Implement interface for data transfer across GPU devices.

Project.toml

@@ -24,8 +24,8 @@ Zygote = "e88e6eb3-aa80-5325-afca-941959d7151f"
 [weakdeps]
 AMDGPU = "21141c5a-9bdb-4563-92ae-f87d6854732e"
 CUDA = "052768ef-5323-5732-b1bb-66c8b64840ba"
-cuDNN = "02a925ec-e4fe-4b08-9a7e-0d78e3d38ccd"
 Metal = "dde4c033-4e86-420c-a63e-0dd931031962"
+cuDNN = "02a925ec-e4fe-4b08-9a7e-0d78e3d38ccd"
 
 [extensions]
 FluxAMDGPUExt = "AMDGPU"
@@ -56,8 +56,8 @@ julia = "1.9"
 [extras]
 AMDGPU = "21141c5a-9bdb-4563-92ae-f87d6854732e"
 BSON = "fbb218c0-5317-5bc6-957e-2ee96dd4b1f0"
-ComponentArrays = "b0b7db55-cfe3-40fc-9ded-d10e2dbeff66"
 CUDA = "052768ef-5323-5732-b1bb-66c8b64840ba"
+ComponentArrays = "b0b7db55-cfe3-40fc-9ded-d10e2dbeff66"
 Documenter = "e30172f5-a6a5-5a46-863b-614d45cd2de4"
 FillArrays = "1a297f60-69ca-5386-bcde-b61e274b549b"
 IterTools = "c8e1da08-722c-5040-9ed9-7db0dc04731e"
@@ -68,6 +68,4 @@ Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
 cuDNN = "02a925ec-e4fe-4b08-9a7e-0d78e3d38ccd"
 
 [targets]
-test = ["Test", "Documenter", "IterTools", "LinearAlgebra",
-        "FillArrays", "ComponentArrays", "BSON", "Pkg",
-        "CUDA", "cuDNN", "Metal", "AMDGPU"]
+test = ["Test", "Documenter", "IterTools", "LinearAlgebra", "FillArrays", "ComponentArrays", "BSON", "Pkg", "CUDA", "cuDNN", "Metal", "AMDGPU"]

docs/src/gpu.md

@@ -311,6 +311,71 @@ julia> device = Flux.get_device(; verbose=true)       # this will resort to auto
 ```
 For detailed information about how the backend is selected, check the documentation for [`Flux.get_device`](@ref).
 
+## Data movement across GPU devices
+
+Flux also supports getting handles to specific GPU devices, and transferring models from one GPU device to another GPU
+device from the same backend. Let's try it out for NVIDIA GPUs. First, we list all the available devices:
+
+```julia-repl
+julia> using Flux, CUDA;
+
+julia> CUDA.devices()
+CUDA.DeviceIterator() for 3 devices:
+0. GeForce RTX 2080 Ti
+1. GeForce RTX 2080 Ti
+2. TITAN X (Pascal)
+
+```
+
+Then, let's select the device with ordinal `0`:
+
+```julia-repl
+julia> device0 = Flux.get_device("CUDA", 0)        # the currently supported values for backend are "CUDA" and "AMD"
+(::Flux.FluxCUDADevice) (generic function with 1 method)
+
+```
+
+Then, let's move a simple dense layer to the GPU represented by `device0`:
+
+```julia-repl
+julia> dense_model = Dense(2 => 3)
+Dense(2 => 3)       # 9 parameters
+
+julia> dense_model = dense_model |> device0;
+
+julia> dense_model.weight
+3×2 CuArray{Float32, 2, CUDA.Mem.DeviceBuffer}:
+  0.695662   0.816299
+ -0.204763  -0.10232
+ -0.955829   0.538412
+
+julia> CUDA.device(dense_model.weight)      # check the GPU to which dense_model is attached
+CuDevice(0): GeForce RTX 2080 Ti
+
+```
+
+Next, we'll get a handle to the device with ordinal `1`, and move `dense_model` to that device:
+
+```julia-repl
+julia> device1 = Flux.get_device("CUDA", 1)
+(::Flux.FluxCUDADevice) (generic function with 1 method)
+
+julia> dense_model = dense_model |> device1;    # don't directly print the model; see warning below
+
+julia> CUDA.device(dense_model.weight)
+CuDevice(1): GeForce RTX 2080 Ti
+
+```
+
+Due to a limitation in `Metal.jl`, currently this kind of data movement across devices is only supported for `CUDA` and `AMD` backends.
+
+!!! warning "Printing models after moving to a different device"
+
+    Due to a limitation in how GPU packages currently work, printing
+    models on the REPL after moving them to a GPU device which is different
+    from the current device will lead to an error.
+
+
 ```@docs
 Flux.AbstractDevice
 Flux.FluxCPUDevice

ext/FluxAMDGPUExt/FluxAMDGPUExt.jl

@@ -17,6 +17,14 @@ const MIOPENFloat = AMDGPU.MIOpen.MIOPENFloat
 # Set to boolean on the first call to check_use_amdgpu
 const USE_AMDGPU = Ref{Union{Nothing, Bool}}(nothing)
 
+function (device::Flux.FluxAMDDevice)(x)
+    if device.deviceID === nothing
+        Flux.gpu(Flux.FluxAMDAdaptor(), x)
+    else
+        return Flux.gpu(Flux.FluxAMDAdaptor(AMDGPU.device_id(device.deviceID) - 1), x)  # subtracting 1, because device_id returns a positive integer
+    end
+end
+Flux._get_device_name(::Flux.FluxAMDDevice) = "AMD"
 Flux._isavailable(::Flux.FluxAMDDevice) = true
 Flux._isfunctional(::Flux.FluxAMDDevice) = AMDGPU.functional()
 

ext/FluxAMDGPUExt/functor.jl

@@ -1,10 +1,41 @@
 # Convert Float64 to Float32, but preserve Float16.
-adapt_storage(::FluxAMDAdaptor, x::T) where T <: AbstractArray =
-    isbits(x) ? x : ROCArray(x)
-adapt_storage(::FluxAMDAdaptor, x::AbstractArray{T, N}) where {T <: AbstractFloat, N} =
-    isbits(x) ? x : ROCArray{Float32, N}(x)
-adapt_storage(::FluxAMDAdaptor, x::AbstractArray{Float16, N}) where N =
-    isbits(x) ? x : ROCArray{Float16, N}(x)
+function adapt_storage(to::FluxAMDAdaptor, x::AbstractArray)
+    if to.ordinal === nothing
+        if (typeof(x) <: AbstractArray{Float16, N} where N)
+            N = length(size(x))
+            return isbits(x) ? x : ROCArray{Float16, N}(x)
+        elseif (typeof(x) <: AbstractArray{T, N} where {T <: AbstractFloat, N})
+            N = length(size(x))
+            return isbits(x) ? x : ROCArray{Float32, N}(x)
+        else
+            return isbits(x) ? x : ROCArray(x)
+        end
+    end
+
+    old_ordinal = AMDGPU.device_id(AMDGPU.device()) - 1     # subtracting 1 because ordinals start from 0
+
+    if !(x isa ROCArray)
+        AMDGPU.device!(AMDGPU.devices()[to.ordinal + 1])    # adding 1 because ordinals start from 0
+        if (typeof(x) <: AbstractArray{Float16, N} where N)
+            N = length(size(x))
+            x_new = isbits(x) ? x : ROCArray{Float16, N}(x)
+        elseif (typeof(x) <: AbstractArray{T, N} where {T <: AbstractFloat, N})
+            N = length(size(x))
+            x_new = isbits(x) ? x : ROCArray{Float32, N}(x)
+        else
+            x_new = isbits(x) ? x : ROCArray(x)
+        end
+        AMDGPU.device!(AMDGPU.devices()[old_ordinal + 1])
+        return x_new
+    elseif AMDGPU.device_id(AMDGPU.device(x)) == to.ordinal
+        return x
+    else
+        AMDGPU.device!(AMDGPU.devices()[to.ordinal + 1])
+        x_new = copy(x)
+        AMDGPU.device!(AMDGPU.devices()[old_ordinal + 1])
+        return x_new
+    end
+end
 
 adapt_storage(::FluxAMDAdaptor, x::Zygote.FillArrays.AbstractFill) =
     ROCArray(collect(x))
@@ -45,10 +76,10 @@ Flux._isleaf(::AMD_CONV) = true
 _exclude(x) = Flux._isleaf(x)
 _exclude(::CPU_CONV) = true
 
-function _amd(x)
+function _amd(ordinal::Union{Nothing, Int}, x)
     check_use_amdgpu()
     USE_AMDGPU[] || return x
-    fmap(x -> Adapt.adapt(FluxAMDAdaptor(), x), x; exclude=_exclude)
+    fmap(x -> Adapt.adapt(FluxAMDAdaptor(ordinal), x), x; exclude=_exclude)
 end
 
 # CPU -> GPU
@@ -74,3 +105,11 @@ function Adapt.adapt_structure(to::FluxCPUAdaptor, m::AMD_CONV)
         Adapt.adapt(to, m.σ), reverse(Adapt.adapt(to, m.weight); dims),
         Adapt.adapt(to, m.bias), m.stride, m.pad, m.dilation, m.groups)
 end
+
+function Flux.get_device(::Val{:AMD}, ordinal::Int)     # ordinal should start from 0
+    old_ordinal = AMDGPU.device_id(AMDGPU.device()) - 1     # subtracting 1 because ordinals start from 0
+    AMDGPU.device!(AMDGPU.devices()[ordinal + 1])           # adding 1 because ordinals start from 0
+    device = Flux.FluxAMDDevice(AMDGPU.device())
+    AMDGPU.device!(AMDGPU.devices()[old_ordinal + 1])
+    return device
+end

ext/FluxCUDAExt/FluxCUDAExt.jl

@@ -14,6 +14,14 @@ import Adapt: adapt_storage
 
 const USE_CUDA = Ref{Union{Nothing, Bool}}(nothing)
 
+function (device::Flux.FluxCUDADevice)(x)
+    if device.deviceID === nothing
+        return Flux.gpu(Flux.FluxCUDAAdaptor(), x)
+    else
+        return Flux.gpu(Flux.FluxCUDAAdaptor(device.deviceID.handle), x)
+    end
+end
+Flux._get_device_name(::Flux.FluxCUDADevice) = "CUDA"
 Flux._isavailable(::Flux.FluxCUDADevice) = true
 Flux._isfunctional(::Flux.FluxCUDADevice) = CUDA.functional()
 

ext/FluxCUDAExt/functor.jl

@@ -1,5 +1,24 @@
-
 adapt_storage(to::FluxCUDAAdaptor, x) = CUDA.cu(x)
+function adapt_storage(to::FluxCUDAAdaptor, x::AbstractArray)
+    to.ordinal === nothing && return CUDA.cu(x)
+
+    # remember current device
+    old_ordinal = CUDA.device().handle
+
+    if !(x isa CuArray)
+        CUDA.device!(to.ordinal)
+        x_new = CUDA.cu(x)
+        CUDA.device!(old_ordinal)
+        return x_new
+    elseif CUDA.device(x).handle == to.ordinal
+        return x
+    else
+        CUDA.device!(to.ordinal)
+        x_new = copy(x)
+        CUDA.device!(old_ordinal)
+        return x_new
+    end
+end
 adapt_storage(to::FluxCUDAAdaptor, x::Zygote.FillArrays.AbstractFill) = CUDA.cu(collect(x))
 adapt_storage(to::FluxCUDAAdaptor, x::Random.TaskLocalRNG) = CUDA.default_rng()
 adapt_storage(to::FluxCUDAAdaptor, x::CUDA.RNG) = x
@@ -25,8 +44,16 @@ ChainRulesCore.rrule(::typeof(adapt), a::FluxCUDAAdaptor, x::AnyCuArray) =
 ChainRulesCore.rrule(::typeof(adapt), a::FluxCUDAAdaptor, x::AbstractArray) =
   adapt(a, x), Δ -> (NoTangent(), NoTangent(), adapt(FluxCPUAdaptor(), unthunk(Δ)))
 
-function _cuda(x)
+function _cuda(ordinal::Union{Nothing, Int}, x)
   check_use_cuda()
   USE_CUDA[] || return x
-  fmap(x -> Adapt.adapt(FluxCUDAAdaptor(), x), x; exclude=Flux._isleaf)
+  fmap(x -> Adapt.adapt(FluxCUDAAdaptor(ordinal), x), x; exclude=Flux._isleaf)
+end
+
+function Flux.get_device(::Val{:CUDA}, ordinal::Int)
+    old_ordinal = CUDA.device().handle
+    CUDA.device!(ordinal)
+    device = Flux.FluxCUDADevice(CUDA.device())
+    CUDA.device!(old_ordinal)
+    return device
 end

ext/FluxMetalExt/FluxMetalExt.jl

@@ -12,6 +12,8 @@ using Zygote
 
 const USE_METAL = Ref{Union{Nothing, Bool}}(nothing)
 
+(::Flux.FluxMetalDevice)(x) = Flux.gpu(Flux.FluxMetalAdaptor(), x)
+Flux._get_device_name(::Flux.FluxMetalDevice) = "Metal"
 Flux._isavailable(::Flux.FluxMetalDevice) = true
 Flux._isfunctional(::Flux.FluxMetalDevice) = Metal.functional()
 

src/functor.jl

@@ -332,13 +332,15 @@ trainable(c::Cholesky) = ()
 
 # CUDA extension. ########
 
-struct FluxCUDAAdaptor end
+Base.@kwdef struct FluxCUDAAdaptor
+    ordinal::Union{Nothing, Int} = nothing
+end
 
 const CUDA_LOADED = Ref{Bool}(false)
 
-function gpu(::FluxCUDAAdaptor, x)
+function gpu(to::FluxCUDAAdaptor, x)
     if CUDA_LOADED[]
-        return _cuda(x)
+        return _cuda(to.ordinal, x)
     else
         @info """
         The CUDA functionality is being called but
@@ -353,13 +355,15 @@ function _cuda end
 
 # AMDGPU extension. ########
 
-struct FluxAMDAdaptor end
+Base.@kwdef struct FluxAMDAdaptor
+    ordinal::Union{Nothing, Int} = nothing
+end
 
 const AMDGPU_LOADED = Ref{Bool}(false)
 
-function gpu(::FluxAMDAdaptor, x)
+function gpu(to::FluxAMDAdaptor, x)
     if AMDGPU_LOADED[]
-        return _amd(x)
+        return _amd(to.ordinal, x)
     else
         @info """
         The AMDGPU functionality is being called but
@@ -500,9 +504,6 @@ Base.@kwdef struct FluxCUDADevice <: AbstractDevice
     deviceID
 end
 
-(::FluxCUDADevice)(x) = gpu(FluxCUDAAdaptor(), x)
-_get_device_name(::FluxCUDADevice) = "CUDA"
-
 """
     FluxAMDDevice <: AbstractDevice
 
@@ -512,9 +513,6 @@ Base.@kwdef struct FluxAMDDevice <: AbstractDevice
     deviceID
 end
 
-(::FluxAMDDevice)(x) = gpu(FluxAMDAdaptor(), x)
-_get_device_name(::FluxAMDDevice) = "AMD"
-
 """
     FluxMetalDevice <: AbstractDevice
 
@@ -524,9 +522,6 @@ Base.@kwdef struct FluxMetalDevice <: AbstractDevice
     deviceID
 end
 
-(::FluxMetalDevice)(x) = gpu(FluxMetalAdaptor(), x)
-_get_device_name(::FluxMetalDevice) = "Metal"
-
 ## device list. order is important
 const DEVICES = Ref{Vector{Union{Nothing, AbstractDevice}}}(Vector{Union{Nothing, AbstractDevice}}(nothing, length(GPU_BACKENDS)))
 DEVICES[][GPU_BACKEND_ORDER["CPU"]] = FluxCPUDevice()
@@ -550,7 +545,7 @@ julia> Flux.supported_devices()
 supported_devices() = GPU_BACKENDS
 
 """
-    Flux.get_device(; verbose=false)::AbstractDevice
+    Flux.get_device(; verbose=false)::Flux.AbstractDevice
 
 Returns a `device` object for the most appropriate backend for the current Julia session. 
 
@@ -653,3 +648,45 @@ function get_device(; verbose=false)::AbstractDevice
         end
     end
 end
+
+"""
+    Flux.get_device(backend::String, ordinal::Int = 0)::Flux.AbstractDevice
+
+Get a device object for a backend specified by the string `backend` and `ordinal`. The currently supported values
+of `backend` are `"CUDA"`, `"AMD"` and `"CPU"`. `ordinal` must be an integer value between `0` and the number of available devices.
+
+# Examples
+
+```julia-repl
+julia> using Flux, CUDA;
+
+julia> CUDA.devices()
+CUDA.DeviceIterator() for 3 devices:
+0. GeForce RTX 2080 Ti
+1. GeForce RTX 2080 Ti
+2. TITAN X (Pascal)
+
+julia> device0 = Flux.get_device("CUDA", 0)
+(::Flux.FluxCUDADevice) (generic function with 1 method)
+
+julia> device0.deviceID
+CuDevice(0): GeForce RTX 2080 Ti
+
+julia> device1 = Flux.get_device("CUDA", 1)
+(::Flux.FluxCUDADevice) (generic function with 1 method)
+
+julia> device1.deviceID
+CuDevice(1): GeForce RTX 2080 Ti
+
+julia> cpu_device = Flux.get_device("CPU")
+(::Flux.FluxCPUDevice) (generic function with 1 method)
+
+```
+"""
+function get_device(backend::String, ordinal::Int = 0)
+    if backend == "CPU"
+        return FluxCPUDevice()
+    else
+        return get_device(Val(Symbol(backend)), ordinal)
+    end
+end