Revert "Multipack parallel bin packing (#2631)"

This reverts commit 8e4158cc0b.

src/axolotl/core/trainers/base.py

@@ -114,8 +114,6 @@ class AxolotlTrainer(
             packing_efficiency_estimate=self.args.sample_packing_efficiency,
             batch_max_len=batch_max_len,
             batch_size=batch_size,
-            group_size=self.args.sample_packing_group_size,
-            bin_size=self.args.sample_packing_bin_size,
             sequential=self.args.sample_packing_sequentially,
             drop_last=True,
         )

src/axolotl/utils/samplers/multipack.py

@@ -1,13 +1,10 @@
+# pylint: skip-file
 """
-Multipack Batch Sampler - An efficient batch sampler for packing variable-length sequences
+Multipack Batch Sampler
-into fixed-capacity batches to optimize memory usage and training throughput.
 """
-
 import logging
 import math
-from concurrent.futures import ProcessPoolExecutor
+from typing import Any, Iterable, List, Union
-from multiprocessing import cpu_count
-from typing import Iterable, Union
 
 import numba
 import numpy as np
@@ -16,39 +13,26 @@ from torch.utils.data import BatchSampler, Sampler, SequentialSampler
 from axolotl.utils.distributed import reduce_and_broadcast
 
 LOG = logging.getLogger(__name__)
+
 LOG.setLevel(logging.INFO)
 
 
 @numba.njit
-def ffd_check(sequence_lengths: np.ndarray, bin_capacity: int, num_bins: int):
+def ffd_check(a: np.ndarray, c: int, n: int):
-    """
+    # First-fit-decreasing bin packing
-    First-fit-decreasing bin packing algorithm check
+    # Check if a[] could fit in n bins with capacity c
+    # https://en.wikipedia.org/wiki/First-fit-decreasing_bin_packing
 
-    Checks if sequences with the given lengths could fit in the specified number of bins
+    a = np.sort(a)[::-1]
-
+    bins = np.full((n,), c, dtype=a.dtype)
-    Args:
+    for size in a:
-        sequence_lengths: Array of sequence lengths
-        bin_capacity: Maximum capacity of each bin
-        num_bins: Number of bins available
-
-    Returns:
-        True if all sequences can be packed, False otherwise
-    """
-    # Sort sequence lengths in descending order for optimal packing
-    sequence_lengths = np.sort(sequence_lengths)[::-1]
-    # Initialize all bins with full capacity
-    bins = np.full((num_bins,), bin_capacity, dtype=sequence_lengths.dtype)
-
-    # Try to place each sequence in the first bin it fits
-    for size in sequence_lengths:
         not_found = True
-        for idx in range(num_bins):
+        for idx in range(n):
             if bins[idx] >= size:
                 bins[idx] -= size
                 not_found = False
                 break
 
-        # If no bin could fit this sequence, packing failed
         if not_found:
             return False
 
@@ -56,132 +40,86 @@ def ffd_check(sequence_lengths: np.ndarray, bin_capacity: int, num_bins: int):
 
 
 @numba.njit
-def pack_group(
+def ffd_with_result(a: np.ndarray, c: int, start_index: int):
-    sequence_lengths: np.ndarray,
+    # First-fit-decreasing bin packing (with result return)
-    group_offset: int,
-    bin_capacity: int,
-    max_bins: int,
-    bin_size: int,
-    safe_mode: bool = True,
-):
-    """
-    Pack a group of sequences into bins using First-Fit Decreasing algorithm
 
-    Args:
+    indices = np.argsort(a)[::-1]
-        sequence_lengths: Array of sequence lengths
+    a = a[indices]
-        group_offset: Offset to apply to indices when returning results
-        bin_capacity: Maximum capacity of each bin
-        max_bins: Maximum number of bins to use
-        bin_size: Maximum number of sequences per bin
-        safe_mode: If True, use a more conservative packing approach
 
-    Returns:
+    bins: List[Any] = []
-        List of bins, where each bin contains indices of sequences assigned to it
+    bins_result: List[Any] = []
-    """
+    for a_id, size in enumerate(a):
-    # Get sorting indices and sort lengths in descending order
+        add_new = True
-    indices = np.argsort(sequence_lengths)[::-1]
+        for idx in range(len(bins)):
-    sorted_lengths = sequence_lengths[indices]
+            if bins[idx] >= size:
-
+                bins[idx] -= size
-    bins_remaining_space: list = []  # Tracks remaining capacity in each bin
+                bins_result[idx].append(indices[a_id] + start_index)
-    bins_assigned_sequences: list = []  # Tracks sequence indices assigned to each bin
+                add_new = False
-
-    for seq_id, size in enumerate(sorted_lengths):
-        global_idx = indices[seq_id] + group_offset
-
-        # Try to place sequence in existing bins
-        add_new_bin = True
-        for bin_idx, _ in enumerate(bins_remaining_space):
-            if (
-                bins_remaining_space[bin_idx] >= size
-                and len(bins_assigned_sequences[bin_idx]) < bin_size
-            ):
-                bins_remaining_space[bin_idx] -= size
-                bins_assigned_sequences[bin_idx].append(global_idx)
-                add_new_bin = False
                 break
 
-        # Create a new bin if needed and if we haven't reached the limit
+        if add_new:
-        if add_new_bin:
+            bins.append(c - size)
-            if len(bins_remaining_space) >= max_bins and safe_mode:
+            bins_result.append([indices[a_id] + start_index])
-                # In safe mode, skip items that would exceed max_bins
-                continue
-            bins_remaining_space.append(bin_capacity - size)
-            bins_assigned_sequences.append([global_idx])
 
-            # Safety check to avoid infinite bins
+    return bins_result
-            if len(bins_remaining_space) > len(sequence_lengths):
-                break
-
-    return bins_assigned_sequences
-
-
-# Define a standalone function for multiprocessing
-def _process_group(args):
-    group_lengths, start_idx, bin_capacity, max_bins, bin_size, safe_mode = args
-    return pack_group(
-        group_lengths, start_idx, bin_capacity, max_bins, bin_size, safe_mode
-    )
-
-
-def pack_parallel(
-    sequence_lengths: np.ndarray,
-    bin_capacity: int,
-    group_size: int,
-    bin_size: int,
-    num_processes: int | None = None,
-    safe_mode: bool = True,
-):
-    """
-    Pack sequences into bins using parallel processing
-
-    Args:
-        sequence_lengths: Array of sequence lengths
-        bin_capacity: Maximum capacity of each bin as total number of tokens
-        group_size: Number of sequences to process in each group
-        bin_size: Maximum number of bins to use
-        num_processes: Number of parallel processes to use
-        safe_mode: If True, use a more conservative packing approach
-
-    Returns:
-        List of bins, where each bin contains indices of sequences assigned to it
-    """
-    num_items = len(sequence_lengths)
-    if num_processes is None:
-        num_processes = max(1, min(num_items // group_size, cpu_count()))
-
-    # Create tasks for parallel processing
-    tasks = []
-    for i in range(0, num_items, group_size):
-        group_lengths = sequence_lengths[i : i + group_size]
-        max_bins = len(group_lengths)  # Allow as many bins as items in the group
-        tasks.append((group_lengths, i, bin_capacity, max_bins, bin_size, safe_mode))
-
-    # Process groups in parallel
-    all_bins = []
-    with ProcessPoolExecutor(max_workers=num_processes) as executor:
-        for group_bins in executor.map(_process_group, tasks):
-            all_bins.extend(group_bins)
-
-    return all_bins
 
 
 @numba.njit
-def allocate_sequentially(
+def allocate(
-    sequence_lengths: np.ndarray, rank: int, bin_capacity: int, num_ranks: int
+    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
+
+
+@numba.njit
+def allocate_sequentially(lengths: np.ndarray, rank: int, c: int, n: int):
     """
     Sequential allocator that preserves example order
 
-    Args:
+    Parameters:
-        sequence_lengths: The lengths of all examples
+    - lengths: The lengths of all examples
-        rank: The current rank (for distributed training)
+    - rank: The current rank (for distributed training)
-        bin_capacity: The capacity of each bin (maximum sequence length)
+    - c: The capacity of each bin (maximum sequence length)
-        num_ranks: Number of ranks (processes/GPUs)
+    - n: Number of ranks
 
     Returns:
-        rank_batches: List of batches for the current rank
+    - result: List of batches for the current rank
-        total_tokens_used: Number of actual example tokens
+    - total_used: Number of actual example tokens
-        total_token_slots: Maximum theoretical number of example tokens (number of bins * bin capacity)
+    - total_slots: Maximum theoretical number of example tokens (number of bins * bin capacity)
     """
     result = []
     total_used = 0
@@ -189,9 +127,9 @@ def allocate_sequentially(
     # First, do sequential packing into bins
     all_bins = []
     current_bin = [0 for i in range(0)]  # numba hint
-    remaining_capacity = bin_capacity
+    remaining_capacity = c
 
-    for idx, size in enumerate(sequence_lengths):
+    for idx, size in enumerate(lengths):
         if size <= remaining_capacity:
             # Example fits in current bin
             current_bin.append(idx)
@@ -202,7 +140,7 @@ def allocate_sequentially(
             if current_bin:  # Add non-empty bin to all_bins
                 all_bins.append(current_bin)
             current_bin = [idx]
-            remaining_capacity = bin_capacity - size
+            remaining_capacity = c - size
             total_used += size
 
     # Add the last bin if not empty
@@ -210,227 +148,132 @@ def allocate_sequentially(
         all_bins.append(current_bin)
 
     # Assign bins to ranks - each rank gets every n-th bin
-    for bin_idx in range(rank, len(all_bins), num_ranks):
+    for bin_idx in range(rank, len(all_bins), n):
         result.append(all_bins[bin_idx])
 
-    return result, total_used, len(all_bins) * bin_capacity
+    return result, total_used, len(all_bins) * c
 
 
 class MultipackBatchSampler(BatchSampler):
-    """
+    """Batch sampler class for multipack"""
-    Batch sampler class for efficient packing of variable-length sequences
-
-    This sampler packs sequences into fixed-capacity bins (batches) to maximize
-    GPU memory utilization and training throughput by reducing padding.
-
-    It supports both parallel packing (using FFD algorithm) and
-    sequential packing (preserving original sequence order).
-    """
 
     def __init__(
         self,
         sampler: Union[Sampler[int], Iterable[int]],
-        batch_size: int,  # Number of bins per batch
+        batch_size: int,
-        batch_max_len: int,  # Maximum sequence length (bin capacity)
+        batch_max_len: int,
-        lengths: np.ndarray,  # Sequence lengths
+        lengths: np.ndarray,
-        packing_efficiency_estimate: float = 1.0,  # Initial efficiency estimate
+        packing_efficiency_estimate: float = 1.0,
-        drop_last: bool = False,  # Whether to drop final batches (might be incomplete)
+        drop_last: bool = False,
-        num_count_samples: int = 16,  # Number of times to estimate batch count
+        num_count_samples: int = 16,
-        sequential: bool = False,  # Whether to use sequential packing
+        sequential: bool = False,
-        group_size: int = 100_000,  # Size of groups for parallel packing
+        **kwargs,
-        bin_size: int = 200,  # The max number of samples that can be packed in a single bin
-        num_processes: int | None = None,  # Number of processes for parallel packing
-        safe_mode: bool = True,  # Conservative packing to prevent training instability
-        **kwargs,  # pylint: disable=unused-argument
     ):
         super().__init__(sampler, batch_size, drop_last)
         self.batch_size = batch_size
         self.batch_max_len = batch_max_len
-        self.lengths = np.array(lengths, dtype=np.int32)
+        self.lengths: np.ndarray = lengths
         self.packing_efficiency_estimate = packing_efficiency_estimate or 1.0
         self.sequential = sequential
-        self.group_size = group_size
-        self.bin_size = bin_size
-        self.num_processes = num_processes
-        self.safe_mode = safe_mode
 
         assert isinstance(self.lengths, np.ndarray)
 
         self.epoch = 0
 
-        # Efficiency statistics tracking
+        # statistics
-        self.total_tokens_used = 0
+        self.eff_total_used = 0
-        self.total_token_slots = 0
+        self.eff_total_slots = 0
 
-        # The number of times to calculate batches to determine minimum packed dataset length
+        # The number of times to calculate the batches to determine the minimum packed dataset length for the local rank
         self.num_count_samples = num_count_samples
-        # Minimum packed dataset length across all ranks (determined by gather/broadcast)
+        # the minimum packed dataset length across all ranks determined by a gather/broadcast
         self.len_across_ranks = None
 
-        # Cache for batches
-        self._batches = None
-
         if self.sequential and not isinstance(sampler, SequentialSampler):
             LOG.warning(
                 "using sequential sample packing with non-sequential sampler, did you want to also enable curriculum_sampling?"
             )
 
     def set_epoch(self, epoch: int):
-        """Set the epoch number, used for reproducible shuffling across epochs"""
         self.epoch = epoch
-        self._batches = None  # Invalidate batch cache
 
     def generate_batches(self, set_stats=False):
-        """
+        indices = [idx for idx in self.sampler]
-        Generate packed batches for training
 
-        Args:
-            set_stats: Whether to update efficiency statistics
-
-        Returns:
-            List of batches, where each batch contains multiple bins,
-            and each bin contains multiple sequence indices
-        """
-        if self._batches is not None:
-            return self._batches
-
-        # Get indices from the sampler
-        indices = [  # pylint: disable=unnecessary-comprehension
-            idx for idx in self.sampler
-        ]
-
-        # Get lengths of the selected sequences
         lengths = self.lengths[indices]
+        lengths_cumsum = np.cumsum(lengths)
 
-        # Pack sequences into bins using either sequential or parallel packing
         if self.sequential:
-            bins, total_used, total_slots = allocate_sequentially(
+            batches, total_used, total_slots = allocate_sequentially(
-                lengths,
+                lengths=lengths,
                 rank=0,
-                bin_capacity=self.batch_max_len,
+                c=self.batch_max_len,
-                num_ranks=1,
+                n=1,
             )
-            # Map bin indices back to original indices
-            bins = [[indices[b_idx] for b_idx in bin_indices] for bin_indices in bins]
         else:
-            # Use parallel packing
+            batches, total_used, total_slots = allocate(
-            all_bins = pack_parallel(
+                lengths=lengths,
-                lengths,
+                lengths_cumsum=lengths_cumsum,
-                bin_capacity=self.batch_max_len,
+                rank=0,
-                group_size=self.group_size,
+                c=self.batch_max_len,
-                bin_size=self.bin_size,
+                n=1,
-                num_processes=self.num_processes,
-                safe_mode=self.safe_mode,
             )
 
-            # Map bin indices back to original indices
-            bins = [
-                [indices[b_idx] for b_idx in bin_indices] for bin_indices in all_bins
-            ]
-
-            # Calculate efficiency statistics
-            total_used = lengths.sum()
-            total_slots = len(all_bins) * self.batch_max_len
-
-        # Group bins into batches (each batch contains batch_size bins)
         batches = [
-            bins[i : i + self.batch_size] for i in range(0, len(bins), self.batch_size)
+            [
+                [indices[b_idx] for b_idx in batch]
+                for batch in batches[i : i + self.batch_size]
+            ]
+            for i in range(0, len(batches), self.batch_size)
         ]
 
-        # Drop last batch if requested and it's incomplete
+        # statistics
-        if self.drop_last and len(batches[-1]) < self.batch_size:
-            batches = batches[:-1]
-            # Adjust total_slots if we dropped a batch
-            if not self.sequential:
-                total_slots -= (self.batch_size - len(batches[-1])) * self.batch_max_len
-
-        # Update statistics if requested
         if set_stats:
-            self.total_tokens_used += total_used
+            self.eff_total_used += total_used
-            self.total_token_slots += total_slots
+            self.eff_total_slots += total_slots
 
-        self._batches = batches
         return batches
 
     def __iter__(self):
-        """
-        Return an iterator over batches
-
-        The batches are truncated to match the minimum number of batches across all ranks
-        to ensure distributed training balance
-        """
         batches = self.generate_batches(set_stats=True)
         if self.len_across_ranks:
-            # Truncate batches to ensure all ranks have the same number of batches
+            # make sure the batches we iterate over is truncated to the same min length across all ranks
             batches = batches[: self.len_across_ranks]
         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
-        Calculate the packing efficiency (ratio of tokens used to total token slots)
-        Higher is better - 1.0 would mean perfect packing with no wasted space
-        """
-        if self.total_token_slots == 0:
-            self.generate_batches(set_stats=True)
-        if self.total_token_slots == 0:
-            return 0.0
-        # Return a Python float instead of potentially a numpy float
-        return float(self.total_tokens_used / self.total_token_slots)
 
     def gather_efficiency(self):
-        """
+        def calc_sample_packing_eff_est(estimates: List[float]):
-        Gather and synchronize packing efficiency estimates across all distributed ranks
-        Returns a conservative efficiency estimate based on the measurements
-        """
-
-        def calc_sample_packing_eff_est(estimates: list[float]):
             LOG.debug(f"sample_packing_eff_est across ranks: {repr(estimates)}")
-            # Use 99.7% of max observed efficiency as a safe estimate
+            return math.floor(0.997 * max(estimates))
-            max_eff = max(float(eff) for eff in estimates)
-            return math.floor(0.997 * max_eff)
 
-        # Gather efficiency from all ranks and apply the calculation function
         sample_packing_actual_eff_all = reduce_and_broadcast(
-            lambda: float(self.efficiency()),  # pylint: disable=unnecessary-lambda
+            lambda: self.efficiency(),  # pylint: disable=unnecessary-lambda
             calc_sample_packing_eff_est,
         )
-
-        # Quantize to 0.5% intervals for stability
         sample_packing_eff_est = (
             math.ceil(sample_packing_actual_eff_all * 200.0) / 200.0
         )
         return sample_packing_eff_est
 
     def gather_len_batches(self, num):
-        """
-        Gather and synchronize batch counts across all distributed ranks
-        Returns the minimum number of batches available on any rank
-        """
-
         def calc_min_len(estimates: list[(int, float)]):
             LOG.info(f"gather_len_batches: {repr(estimates)}")
             return math.floor(min(estimates))
 
-        # Find minimum batch count across ranks to ensure balance
         min_len_batches = reduce_and_broadcast(lambda: num, calc_min_len)
         return min_len_batches
 
     def __len__(self):
-        """
+        if not self.len_across_ranks:
-        Return the total number of batches that will be yielded by this sampler
+            len_batches = min(
-
+                [self.num_batches() for _ in range(self.num_count_samples)]
-        This is calculated as the minimum number of batches available on any rank
-        to ensure balanced distributed training
-        """
-        if self._batches is None:
-            self._batches = self.generate_batches(set_stats=True)
-
-        if self.len_across_ranks is None:
-            # Sample multiple times to get stable estimate
-            len_batches = min(  # pylint: disable=consider-using-generator
-                [len(self._batches) for _ in range(self.num_count_samples)]
             )
-            # Gather minimum across all ranks
             self.len_across_ranks = self.gather_len_batches(len_batches)
-
         return self.len_across_ranks