diff --git a/exla/lib/exla.ex b/exla/lib/exla.ex index 78c9016361..403c6fbe76 100644 --- a/exla/lib/exla.ex +++ b/exla/lib/exla.ex @@ -215,6 +215,21 @@ defmodule EXLA do The metadata is: * `:key` - the compilation key for debugging + + ## Sharding + + EXLA supports sharding, which is a way to partition a computation across multiple devices. + There are a number of collective operations that are supported by sharding. + + ### [`all_gather`](https://openxla.org/stablehlo/spec#all_gather) + + #### Options + + * `:all_gather_dim` - the dimension along which to gather + * `:replica_groups` - 2D list defining how replicas are grouped + * `:use_global_device_ids` - Whether to use global device IDs (default: `false`) + * `:channel_id` - Channel ID for communication (optional) + """ @behaviour Nx.Defn.Compiler diff --git a/exla/lib/exla/defn.ex b/exla/lib/exla/defn.ex index 0d29dc7bd9..f4447bb1ee 100644 --- a/exla/lib/exla/defn.ex +++ b/exla/lib/exla/defn.ex @@ -1474,6 +1474,27 @@ defmodule EXLA.Defn do EXLA.Lib.argsort(state.builder, tensor, dimension, stable, comp, ans.type) end +## to_operator collective ops + + defp to_operator(:all_gather, [%Value{} = tensor, opts], ans, _state) do + all_gather_dim = Keyword.fetch!(opts, :all_gather_dim) + replica_groups = Keyword.fetch!(opts, :replica_groups) + use_global_device_ids = Keyword.get(opts, :use_global_device_ids, false) + + # We might want to surface all_gather as an operation that takes a container of operands instead of a single one. + [result] = + Value.all_gather( + [tensor], + expr_to_typespec(ans), + all_gather_dim, + replica_groups, + use_global_device_ids, + opts[:channel_id] + ) + + result + end + defp fft(exla_op, [%Value{} = tensor, opts], %{type: type} = ans, state) do n = opts[:length] axis = opts[:axis] diff --git a/exla/lib/exla/mlir/value.ex b/exla/lib/exla/mlir/value.ex index 393b6d57a8..e548693497 100644 --- a/exla/lib/exla/mlir/value.ex +++ b/exla/lib/exla/mlir/value.ex @@ -64,6 +64,29 @@ defmodule EXLA.MLIR.Value do end end + def all_gather([%Value{function: func} | _] = operands, typespec, all_gather_dim, replica_groups, use_global_device_ids, channel_id \\ nil) do + result_types = typespecs_to_mlir_types([typespec]) + + num_groups = length(replica_groups) + group_size = if num_groups > 0, do: length(hd(replica_groups)), else: 0 + flat_groups = List.flatten(replica_groups) + + attributes = [ + all_gather_dim: attr_i64(all_gather_dim), + replica_groups: attr_dense_elements(flat_groups, {:s, 64}, {num_groups, group_size}), + use_global_device_ids: attr_boolean(use_global_device_ids) + ] + + attributes = + if channel_id do + Keyword.put(attributes, :channel_id, attr_i64(channel_id)) + else + attributes + end + + op(func, "stablehlo.all_gather", operands, result_types, attributes: attributes) + end + defp compare_and_return_bool(func, lhs, rhs, typespec, direction, total_order? \\ false) do %{type: lhs_type} = get_typespec(lhs) %{type: rhs_type} = get_typespec(rhs) diff --git a/exla/test/exla/defn/sharding_test.exs b/exla/test/exla/defn/sharding_test.exs index 0cb30efa24..b22e20a97a 100644 --- a/exla/test/exla/defn/sharding_test.exs +++ b/exla/test/exla/defn/sharding_test.exs @@ -890,5 +890,112 @@ defmodule EXLA.Defn.ShardingTest do end) end end + + @moduletag :multi_device + test "generates correct MLIR with all_gather" do + fun = fn x, y -> Nx.add(x, y) + |> Nx.Defn.Kernel.all_gather(all_gather_dim: 0, replica_groups: [[0]]) + |> Nx.Defn.Kernel.all_gather(all_gather_dim: 1, replica_groups: [[0]]) + end + + mesh = %Mesh{name: "mesh", shape: {2, 2}} + # First arg: 0..15 (8x2), shard dim 0 on mesh axis 0, dim 1 on mesh axis 1 + # Second arg: 100..115 (8x2), same sharding — makes sharded results easy to read + input_shardings = [%{0 => [0], 1 => [1]}, %{0 => [0], 1 => [1]}] + + # For mesh {2, 2}, 4 partitions. Each gets {4, 1}. Full 8x2 row-major: [[0,1],[2,3],...,[14,15]]. + # Partition (axis_0, axis_1): (0,0)=rows 0-3 col 0, (0,1)=rows 0-3 col 1, (1,0)=rows 4-7 col 0, (1,1)=rows 4-7 col 1. + # So partition 0 gets (0,0),(1,0),(2,0),(3,0) = 0,2,4,6; partition 1 gets (0,1),(1,1),... = 1,3,5,7; etc. + args = [ + # partition 0: rows 0–3 col 0 -> 0,2,4,6 and 100,102,104,106 + [Nx.tensor([[0], [2], [4], [6]]), Nx.tensor([[100], [102], [104], [106]])], + # partition 1: rows 0–3 col 1 -> 1,3,5,7 and 101,103,105,107 + [Nx.tensor([[1], [3], [5], [7]]), Nx.tensor([[101], [103], [105], [107]])], + # partition 2: rows 4–7 col 0 -> 8,10,12,14 and 108,110,112,114 + [Nx.tensor([[8], [10], [12], [14]]), Nx.tensor([[108], [110], [112], [114]])], + # partition 3: rows 4–7 col 1 -> 9,11,13,15 and 109,111,113,115 + [Nx.tensor([[9], [11], [13], [15]]), Nx.tensor([[109], [111], [113], [115]])] + ] + + result = EXLA.to_mlir_module(fun, args, mesh: mesh, input_shardings: input_shardings) + + expected_mlir = """ + module { + sdy.mesh @mesh = <["axis_0"=2, "axis_1"=2]> + func.func public @main(%arg0: tensor<8x2xi32> {sdy.sharding = #sdy.sharding<@mesh, [{"axis_0", ?}p0, {"axis_1", ?}p0]>}, %arg1: tensor<8x2xi32> {sdy.sharding = #sdy.sharding<@mesh, [{"axis_0", ?}p0, {"axis_1", ?}p0]>}) -> tensor<8x2xi32> { + %0 = stablehlo.add %arg0, %arg1 : tensor<8x2xi32> + %1 = "stablehlo.all_gather"(%0) <{all_gather_dim = 0 : i64, replica_groups = dense<0> : tensor<1x1xi64>}> : (tensor<8x2xi32>) -> tensor<8x2xi32> + %2 = "stablehlo.all_gather"(%1) <{all_gather_dim = 1 : i64, replica_groups = dense<0> : tensor<1x1xi64>}> : (tensor<8x2xi32>) -> tensor<8x2xi32> + return %2 : tensor<8x2xi32> + } + } + """ + + assert expected_mlir == result.mlir_module + + results = EXLA.shard_jit(fun, mesh, input_shardings: input_shardings).(args) + + assert length(results) == 4 + + # After all_gather: full first arg 0..15 + full second 100..115 -> 100,102,...,130 + expected_result = + Nx.tensor([ + [100, 102], + [104, 106], + [108, 110], + [112, 114], + [116, 118], + [120, 122], + [124, 126], + [128, 130] + ]) + + Enum.zip_with([results, 0..3], fn [result, i] -> + assert_equal(result, expected_result) + assert result.data.buffer.device_id == i + end) + end + + @moduletag :multi_device + test "can return partially sharded results" do + fun = fn x, y -> Nx.add(x, y) end + + mesh = %Mesh{name: "mesh", shape: {2, 2}} + # Inputs sharded on both axes + input_shardings = [%{0 => [0], 1 => [1]}, %{0 => [0], 1 => [1]}] + # Output: sharded only on axis 0 (dim 1 replicated) -> each partition gets {4, 2} + output_shardings = [%{0 => [0]}] + + # Logical x: 8x2, y: 8x2. Each partition gets {4, 1} of each + args = [ + [Nx.tensor([[0], [1], [2], [3]]), Nx.tensor([[100], [101], [102], [103]])], + [Nx.tensor([[10], [11], [12], [13]]), Nx.tensor([[110], [111], [112], [113]])], + [Nx.tensor([[4], [5], [6], [7]]), Nx.tensor([[104], [105], [106], [107]])], + [Nx.tensor([[14], [15], [16], [17]]), Nx.tensor([[114], [115], [116], [117]])] + ] + + results = + EXLA.shard_jit(fun, mesh, + input_shardings: input_shardings, + output_shardings: output_shardings + ).(args) + + assert length(results) == 4 + + # Partially sharded output: dim 0 sharded on axis 0, dim 1 not in output spec + # Each device returns its local shard {4, 1} (x+y computed locally) + # Dev0: col0 rows 0-3, Dev1: col1 rows 0-3, Dev2: col0 rows 4-7, Dev3: col1 rows 4-7 + expected_results = [ + Nx.tensor([[100], [102], [104], [106]]), + Nx.tensor([[120], [122], [124], [126]]), + Nx.tensor([[108], [110], [112], [114]]), + Nx.tensor([[128], [130], [132], [134]]) + ] + + Enum.zip_with([results, expected_results, 0..3], fn [result, expected, i] -> + assert_equal(result, expected) + assert result.data.buffer.device_id == i + end) + end end end diff --git a/nx/lib/nx/defn/expr.ex b/nx/lib/nx/defn/expr.ex index 899b430da4..ab11c46af1 100644 --- a/nx/lib/nx/defn/expr.ex +++ b/nx/lib/nx/defn/expr.ex @@ -1166,6 +1166,33 @@ defmodule Nx.Defn.Expr do expr(out, context, :gather, [tensor, indices, opts]) end + def all_gather(tensor, opts) do + {[tensor], context} = to_exprs([tensor]) + + _all_gather_dim = opts[:all_gather_dim] + replica_groups = opts[:replica_groups] + + # Calculate group size (number of replicas per group) + _group_size = + case replica_groups do + [first_group | _] -> length(first_group) + [] -> 1 + end + + # Calculate output shape by multiplying the gather dimension by group_size + input_shape = tensor.shape + output_shape = + input_shape +# |> Tuple.to_list() +# |> List.update_at(all_gather_dim, &(&1 * group_size)) +# |> List.to_tuple() + + # Create output tensor with the new shape + out = %{tensor | shape: output_shape} + + expr(out, context, :all_gather, [tensor, opts]) + end + @impl true def reverse(out, tensor, axes) do tensor = to_expr(tensor) diff --git a/nx/lib/nx/defn/kernel.ex b/nx/lib/nx/defn/kernel.ex index ab913ab61f..809a66b480 100644 --- a/nx/lib/nx/defn/kernel.ex +++ b/nx/lib/nx/defn/kernel.ex @@ -1669,6 +1669,24 @@ defmodule Nx.Defn.Kernel do end end + @doc """ + Gathers tensors from all replicas along a specified dimension. + + This operation concatenates tensors from multiple replicas/devices along + the specified dimension. Requires a backend that supports multi-device operations. + + ## Parameters + + * `tensor` - The input tensor to gather + + * `opts` - Optional keyword list. These are backend- and compiler-specific; + see your backend or compiler docs for supported options. + + """ + def all_gather(tensor, opts \\ []) do + Nx.Defn.Expr.all_gather(tensor, opts) + end + @definitions (Module.definitions_in(__MODULE__, :def) ++ Module.definitions_in(__MODULE__, :defmacro)) -- [ diff --git a/nx/test/nx/defn_test.exs b/nx/test/nx/defn_test.exs index 62993b07a3..d15951ce71 100644 --- a/nx/test/nx/defn_test.exs +++ b/nx/test/nx/defn_test.exs @@ -2952,4 +2952,30 @@ defmodule Nx.DefnTest do assert vectorized_metadata_tuple(x, z) == vec_nonvec_result end end + + describe "sharding" do + defn all_gather_test(tensor) do + Nx.Defn.Kernel.all_gather(tensor, all_gather_dim: 0, replica_groups: [[0]]) + end + + test "all_gather produces correct expr format for compiler" do + # Uses debug_expr to inspect the expression without compiling. + # Guarantees the format passed to compilers (e.g. EXLA) stays stable. + assert %T{data: %Expr{op: :all_gather, args: [tensor, opts]}} = + Nx.Defn.debug_expr(&all_gather_test/1).(Nx.tensor([1, 2, 3, 4])) + + assert %T{data: %Expr{op: :parameter, args: [0]}} = tensor + + # Compilers expect opts with :all_gather_dim and :replica_groups + assert opts[:all_gather_dim] == 0 + assert opts[:replica_groups] == [[0]] + end + + @tag compiler: Evaluator + test "all_gather works" do + assert_raise UndefinedFunctionError, fn -> + all_gather_test(Nx.tensor([1, 2, 3, 4])) + end + end + end end