Differentiation (AD), we would be able to save a lot of time in finding parameter values that best fit data.

I have read about AD in Julia, but I am not deeply familiar with its limitations. I tried to differentiate a simple ABM (Agent.jl's wealth distribution example). But I get the following error. I am not sure what part of the data structure we use are not supported.

using Agents
using Zygote
using Statistics: std

mutable struct WealthAgent <: AbstractAgent
    id::Int
    wealth::Int
end

function wealth_model(;numagents = 100, initwealth = 1)
    model = ABM(WealthAgent, scheduler=random_activation)
    for i in 1:numagents
        add_agent!(model, initwealth)
    end
    return model
end

function agent_step!(agent, model)
    agent.wealth == 0 && return # do nothing
    ragent = random_agent(model)
    agent.wealth -= 1
    ragent.wealth += 1
end

function costWealth(M)
  agent_properties = [:wealth]
  model = wealth_model(numagents=M)
  step!(model, agent_step!, 5)
  std([agent.wealth for agent in values(model.agents)])
end

gradient(N ->costWealth(N), 1000)

ERROR: InexactError: Int64(0.002002002002002002)
Stacktrace:
 [1] Int64 at .\float.jl:709 [inlined]
 [2] convert at .\number.jl:7 [inlined]
 [3] _backvar at C:\Users\Ali\.julia\packages\Zygote\KNUTW\src\lib\array.jl:270 
[inlined]
 [4] _backvar at C:\Users\Ali\.julia\packages\Zygote\KNUTW\src\lib\array.jl:269 
[inlined]
 [5] #1264 at C:\Users\Ali\.julia\packages\Zygote\KNUTW\src\lib\array.jl:274 [inlined]
 [6] (::Zygote.var"#3219#back#1266"{Zygote.var"#1264#1265"{Bool,Colon,Float64,Array{Int64,1},Float64}})(::Float64) at C:\Users\Ali\.julia\packages\ZygoteRules\6nssF\src\adjoint.jl:49
 [7] costWealth at .\REPL[115]:7 [inlined]
 [8] (::typeof(∂(costWealth)))(::Float64) at C:\Users\Ali\.julia\packages\Zygote\KNUTW\src\compiler\interface2.jl:0
 [9] #17 at .\REPL[119]:1 [inlined]
 [10] (::Zygote.var"#38#39"{typeof(∂(#17))})(::Float64) at C:\Users\Ali\.julia\packages\Zygote\KNUTW\src\compiler\interface.jl:36
 [11] gradient(::Function, ::Int64, ::Vararg{Int64,N} where N) at C:\Users\Ali\.julia\packages\Zygote\KNUTW\src\compiler\interface.jl:45
 [12] top-level scope at REPL[119]:1

The error varies depending of the model though.

[Libbum]

I'm getting a core dump when I try this example

julia: /buildworker/worker/package_linux64/build/src/codegen.cpp:4357: jl_cgval_t emit_expr(jl_codectx_t&, jl_value_t*, ssize_t): Assertion `token.V->getType()->isTokenTy()' failed.
so I can't help troubleshoot until I figure that out.

I haven't worked with AD in Julia that much myself, but I've never considered using it on discrete variables. What if you changed wealth to Float64?

[kavir1698]

Thanks for the reply. Good point. I get the error below with Float64 for wealth. But this error seems more promising to be solvable.

ERROR: Need an adjoint for constructor Base.ValueIterator{Dict{Int64,WealthAgent}}. Gradient is of type Array{Base.RefValue{Any},1}
Stacktrace:
 [1] error(::String) at .\error.jl:33
 [2] (::Zygote.Jnew{Base.ValueIterator{Dict{Int64,WealthAgent}},Nothing,false})(::Array{Base.RefValue{Any},1}) at C:\Users\Ali\.julia\packages\Zygote\KNUTW\src\lib\lib.jl:294
 [3] (::Zygote.var"#378#back#196"{Zygote.Jnew{Base.ValueIterator{Dict{Int64,WealthAgent}},Nothing,false}})(::Array{Base.RefValue{Any},1}) at C:\Users\Ali\.julia\packages\ZygoteRules\6nssF\src\adjoint.jl:49
 [4] ValueIterator at .\abstractdict.jl:44 [inlined]
 [5] (::typeof(∂(Base.ValueIterator{Dict{Int64,WealthAgent}})))(::Array{Base.RefValue{Any},1}) at C:\Users\Ali\.julia\packages\Zygote\KNUTW\src\compiler\interface2.jl:0
 [6] ValueIterator at .\abstractdict.jl:44 [inlined]
 [7] (::typeof(∂(Base.ValueIterator)))(::Array{Base.RefValue{Any},1}) at C:\Users\Ali\.julia\packages\Zygote\KNUTW\src\compiler\interface2.jl:0
 [8] values at .\abstractdict.jl:123 [inlined]
 [9] (::typeof(∂(values)))(::Array{Base.RefValue{Any},1}) at C:\Users\Ali\.julia\packages\Zygote\KNUTW\src\compiler\interface2.jl:0
 [10] costWealth at .\REPL[8]:5 [inlined]
 [11] (::typeof(∂(costWealth)))(::Float64) at C:\Users\Ali\.julia\packages\Zygote\KNUTW\src\compiler\interface2.jl:0
 [12] #6 at .\REPL[10]:1 [inlined]
 [13] (::Zygote.var"#38#39"{typeof(∂(#6))})(::Float64) at C:\Users\Ali\.julia\packages\Zygote\KNUTW\src\compiler\interface.jl:36
 [14] gradient(::Function, ::Int64, ::Vararg{Int64,N} where N) at C:\Users\Ali\.julia\packages\Zygote\KNUTW\src\compiler\interface.jl:45
 [15] top-level scope at REPL[10]:1

[Libbum]

I'm going to try and dig into this a little deeper. Completely agree it would be good to have this working. Luckily I've had a need to start working with Zygote myself recently, so hopefully we can get this done for v3

[ChrisRackauckas]

You could attempt to get the strong solution of some models, but in many ABMs the discreteness is inherently non-differentiable through AD, and you'd have to move to the continuous master equation PDE to get a differentiable form.

[kavir1698]

Thank you, @ChrisRackauckas , for your reply. It is great to have the input of an expert on this issue. Is is possible to know _when_ the discreteness is not a problem?
And if not through AD, could numerical differentiation be applied to ABMs?