fsdp_ulysses.py

# Copyright 2024 Bytedance Ltd. and/or its affiliates
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
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"""
Contains a resharding manager that binds weights from FSDP zero3 to XPerfGPT
"""

from torch.distributed.device_mesh import DeviceMesh

from verl import DataProto
from verl.utils.ulysses import get_ulysses_sequence_parallel_group, set_ulysses_sequence_parallel_group

from .base import BaseShardingManager


class FSDPUlyssesShardingManager(BaseShardingManager):
    """
    Sharding manager to support data resharding when using FSDP + Ulysses
    """

    def __init__(self, device_mesh: DeviceMesh):
        super().__init__()
        self.device_mesh = device_mesh

    def __enter__(self):
        if self.device_mesh is not None:
            self.prev_sp_group = get_ulysses_sequence_parallel_group()
            set_ulysses_sequence_parallel_group(self.device_mesh["sp"].get_group())

    def __exit__(self, exc_type, exc_value, traceback):
        if self.device_mesh is not None:
            set_ulysses_sequence_parallel_group(self.prev_sp_group)

    def preprocess_data(self, data: DataProto) -> DataProto:
        """
        AllGather data from sp region
        This is because the data is first sharded along the FSDP dimension as we utilize the DP_COMPUTE
        In Ulysses, we need to make sure the same data is used across a SP group
        """
        if self.device_mesh is not None:
            sp_group = self.device_mesh["sp"].get_group()
            data = data.to("cuda")
            data.all_gather(sp_group)

        return data

    def postprocess_data(self, data: DataProto) -> DataProto:
        """
        Split the data to follow FSDP partition
        """
        if self.device_mesh is not None:
            sp_size = self.device_mesh["sp"].size()
            sp_rank = self.device_mesh["sp"].get_local_rank()
            data = data.chunk(chunks=sp_size)[sp_rank]

        return data