Freezing Model Parameters

Warning

API for freezing parameters should be considered experimental at this point.

In this manual entry, we will go over how to freeze certain parameters in a model.

Freezing Layers of a Particular Kind

To freeze a particular kind of layer, let's say Dense in the following example. We can use Lux.Experimental.@layer_map and freeze layers if they are of type Dense.

using Lux, Random

rng = Random.default_rng()
Random.seed!(rng, 0)

model = Chain(Dense(3, 4), Chain(Dense(4, 4), Dropout(0.5f0), BatchNorm(4)),
    Dense(4, 1); disable_optimizations=true)

ps, st = Lux.setup(rng, model)

x = randn(rng, Float32, 3, 2)

model(x, ps, st)

function freeze_dense(d::Lux.Dense, ps, st, ::String)
    return Lux.freeze(d, ps, st, (:weight, :bias))
end
freeze_dense(l, ps, st, name) = (l, ps, st)

model_frozen, ps_frozen, st_frozen = Lux.Experimental.@layer_map freeze_dense model ps st

model_frozen(x, ps_frozen, st_frozen)

(Float32[1.6873633 -1.6873633], (layer_1 = (frozen_params = (weight = Float32[-0.026350189 -0.5554656 -0.35653266; -0.17461072 0.6705545 0.29924855; -0.8935247 -0.42453378 -0.3020351; -0.7988979 -0.7666331 -0.7104237], bias = Float32[0.0; 0.0; 0.0; 0.0;;]), states = NamedTuple()), layer_2 = (layer_1 = (frozen_params = (weight = Float32[-0.47289538 -0.680748 0.1764085 0.34383082; 0.42747158 -0.13819042 -0.109261915 -0.6143286; -0.35790488 -0.20881107 0.70390546 0.48137343; 0.82561636 0.38187847 0.05779423 -0.35181466], bias = Float32[0.0; 0.0; 0.0; 0.0;;]), states = NamedTuple()), layer_2 = (rng = Random.TaskLocalRNG(), training = Val{true}()), layer_3 = (running_mean = Float32[0.04584409, 0.03484953, 0.0, 0.0074841487], running_var = Float32[0.9420336, 0.92428976, 0.9, 0.90112025], training = Val{true}())), layer_3 = (frozen_params = (weight = Float32[0.3981135 0.45468387 -0.07694905 0.8353388], bias = Float32[0.0;;]), states = NamedTuple())))

Freezing by Layer Name

When the function in layer_map is called, the 4th argument is the name of the layer. For example, if you want to freeze the 1st layer inside the inner Chain. The name for this would be <model>.layer_2.layer_1.

Freezing by Layer Name

function freeze_by_name(d, ps, st, name::String)
    if name == "model.layer_2.layer_1"
        return Lux.Experimental.freeze(d, ps, st, (:weight, :bias))
    else
        return d, ps, st
    end
end

Freezing by Layer Type

function freeze_dense(d::Dense, ps, st, ::String)
    return Lux.Experimental.freeze(d, ps, st, (:weight, :bias))
end
freeze_dense(l, ps, st, _) = (l, ps, st)

Freezing Part of the Parameters

Instead of freezing all the parameters, we can simply specify (:weight,) to freeze only the weight parameter while training the bias parameter.

Freezing Some Parameters of a Layer

function freeze_by_name(d, ps, st, name::String)
    if name == "model.layer_2.layer_1"
        return Lux.freeze(d, ps, st, (:weight,))
    else
        return d, ps, st
    end
end

Freezing All Parameters of a Layer

function freeze_by_name(d, ps, st, name::String)
    if name == "model.layer_2.layer_1"
        return Lux.freeze(d, ps, st, (:weight, :bias))
    else
        return d, ps, st
    end
end

Freezing Part of a Chain

Starting v0.4.22, we can directly index into a Chain. So freezing a part of a Chain, is extremely easy.

using Lux, Random

rng = Random.default_rng()
Random.seed!(rng, 0)

model = Chain(Dense(3, 4), Dense(4, 4), Dropout(0.5f0), BatchNorm(4), Dense(4, 1))

model_frozen = Chain(model[1:2], Lux.freeze(model[3:4]), model[5])
ps, st = Lux.setup(rng, model_frozen)

x = randn(rng, Float32, 3, 2)

model_frozen(x, ps, st)

(Float32[-0.53158027 0.53158027], (layer_1 = NamedTuple(), layer_2 = NamedTuple(), layer_3 = (frozen_params = (layer_3 = NamedTuple(), layer_4 = (scale = Float32[1.0, 1.0, 1.0, 1.0], bias = Float32[0.0, 0.0, 0.0, 0.0])), states = (layer_3 = (rng = Random.TaskLocalRNG(), training = Val{true}()), layer_4 = (running_mean = Float32[0.0, 0.021013658, -0.057823665, 0.0], running_var = Float32[0.9, 0.9088315, 0.9668715, 0.9], training = Val{true}()))), layer_4 = NamedTuple()))