[cp] set up load balancing testbed

ghstack-source-id: f28b448bab842304813ff7464ca61e71494ec552
Pull Request resolved: #120

Author: XilunWu

attn_gym/load_balance/__init__.py

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+from attn_gym.load_balance.load_balancer import load_balance_algo
+
+__all__ = ["load_balance_algo"]

attn_gym/load_balance/load_balancer.py

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+from abc import ABC, abstractmethod
+
+import torch
+
+
+__all__ = ["load_balance_algo"]
+
+
+def load_balance_algo(S: int, size: int, block_size: int) -> torch.Tensor:
+    return HeadTail.gen_load_balance_plan(S, size, block_size)
+
+
+class LoadAlgorithm(ABC):
+    @classmethod
+    @abstractmethod
+    def gen_load_balance_plan(cls, S: int, size: int, block_size: int) -> torch.Tensor:
+        pass
+
+
+class Noop(LoadAlgorithm):
+    @classmethod
+    def gen_load_balance_plan(cls, S: int, size: int, block_size: int) -> torch.Tensor:
+        total_num_blk = S // block_size
+        assert S % (size * block_size) == 0
+        local_num_blk = total_num_blk // size
+        return torch.arange(total_num_blk, device="cuda").view(size, local_num_blk)
+
+
+class HeadTail(LoadAlgorithm):
+    @classmethod
+    def gen_load_balance_plan(cls, S: int, size: int, block_size: int) -> torch.Tensor:
+        total_num_blk = S // block_size
+        assert S % (size * 2 * block_size) == 0
+        local_num_blk_pair = total_num_blk // (size * 2)
+        plan_tensor = torch.arange(total_num_blk, device="cuda").view(
+            -1, local_num_blk_pair
+        )
+        return torch.stack(
+            (
+                plan_tensor[:size],
+                plan_tensor[size:].flip(dims=(0,)),
+            ),
+            dim=1,
+        ).view(size, -1)
+
+
+if __name__ == "__main__":
+    print(HeadTail.gen_load_balance_plan(32, 4, 1))

examples/distributed_benchmark.py

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+from functools import lru_cache
+from typing import Optional
+
+import os
+import torch
+import torch.distributed as dist
+from torch.distributed.device_mesh import init_device_mesh
+from torch.distributed.tensor import distribute_tensor, DTensor, DeviceMesh, Replicate, Shard
+
+
+from torch.nn.attention.flex_attention import (
+    _DEFAULT_SPARSE_BLOCK_SIZE,
+    create_block_mask,
+    flex_attention,
+    _mask_mod_signature,
+)
+
+from attn_gym.masks.document_mask import length_to_offsets
+from attn_gym.masks import (
+    causal_mask,
+    generate_doc_mask_mod,
+)
+from attn_gym.load_balance import load_balance_algo
+
+
+def get_device_type() -> str:
+    return "cuda"
+
+
+@lru_cache
+def create_block_mask_cached(score_mod, B, H, M, N, device="cuda"):
+    block_mask = create_block_mask(score_mod, B, H, M, N, device=device)
+    return block_mask
+
+
+# TODO: re-write it into a wrapper???
+def rewrite_mask_mod_for_cp(
+    mask_mod: _mask_mod_signature,
+    rank: int,
+    block_size: int,
+    load_balancer_output: torch.Tensor,
+) -> _mask_mod_signature:
+    def local_q_idx_to_q_idx(local_q_idx) -> int:
+        # calculate local block_idx and block_offset
+        local_blk_idx, local_blk_offset = (
+            local_q_idx // block_size, local_q_idx % block_size
+        )
+        current_rank_blk_list = load_balancer_output[rank]
+        blk_idx = current_rank_blk_list[local_blk_idx]
+        return blk_idx * block_size + local_blk_offset
+
+    return lambda b, h, q_idx, kv_idx: mask_mod(
+        b, h, local_q_idx_to_q_idx(q_idx), kv_idx
+    )
+
+
+def run_document_masking(device_mesh, max_seq_len, num_docs):
+    # initialize the document lengths
+    import random
+
+    random.seed(0)
+    torch.cuda.manual_seed(0)
+
+    def generate_random_lengths(total_length, num_documents):
+        # Initialize all lengths to 1 to ensure each document has at least one token
+        lengths = [1] * num_documents
+        remaining_length = total_length - num_documents
+
+        # Randomly distribute the remaining length
+        for _ in range(remaining_length):
+            index = random.randint(0, num_documents - 1)
+            lengths[index] += 1
+
+        return lengths
+
+    lengths = generate_random_lengths(max_seq_len, num_docs)
+    offsets = length_to_offsets(lengths, torch.device(f'cuda:{torch.cuda.current_device():d}'))  # TODO: replace with a device mesh call
+    document_causal_mask = generate_doc_mask_mod(causal_mask, offsets)
+    test_mask_with_load_balance(device_mesh, mask_mod=document_causal_mask, S=max_seq_len)
+
+
+def test_mask_with_load_balance(
+    device_mesh: DeviceMesh,
+    mask_mod: Optional[_mask_mod_signature] = None,
+    B: int = 16,
+    H: int = 16,
+    S: int = 8192,
+    D: int = 64,
+    skip_correctness: bool = False,
+    print_mask: bool = True,
+    device: str = "cuda",
+):
+    data_type = torch.float16
+
+    # create block mask
+    block_mask = create_block_mask_cached(mask_mod, 1, 1, S, S, device=device)
+    block_size = _DEFAULT_SPARSE_BLOCK_SIZE  # TODO: get block size from block mask
+
+    # input initialization
+    qkv = [
+        torch.rand(
+            (B, H, S, D),
+            device=device_mesh.device_type,
+            dtype=data_type,
+            requires_grad=True,
+        )
+        for _ in range(3)
+    ]
+
+    # NOTE: this shuffle op can be done in other ways
+    def shuffle_tensor_for_load_balancing(
+        x: torch.Tensor, shuffle_tensor: torch.Tensor, dim: int
+    ) -> torch.Tensor:
+        # shuffle the tensor
+        num_chunks = shuffle_tensor.numel()
+        x_chunk_list = torch.chunk(x, num_chunks, dim=dim)
+        assert len(x_chunk_list) == num_chunks
+        new_x_chunk_list = [None] * num_chunks
+        for blk_idx in range(num_chunks):
+            new_x_chunk_list[blk_idx] = x_chunk_list[shuffle_tensor[blk_idx].item()]
+
+        return torch.cat(new_x_chunk_list, dim=dim)
+
+    def interchange_index_value_2d(tensor: torch.Tensor) -> torch.Tensor:
+        """
+        Interchange the index and value in a PyTorch tensor. The input tensor has
+        structure: rank -> [block_idx, ...] and the output tensor will be:
+        block_idx -> block_idx_in_shuffled_tensor
+        """
+        flattened_tensor = tensor.view(-1)
+        indices = torch.arange(
+            flattened_tensor.numel(), device=flattened_tensor.device
+        )
+        revert_tensor = torch.empty_like(flattened_tensor)
+        revert_tensor[flattened_tensor] = indices
+
+        return revert_tensor
+
+    cp_mesh_size = device_mesh.size()
+    load_balancer_output = load_balance_algo(S, cp_mesh_size, block_size)
+
+    seq_dim = 2
+    # copy QKV
+    qkv_copy = [t.detach().clone() for t in qkv]
+    # shuffle Q
+    qkv_copy[0] = shuffle_tensor_for_load_balancing(
+        qkv_copy[0], load_balancer_output.view(-1), dim=seq_dim
+    )
+    qkv_dist = [
+        distribute_tensor(
+            t.requires_grad_(), device_mesh, [
+                Shard(seq_dim) if i == 0 else Replicate()
+            ]
+        )
+        for (i, t) in enumerate(qkv)
+    ]
+
+    q_local, k_full, v_full = (dt.to_local() for dt in qkv_dist)
+
+    # rewrite `block_mask`
+    mask_mod: _mask_mod_signature = block_mask.mask_mod
+    cp_rank = device_mesh.get_local_rank()
+    cp_mask_mod = rewrite_mask_mod_for_cp(
+        mask_mod, cp_rank, block_size, load_balancer_output
+    )
+    cp_block_mask = create_block_mask_cached(
+        cp_mask_mod, B=1, H=1, M=S // cp_mesh_size, N=S, device=device
+    )
+
+    # Compile the flex_attention function
+    compiled_flex_attention = torch.compile(flex_attention, dynamic=False)
+
+    # TODO: this doesn't address the return_lse=True case
+    cp_out = compiled_flex_attention(
+        q_local,
+        k_full,
+        v_full,
+        score_mod=None,
+        block_mask=cp_block_mask,
+    )
+    assert isinstance(cp_out, torch.Tensor)
+
+    # unshard
+    cp_out_dist = DTensor.from_local(cp_out, device_mesh, [Shard(seq_dim)])
+    full_cp_out_dist = cp_out_dist.full_tensor()
+    # rearrange
+    blk_idx_shuffled = interchange_index_value_2d(load_balancer_output)
+    full_cp_out_dist = shuffle_tensor_for_load_balancing(
+        full_cp_out_dist, blk_idx_shuffled, dim=seq_dim
+    )
+
+    # local flex attention
+    expect_out = flex_attention(*qkv, block_mask=block_mask)
+    torch.testing.assert_close(full_cp_out_dist, expect_out, atol=1e-1, rtol=1e-2)
+
+
+def load_balancing_example(world_size: int, rank: int) -> None:
+    device_type = get_device_type()
+    device_handle = getattr(torch, device_type, None)
+    assert device_handle is not None, f"Unsupported device type: {device_type}"
+    num_devices_per_host = device_handle.device_count()
+    device_handle.set_device(rank % num_devices_per_host)
+    torch._dynamo.config.cache_size_limit = 1000
+
+    # init device mesh
+    device_mesh = init_device_mesh(device_type=device_type, mesh_shape=(world_size,))
+
+    run_document_masking(device_mesh, max_seq_len=4096, num_docs=12)
+
+
+if __name__ == "__main__":
+    # this script is launched via torchrun which automatically manages ProcessGroup
+    rank = int(os.environ["RANK"])
+    world_size = int(os.environ["WORLD_SIZE"])
+    # assert world_size == 4  # our example uses 4 worker ranks
+
+    try:
+        load_balancing_example(world_size, rank)
+    finally:
+        dist.barrier()
+        dist.destroy_process_group()