multipack w batch sampler (#795)

* test batch sampler w varying batch lens

* wip

* multipack batchsampler wip

* wip

* fix for prepare data loader to get correct # of steps based on gpues

* lint and clean up

* calculate len estimate

* fix total num steps calc

* add options for dataloader_num_workers and dataloader_pin_memory

* remove gitbook

* support prefetch_factor for dataloader optimization

* fix the kwarg

src/axolotl/utils/samplers/multipack.py

@@ -0,0 +1,193 @@
+# 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 (
+            world_size
+            * math.floor(
+                0.99
+                * lengths_sum_per_device
+                / self.packing_efficiency_estimate
+                // self.batch_max_len
+            )
+            - 1
+        )