# Copyright (c) Facebook, Inc. and its affiliates.
# Modified by BaseDetection, Inc. and its affiliates.
import itertools
import math
from collections import defaultdict
import numpy as np
import torch
from torch.utils.data.sampler import Sampler
from cvpods.utils import comm
from ..registry import SAMPLERS
[docs]@SAMPLERS.register()
class DistributedGroupSampler(Sampler):
"""Sampler that restricts data loading to a subset of the dataset.
It is especially useful in conjunction with
:class:`torch.nn.parallel.DistributedDataParallel`. In such case, each
process can pass a DistributedSampler instance as a DataLoader sampler,
and load a subset of the original dataset that is exclusive to it.
.. note::
Dataset is assumed to be of constant size.
"""
[docs] def __init__(self,
dataset,
samples_per_gpu=1,
num_replicas=None,
rank=None):
"""
Args:
dataset (Dataset): Dataset used for sampling.
num_replicas (optional): Number of processes participating in
distributed training.
rank (optional): Rank of the current process within num_replicas.
"""
_rank = comm.get_rank()
_num_replicas = comm.get_world_size()
if num_replicas is None:
num_replicas = _num_replicas
if rank is None:
rank = _rank
self.dataset = dataset
self.samples_per_gpu = samples_per_gpu
self.num_replicas = num_replicas
self.rank = rank
self.epoch = 0
assert hasattr(self.dataset, 'aspect_ratios')
self.aspect_ratios = self.dataset.aspect_ratios
self.group_sizes = np.bincount(self.aspect_ratios)
self.num_samples = 0
for i, j in enumerate(self.group_sizes):
self.num_samples += int(
math.ceil(
self.group_sizes[i] * 1.0 / self.samples_per_gpu / self.num_replicas
)
) * self.samples_per_gpu
self.total_size = self.num_samples * self.num_replicas
def __iter__(self):
# deterministically shuffle based on epoch
g = torch.Generator()
g.manual_seed(self.epoch)
indices = []
for i, size in enumerate(self.group_sizes):
if size > 0:
indice = np.where(self.aspect_ratios == i)[0]
assert len(indice) == size
indice = indice[list(torch.randperm(int(size),
generator=g))].tolist()
extra = int(
math.ceil(
size * 1.0 / self.samples_per_gpu / self.num_replicas)
) * self.samples_per_gpu * self.num_replicas - len(indice)
# pad indice
tmp = indice.copy()
for _ in range(extra // size):
indice.extend(tmp)
indice.extend(tmp[:extra % size])
indices.extend(indice)
assert len(indices) == self.total_size
indices = [
indices[j] for i in list(
torch.randperm(
len(indices) // self.samples_per_gpu, generator=g))
for j in range(i * self.samples_per_gpu, (i + 1) * self.samples_per_gpu)
]
# subsample
offset = self.num_samples * self.rank
indices = indices[offset:offset + self.num_samples]
assert len(indices) == self.num_samples
return iter(indices)
def __len__(self):
return self.num_samples
[docs] def set_epoch(self, epoch):
self.epoch = epoch
[docs]@SAMPLERS.register()
class RepeatFactorTrainingSampler(Sampler):
"""
Similar to TrainingSampler, but suitable for training on class imbalanced datasets
like LVIS. In each epoch, an image may appear multiple times based on its "repeat
factor". The repeat factor for an image is a function of the frequency the rarest
category labeled in that image. The "frequency of category c" in [0, 1] is defined
as the fraction of images in the training set (without repeats) in which category c
appears.
See https://arxiv.org/abs/1908.03195 (>= v2) Appendix B.2.
"""
[docs] def __init__(self, dataset, repeat_thresh, shuffle=True, seed=None):
"""
Args:
dataset (Dataset): dataset used for sampling.
repeat_thresh (float): frequency threshold below which data is repeated.
shuffle (bool): whether to shuffle the indices or not.
seed (int): the initial seed of the shuffle. Must be the same
across all workers. If None, will use a random seed shared
among workers (require synchronization among all workers).
"""
self._shuffle = shuffle
if seed is None:
seed = comm.shared_random_seed()
self._seed = int(seed)
self._rank = comm.get_rank()
self._world_size = comm.get_world_size()
dataset_dicts = []
if hasattr(dataset, "datasets"):
for d in dataset.datasets:
dataset_dicts += d.dataset_dicts
else:
dataset_dicts = dataset.dataset_dicts
# Get fractional repeat factors and split into whole number (_int_part)
# and fractional (_frac_part) parts.
rep_factors = self._get_repeat_factors(dataset_dicts, repeat_thresh)
self._int_part = torch.trunc(rep_factors)
self._frac_part = rep_factors - self._int_part
def _get_repeat_factors(self, dataset_dicts, repeat_thresh):
"""
Compute (fractional) per-image repeat factors.
Args:
See __init__.
Returns:
torch.Tensor: the i-th element is the repeat factor for the dataset image
at index i.
"""
# 1. For each category c, compute the fraction of images that contain it: f(c)
category_freq = defaultdict(int)
for dataset_dict in dataset_dicts: # For each image (without repeats)
cat_ids = {ann["category_id"] for ann in dataset_dict["annotations"]}
for cat_id in cat_ids:
category_freq[cat_id] += 1
num_images = len(dataset_dicts)
for k, v in category_freq.items():
category_freq[k] = v / num_images
# 2. For each category c, compute the category-level repeat factor:
# r(c) = max(1, sqrt(t / f(c)))
category_rep = {
cat_id: max(1.0, math.sqrt(repeat_thresh / cat_freq))
for cat_id, cat_freq in category_freq.items()
}
# 3. For each image I, compute the image-level repeat factor:
# r(I) = max_{c in I} r(c)
rep_factors = []
for dataset_dict in dataset_dicts:
cat_ids = {ann["category_id"] for ann in dataset_dict["annotations"]}
rep_factor = max({category_rep[cat_id] for cat_id in cat_ids})
rep_factors.append(rep_factor)
return torch.tensor(rep_factors, dtype=torch.float32)
def _get_epoch_indices(self, generator):
"""
Create a list of dataset indices (with repeats) to use for one epoch.
Args:
generator (torch.Generator): pseudo random number generator used for
stochastic rounding.
Returns:
torch.Tensor: list of dataset indices to use in one epoch. Each index
is repeated based on its calculated repeat factor.
"""
# Since repeat factors are fractional, we use stochastic rounding so
# that the target repeat factor is achieved in expectation over the
# course of training
rands = torch.rand(len(self._frac_part), generator=generator)
rep_factors = self._int_part + (rands < self._frac_part).float()
# Construct a list of indices in which we repeat images as specified
indices = []
for dataset_index, rep_factor in enumerate(rep_factors):
indices.extend([dataset_index] * int(rep_factor.item()))
return torch.tensor(indices, dtype=torch.int64)
def __iter__(self):
start = self._rank
yield from itertools.islice(self._infinite_indices(), start, None, self._world_size)
def _infinite_indices(self):
g = torch.Generator()
g.manual_seed(self._seed)
while True:
# Sample indices with repeats determined by stochastic rounding; each
# "epoch" may have a slightly different size due to the rounding.
indices = self._get_epoch_indices(g)
if self._shuffle:
randperm = torch.randperm(len(indices), generator=g)
yield from indices[randperm]
else:
yield from indices
[docs]@SAMPLERS.register()
class InferenceSampler(Sampler):
"""
Produce indices for inference.
Inference needs to run on the __exact__ set of samples,
therefore when the total number of samples is not divisible by the number of workers,
this sampler produces different number of samples on different workers.
"""
[docs] def __init__(self, size: int):
"""
Args:
size (int): the total number of data of the underlying dataset to sample from
"""
self._size = size
assert size > 0
self._rank = comm.get_rank()
self._world_size = comm.get_world_size()
shard_size = (self._size - 1) // self._world_size + 1
begin = shard_size * self._rank
end = min(shard_size * (self._rank + 1), self._size)
self._local_indices = range(begin, end)
def __iter__(self):
yield from self._local_indices
def __len__(self):
return len(self._local_indices)