src/axolotl/utils/samplers/multipack.py

# pylint: skip-file
"""
Multipack Batch Sampler
"""
import logging
import math
import os
from typing import Any, Iterable, List, Union

import numba
import numpy as np
from torch.utils.data import BatchSampler, Sampler

LOG = logging.getLogger("axolotl.utils.samplers.multipack")


@numba.njit
def ffd_check(a: np.ndarray, c: int, n: int):
    # First-fit-decreasing bin packing
    # Check if a[] could fit in n bins with capacity c
    # https://en.wikipedia.org/wiki/First-fit-decreasing_bin_packing

    a = np.sort(a)[::-1]
    bins = np.full((n,), c, dtype=a.dtype)
    for size in a:
        not_found = True
        for idx in range(n):
            if bins[idx] >= size:
                bins[idx] -= size
                not_found = False
                break

        if not_found:
            return False

    return True


@numba.njit
def ffd_with_result(a: np.ndarray, c: int, start_index: int):
    # First-fit-decreasing bin packing (with result return)

    indices = np.argsort(a)[::-1]
    a = a[indices]

    bins: List[Any] = []
    bins_result: List[Any] = []
    for a_id, size in enumerate(a):
        add_new = True
        for idx in range(len(bins)):
            if bins[idx] >= size:
                bins[idx] -= size
                bins_result[idx].append(indices[a_id] + start_index)
                add_new = False
                break

        if add_new:
            bins.append(c - size)
            bins_result.append([indices[a_id] + start_index])

    return bins_result


@numba.njit
def allocate(
    lengths: np.ndarray, lengths_cumsum: np.ndarray, rank: int, c: int, n: int
):
    # Dynamic batch allocator, similar to Multifit
    # https://en.wikipedia.org/wiki/Multifit_algorithm
    # ~99.5% efficiency on OpenChat training set (12 * 2048 ctx len)

    s = 0
    start_index = 0
    result = []

    while True:
        # binary search [l, r)
        left = 1
        right = 1 + np.searchsorted(lengths_cumsum[start_index:], s + c * n, "right")

        while right - left > 1:
            mid = (left + right) // 2
            if ffd_check(lengths[start_index : start_index + mid], c, n):
                left = mid
            else:
                right = mid

        # use length l
        batch = ffd_with_result(
            lengths[start_index : start_index + left], c, start_index
        )
        assert len(batch) <= n
        if len(batch) < n:
            break

        start_index += left
        s = lengths_cumsum[start_index - 1]

        # add local rank
        result.append(batch[rank])

    return result, s, len(result) * c * n


class MultipackBatchSampler(BatchSampler):
    """
    Batch Sampler class for multipack
    """

    def __init__(
        self,
        sampler: Union[Sampler[int], Iterable[int]],
        batch_size: int,
        drop_last: bool,
        batch_max_len: int,
        lengths: np.ndarray,
        packing_efficiency_estimate: float = 1.0,
    ):
        super().__init__(sampler, batch_size, drop_last)
        self.batch_size = None
        self.batch_max_len = batch_max_len
        self.lengths: np.ndarray = lengths
        self.packing_efficiency_estimate = packing_efficiency_estimate or 1.0

        assert isinstance(self.lengths, np.ndarray)

        self.epoch = 0

        # statistics
        self.eff_total_used = 0
        self.eff_total_slots = 0

    def set_epoch(self, epoch: int):
        self.epoch = epoch

    def generate_batches(self, set_stats=False):
        indices = [idx for idx in self.sampler]

        lengths = self.lengths[indices]
        lengths_cumsum = np.cumsum(lengths)

        batches, total_used, total_slots = allocate(
            lengths=lengths,
            lengths_cumsum=lengths_cumsum,
            rank=0,
            c=self.batch_max_len,
            n=1,
        )

        batches = [[indices[b_idx] for b_idx in batch] for batch in batches]

        # statistics
        if set_stats:
            self.eff_total_used += total_used
            self.eff_total_slots += total_slots

        return batches

    def __iter__(self):
        batches = self.generate_batches(set_stats=True)
        return iter(batches)

    def num_batches(self):
        batches = self.generate_batches(set_stats=True)
        return len(batches)

    def efficiency(self):
        return self.eff_total_used / self.eff_total_slots

    def __len__(self):
        self.num_batches()
        return self._len_est()

    def _len_est(self):
        world_size = int(os.getenv("WORLD_SIZE", "1"))
        lengths_sum = np.sum(self.lengths)
        lengths_sum_per_device = lengths_sum // world_size
        LOG.info(
            f"packing_efficiency_estimate: {self.packing_efficiency_estimate} "
            f"total_num_tokens per device: {lengths_sum_per_device}"
        )

        # shave off 1% + 1 for dealing with variance in packing from random sampler to sampler
        return max(
            0,
            (
                world_size
                * math.floor(
                    0.99
                    * lengths_sum_per_device
                    / self.packing_efficiency_estimate
                    // self.batch_max_len
                )
                - 1
            ),
        )