verl/utils/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 typing import List, Tuple

import torch
from torch import distributed as dist


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):
    """get order of seq lengths to make partitions balanced, this is
        used in balacing sum of seqlength across dp ranks and microbatches
    Parameters:
        seqlen_list (List[int]):
            seq lengths of each items
        k_partitions (int):
            resulting number of partitions
        equal_size (bool):
            if True, number of items in each partitions must be equal.
            if False, only consider balancing the sum, each partition can have
            variable number of items
    Returns:
        partitions (List[List[int]]):
            return k_partitions list containing the index of items.
    """
    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):
    # add some metrics of seqlen sum on dp ranks
    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
    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 rearrange_micro_batches(batch, max_token_len, dp_group=None, same_micro_num_in_dp=True, min_num_micro_batch=None):
    """
    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.
        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).

    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="cuda")
        dist.all_reduce(num_micro_batches, op=dist.ReduceOp.MAX, group=dp_group)
        num_micro_batches = num_micro_batches.cpu().item()

    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)

    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):
    reverse_idx_map = copy.deepcopy(idx_map)

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

    return reverse_idx_map