seqlen_balancing.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
# limitations under the License.

import copy
import heapq
from itertools import chain

import torch
from torch import distributed as dist

from verl.protocol import DataProto
from verl.utils.device import get_device_name


def karmarkar_karp(seqlen_list: list[int], k_partitions: int, equal_size: bool):
    # see: https://en.wikipedia.org/wiki/Largest_differencing_method
    class Set:
        def __init__(self) -> None:
            self.sum = 0
            self.items = []

        def add(self, idx: int, val: int):
            self.items.append((idx, val))
            self.sum += val

        def merge(self, other):
            for idx, val in other.items:
                self.items.append((idx, val))
                self.sum += val

        def __lt__(self, other):
            if self.sum != other.sum:
                return self.sum < other.sum
            if len(self.items) != len(other.items):
                return len(self.items) < len(other.items)
            return self.items < other.items

    class State:
        def __init__(self, items: list[tuple[int, int]], k: int) -> None:
            self.k = k
            # sets should always be decreasing order
            self.sets = [Set() for _ in range(k)]
            assert len(items) in [1, k], f"{len(items)} not in [1, {k}]"
            for i, (idx, seqlen) in enumerate(items):
                self.sets[i].add(idx=idx, val=seqlen)
            self.sets = sorted(self.sets, reverse=True)

        def get_partitions(self):
            partitions = []
            for i in range(len(self.sets)):
                cur_partition = []
                for idx, _ in self.sets[i].items:
                    cur_partition.append(idx)
                partitions.append(cur_partition)
            return partitions

        def merge(self, other):
            for i in range(self.k):
                self.sets[i].merge(other.sets[self.k - 1 - i])
            self.sets = sorted(self.sets, reverse=True)

        @property
        def spread(self) -> int:
            return self.sets[0].sum - self.sets[-1].sum

        def __lt__(self, other):
            # least heap, let the state with largest spread to be popped first,
            # if the spread is the same, let the state who has the largest set
            # to be popped first.
            if self.spread != other.spread:
                return self.spread > other.spread
            return self.sets[0] > other.sets[0]

        def __repr__(self) -> str:
            repr_str = "["
            for i in range(self.k):
                if i > 0:
                    repr_str += ","
                repr_str += "{"
                for j, (_, seqlen) in enumerate(self.sets[i].items):
                    if j > 0:
                        repr_str += ","
                    repr_str += str(seqlen)
                repr_str += "}"
            repr_str += "]"
            return repr_str

    sorted_seqlen_list = sorted([(seqlen, i) for i, seqlen in enumerate(seqlen_list)])
    states_pq = []
    if equal_size:
        assert len(seqlen_list) % k_partitions == 0, f"{len(seqlen_list)} % {k_partitions} != 0"
        for offset in range(0, len(sorted_seqlen_list), k_partitions):
            items = []
            for i in range(k_partitions):
                seqlen, idx = sorted_seqlen_list[offset + i]
                items.append((idx, seqlen))
            heapq.heappush(states_pq, State(items=items, k=k_partitions))
    else:
        for seqlen, idx in sorted_seqlen_list:
            heapq.heappush(states_pq, State(items=[(idx, seqlen)], k=k_partitions))

    while len(states_pq) > 1:
        state0 = heapq.heappop(states_pq)
        state1 = heapq.heappop(states_pq)
        # merge states
        state0.merge(state1)
        heapq.heappush(states_pq, state0)

    final_state = states_pq[0]
    partitions = final_state.get_partitions()
    if equal_size:
        for i, partition in enumerate(partitions):
            assert len(partition) * k_partitions == len(seqlen_list), (
                f"{len(partition)} * {k_partitions} != {len(seqlen_list)}"
            )
    return partitions


def greedy_partition(seqlen_list: list[int], k_partitions: int, equal_size: bool):
    bias = sum(seqlen_list) + 1 if equal_size else 0
    sorted_seqlen = [(seqlen + bias, i) for i, seqlen in enumerate(seqlen_list)]
    partitions = [[] for _ in range(k_partitions)]
    partition_sums = [0 for _ in range(k_partitions)]
    for seqlen, i in sorted_seqlen:
        min_idx = None
        for j in range(k_partitions):
            if min_idx is None or partition_sums[j] < partition_sums[min_idx]:
                min_idx = j
        partitions[min_idx].append(i)
        partition_sums[min_idx] += seqlen
    if equal_size:
        for i, partition in enumerate(partitions):
            assert len(partition) * k_partitions == len(seqlen_list), (
                f"{len(partition)} * {k_partitions} != {len(seqlen_list)}"
            )
    return partitions


def get_seqlen_balanced_partitions(seqlen_list: list[int], k_partitions: int, equal_size: bool):
    """
    Calculates partitions of indices from seqlen_list such that the sum of sequence lengths
    in each partition is balanced. Uses the Karmarkar-Karp differencing method.

    This is useful for balancing workload across devices or batches, especially when
    dealing with variable sequence lengths.

    Args:
        seqlen_list (List[int]): A list of sequence lengths for each item.
        k_partitions (int): The desired number of partitions.
        equal_size (bool): If True, ensures that each partition has the same number of items.
                           Requires len(seqlen_list) to be divisible by k_partitions.
                           If False, partitions can have varying numbers of items, focusing
                           only on balancing the sum of sequence lengths.

    Returns:
        List[List[int]]: A list containing k_partitions lists. Each inner list contains the
                         original indices of the items assigned to that partition. The indices
                         within each partition list are sorted.

    Raises:
        AssertionError: If len(seqlen_list) < k_partitions.
        AssertionError: If equal_size is True and len(seqlen_list) is not divisible by k_partitions.
        AssertionError: If any resulting partition is empty.
    """
    assert len(seqlen_list) >= k_partitions, f"number of items:[{len(seqlen_list)}] < k_partitions:[{k_partitions}]"

    def _check_and_sort_partitions(partitions):
        assert len(partitions) == k_partitions, f"{len(partitions)} != {k_partitions}"
        seen_idx = set()
        sorted_partitions = [None] * k_partitions
        for i, partition in enumerate(partitions):
            assert len(partition) > 0, f"the {i}-th partition is empty"
            for idx in partition:
                seen_idx.add(idx)
            sorted_partitions[i] = sorted(partition)
        assert seen_idx == set(range(len(seqlen_list)))
        return sorted_partitions

    partitions = karmarkar_karp(seqlen_list=seqlen_list, k_partitions=k_partitions, equal_size=equal_size)
    return _check_and_sort_partitions(partitions)


def log_seqlen_unbalance(seqlen_list: list[int], partitions: list[list[int]], prefix):
    """
    Calculate and log metrics related to sequence length imbalance before and after partitioning.

    Args:
        seqlen_list (List[int]): A list of sequence lengths for each item.
        partitions (List[List[int]]): A list of partitions, where each inner list contains indices
                                      from seqlen_list assigned to that partition.
        prefix (str): A prefix to be added to each metric key in the returned dictionary.

    Returns:
        dict: A dictionary containing metrics related to sequence length imbalance.
    """
    # Get the number of partitions
    k_partition = len(partitions)
    # assert len(seqlen_list) % k_partition == 0
    batch_size = len(seqlen_list) // k_partition
    min_sum_seqlen = None
    max_sum_seqlen = None
    total_sum_seqlen = 0

    # Iterate over each batch of sequence lengths
    for offset in range(0, len(seqlen_list), batch_size):
        cur_sum_seqlen = sum(seqlen_list[offset : offset + batch_size])
        if min_sum_seqlen is None or cur_sum_seqlen < min_sum_seqlen:
            min_sum_seqlen = cur_sum_seqlen
        if max_sum_seqlen is None or cur_sum_seqlen > max_sum_seqlen:
            max_sum_seqlen = cur_sum_seqlen
        total_sum_seqlen += cur_sum_seqlen

    balanced_sum_seqlen_list = []
    for partition in partitions:
        cur_sum_seqlen_balanced = sum([seqlen_list[i] for i in partition])
        balanced_sum_seqlen_list.append(cur_sum_seqlen_balanced)
    # print("balanced_sum_seqlen_list: ", balanced_sum_seqlen_list)
    min_sum_seqlen_balanced = min(balanced_sum_seqlen_list)
    max_sum_seqlen_balanced = max(balanced_sum_seqlen_list)

    return {
        f"{prefix}/min": min_sum_seqlen,
        f"{prefix}/max": max_sum_seqlen,
        f"{prefix}/minmax_diff": max_sum_seqlen - min_sum_seqlen,
        f"{prefix}/balanced_min": min_sum_seqlen_balanced,
        f"{prefix}/balanced_max": max_sum_seqlen_balanced,
        f"{prefix}/mean": total_sum_seqlen / len(partitions),
    }


def ceildiv(a, b):
    return -(a // -b)


def roundup_divisible(a, b):
    return ((a + b - 1) // b) * b


def rearrange_micro_batches(
    batch,
    max_token_len,
    dp_group=None,
    num_batches_divided_by=None,
    same_micro_num_in_dp=True,
    min_num_micro_batch=None,
    use_dynamic_bsz_balance=True,
):
    """
    Split a batch into micro-batches by total token count, with optional DP sync and padding.

    Args:
        batch (TensorDict): must include "attention_mask" (B*S); other fields are sliced similarly.
        max_token_len (int): max sum of attention_mask per micro-batch.
        dp_group (optional): torch.distributed group for data-parallel sync.
        num_batches_divided_by (optional): virtual pipeline parallel size, for megatron.
        same_micro_num_in_dp (bool): if True and dp_group set, pad all ranks to the same count.
        min_num_micro_batch (int, optional): force at least this many splits (pads empty ones).
        use_dynamic_bsz_balance (bool, optional): balance the computational workload between micro-batches

    Returns:
        List[TensorDict]: the micro-batches.
        List[List[int]]: index lists mapping each micro-batch back to original positions.
    """
    # this is per local micro_bsz
    max_seq_len = batch["attention_mask"].shape[-1]
    assert max_token_len >= max_seq_len, (
        f"max_token_len must be greater than the sequence length. Got {max_token_len=} and {max_seq_len=}"
    )
    seq_len_effective: torch.Tensor = batch["attention_mask"].sum(dim=1)
    total_seqlen = seq_len_effective.sum().item()
    # NOTE: num_microbatches <= batch_size, so take the min of this two.
    num_micro_batches = min(len(seq_len_effective), ceildiv(total_seqlen, max_token_len))
    if min_num_micro_batch is not None:
        # used to support pp
        num_micro_batches = max(min_num_micro_batch, num_micro_batches)
    if dist.is_initialized() and same_micro_num_in_dp:
        num_micro_batches = torch.tensor([num_micro_batches], device=get_device_name())
        dist.all_reduce(num_micro_batches, op=dist.ReduceOp.MAX, group=dp_group)
        num_micro_batches = num_micro_batches.cpu().item()
    if num_batches_divided_by is not None:
        num_micro_batches = roundup_divisible(num_micro_batches, num_batches_divided_by)

    seq_len_effective = seq_len_effective.tolist()
    assert num_micro_batches <= len(seq_len_effective)

    micro_bsz_idx = get_seqlen_balanced_partitions(seq_len_effective, num_micro_batches, equal_size=False)

    if use_dynamic_bsz_balance:
        # Use the sum of squared sequence lengths to approximate attention computation workload
        micro_bsz_idx.sort(
            key=lambda partition: (
                sum(seq_len_effective[idx] ** 2 for idx in partition),
                min(partition) if partition else 0,
            ),
            reverse=True,
        )

    micro_batches = []

    for partition in micro_bsz_idx:
        curr_micro_batch = []
        for idx in partition:
            curr_micro_batch.append(batch[idx : idx + 1])
        curr_micro_batch = torch.cat(curr_micro_batch)

        micro_batches.append(curr_micro_batch)

    return micro_batches, micro_bsz_idx


def get_reverse_idx(idx_map):
    """
    Build the inverse of an index mapping.

    Args:
        idx_map (Sequence[int]): Sequence where idx_map[i] = j.

    Returns:
        List[int]: Inverse mapping list such that output[j] = i for each i.
    """
    reverse_idx_map = copy.deepcopy(idx_map)

    for i, idx in enumerate(idx_map):
        reverse_idx_map[idx] = i

    return reverse_idx_map


def prepare_dynamic_batch(data: DataProto, max_token_len: int) -> tuple[list[DataProto], list[list[int]]]:
    """
    Prepare a batch for dynamic batching.

    Args:
        data (DataProto): The input data.
        max_token_len (int): The maximum token length for dynamic batching.

    Returns:
        Tuple[List[DataProto], List[List[int]]]: A tuple containing a list of DataProto objects
        and a list of index lists.
    """
    batch, batch_idx_list = rearrange_micro_batches(data.batch, max_token_len=max_token_len)
    micro_batches = []
    for i, batch_idx in enumerate(batch_idx_list):
        tensors = dict(batch[i])
        non_tensors = {key: value[batch_idx] for key, value in data.non_tensor_batch.items()}
        micro_batches.append(DataProto.from_dict(tensors, non_tensors))

    return micro_batches, batch_idx_list


def restore_dynamic_batch(data: torch.Tensor, batch_idx_list: list[list[int]]) -> torch.Tensor:
    """
    Restore a batch from dynamic batching.

    Args:
        data (torch.Tensor): The input data.
        batch_idx_list (List[List[int]]): The list of index lists.

    Returns:
        torch.Tensor: The restored data.
    """
    indices = list(chain.from_iterable(batch_idx_list))
    revert_indices = torch.tensor(get_reverse_idx(indices), dtype=torch.long)
    return data[revert_indices]