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# pylint: disable=too-many-instance-attributes, too-many-arguments, protected-access, too-many-branches
# pylint: disable=too-many-public-methods
"""A `Module` implement the `BaseModule` API by wrapping a `Symbol` and one or
more `Executor` for data parallelization.
"""
import logging
import warnings
from .. import context as ctx
from .. import optimizer as opt
from .. import ndarray as nd
from .executor_group import DataParallelExecutorGroup
from ..model import _create_kvstore, _initialize_kvstore, _update_params, _update_params_on_kvstore
from ..model import load_checkpoint
from ..initializer import Uniform, InitDesc
from ..io import DataDesc
from ..ndarray import zeros
from .base_module import BaseModule, _check_input_names, _parse_data_desc
[docs]class Module(BaseModule):
"""Module is a basic module that wrap a `Symbol`. It is functionally the same
as the `FeedForward` model, except under the module API.
Parameters
----------
symbol : Symbol
data_names : list of str
Defaults to `('data')` for a typical model used in image classification.
label_names : list of str
Defaults to `('softmax_label')` for a typical model used in image
classification.
logger : Logger
Defaults to `logging`.
context : Context or list of Context
Defaults to ``mx.cpu()``.
work_load_list : list of number
Default ``None``, indicating uniform workload.
fixed_param_names: list of str
Default ``None``, indicating no network parameters are fixed.
state_names : list of str
states are similar to data and label, but not provided by data iterator.
Instead they are initialized to 0 and can be set by `set_states()`.
group2ctxs : dict of str to context or list of context,
or list of dict of str to context
Default is `None`. Mapping the `ctx_group` attribute to the context assignment.
compression_params : dict
Specifies type of gradient compression and additional arguments depending
on the type of compression being used. For example, 2bit compression requires a threshold.
Arguments would then be {'type':'2bit', 'threshold':0.5}
See mxnet.KVStore.set_gradient_compression method for more details on gradient compression.
"""
def __init__(self, symbol, data_names=('data',), label_names=('softmax_label',),
logger=logging, context=ctx.cpu(), work_load_list=None,
fixed_param_names=None, state_names=None, group2ctxs=None,
compression_params=None):
super(Module, self).__init__(logger=logger)
if isinstance(context, ctx.Context):
context = [context]
self._context = context
if work_load_list is None:
work_load_list = [1] * len(self._context)
assert len(work_load_list) == len(self._context)
self._work_load_list = work_load_list
self._group2ctxs = group2ctxs
self._symbol = symbol
data_names = list(data_names) if data_names is not None else []
label_names = list(label_names) if label_names is not None else []
state_names = list(state_names) if state_names is not None else []
fixed_param_names = list(fixed_param_names) if fixed_param_names is not None else []
_check_input_names(symbol, data_names, "data", True)
_check_input_names(symbol, label_names, "label", False)
_check_input_names(symbol, state_names, "state", True)
_check_input_names(symbol, fixed_param_names, "fixed_param", True)
arg_names = symbol.list_arguments()
input_names = data_names + label_names + state_names
self._param_names = [x for x in arg_names if x not in input_names]
self._fixed_param_names = fixed_param_names
self._aux_names = symbol.list_auxiliary_states()
self._data_names = data_names
self._label_names = label_names
self._state_names = state_names
self._output_names = symbol.list_outputs()
self._arg_params = None
self._aux_params = None
self._params_dirty = False
self._compression_params = compression_params
self._optimizer = None
self._kvstore = None
self._update_on_kvstore = None
self._updater = None
self._preload_opt_states = None
self._grad_req = None
self._exec_group = None
self._data_shapes = None
self._label_shapes = None
@staticmethod
[docs] def load(prefix, epoch, load_optimizer_states=False, **kwargs):
"""Creates a model from previously saved checkpoint.
Parameters
----------
prefix : str
path prefix of saved model files. You should have
"prefix-symbol.json", "prefix-xxxx.params", and
optionally "prefix-xxxx.states", where xxxx is the
epoch number.
epoch : int
epoch to load.
load_optimizer_states : bool
whether to load optimizer states. Checkpoint needs
to have been made with save_optimizer_states=True.
data_names : list of str
Default is `('data')` for a typical model used in image classification.
label_names : list of str
Default is `('softmax_label')` for a typical model used in image
classification.
logger : Logger
Default is `logging`.
context : Context or list of Context
Default is ``cpu()``.
work_load_list : list of number
Default ``None``, indicating uniform workload.
fixed_param_names: list of str
Default ``None``, indicating no network parameters are fixed.
"""
sym, args, auxs = load_checkpoint(prefix, epoch)
mod = Module(symbol=sym, **kwargs)
mod._arg_params = args
mod._aux_params = auxs
mod.params_initialized = True
if load_optimizer_states:
mod._preload_opt_states = '%s-%04d.states'%(prefix, epoch)
return mod
[docs] def save_checkpoint(self, prefix, epoch, save_optimizer_states=False):
"""Saves current progress to checkpoint.
Use `mx.callback.module_checkpoint` as `epoch_end_callback` to save during training.
Parameters
----------
prefix : str
The file prefix to checkpoint to.
epoch : int
The current epoch number.
save_optimizer_states : bool
Whether to save optimizer states to continue training.
"""
self._symbol.save('%s-symbol.json'%prefix)
param_name = '%s-%04d.params' % (prefix, epoch)
self.save_params(param_name)
logging.info('Saved checkpoint to \"%s\"', param_name)
if save_optimizer_states:
state_name = '%s-%04d.states' % (prefix, epoch)
self.save_optimizer_states(state_name)
logging.info('Saved optimizer state to \"%s\"', state_name)
def _reset_bind(self):
"""Internal function to reset binded state."""
self.binded = False
self._exec_group = None
self._data_shapes = None
self._label_shapes = None
@property
def data_names(self):
"""A list of names for data required by this module."""
return self._data_names
@property
def label_names(self):
"""A list of names for labels required by this module."""
return self._label_names
@property
def output_names(self):
"""A list of names for the outputs of this module."""
return self._output_names
@property
def data_shapes(self):
"""Gets data shapes.
Returns
-------
A list of `(name, shape)` pairs.
"""
assert self.binded
return self._data_shapes
@property
def label_shapes(self):
"""Gets label shapes.
Returns
-------
A list of `(name, shape)` pairs.
The return value could be ``None`` if
the module does not need labels, or if the module is not bound for
training (in this case, label information is not available).
"""
assert self.binded
return self._label_shapes
@property
def output_shapes(self):
"""Gets output shapes.
Returns
-------
A list of `(name, shape)` pairs.
"""
assert self.binded
return self._exec_group.get_output_shapes()
[docs] def get_params(self):
"""Gets current parameters.
Returns
-------
`(arg_params, aux_params)`
A pair of dictionaries each mapping parameter names to NDArray values.
"""
assert self.binded and self.params_initialized
if self._params_dirty:
self._sync_params_from_devices()
return (self._arg_params, self._aux_params)
[docs] def init_params(self, initializer=Uniform(0.01), arg_params=None, aux_params=None,
allow_missing=False, force_init=False, allow_extra=False):
"""Initializes the parameters and auxiliary states.
Parameters
----------
initializer : Initializer
Called to initialize parameters if needed.
arg_params : dict
If not ``None``, should be a dictionary of existing arg_params. Initialization
will be copied from that.
aux_params : dict
If not ``None``, should be a dictionary of existing aux_params. Initialization
will be copied from that.
allow_missing : bool
If ``True``, params could contain missing values, and the initializer will be
called to fill those missing params.
force_init : bool
If ``True``, will force re-initialize even if already initialized.
allow_extra : boolean, optional
Whether allow extra parameters that are not needed by symbol.
If this is True, no error will be thrown when arg_params or aux_params
contain extra parameters that is not needed by the executor.
"""
if self.params_initialized and not force_init:
warnings.warn("Parameters already initialized and force_init=False. "
"init_params call ignored.", stacklevel=2)
return
assert self.binded, 'call bind before initializing the parameters'
def _impl(name, arr, cache):
"""Internal helper for parameter initialization"""
if cache is not None:
if name in cache:
cache_arr = cache[name]
# just in case the cached array is just the target itself
if cache_arr is not arr:
cache_arr.copyto(arr)
else:
if not allow_missing:
raise RuntimeError("%s is not presented" % name)
if initializer is not None:
initializer(name, arr)
else:
initializer(name, arr)
attrs = self._symbol.attr_dict()
for name, arr in sorted(self._arg_params.items()):
desc = InitDesc(name, attrs.get(name, None))
_impl(desc, arr, arg_params)
for name, arr in sorted(self._aux_params.items()):
desc = InitDesc(name, attrs.get(name, None))
_impl(desc, arr, aux_params)
self.params_initialized = True
self._params_dirty = False
# copy the initialized parameters to devices
self._exec_group.set_params(self._arg_params, self._aux_params,
allow_extra=allow_extra)
[docs] def set_params(self, arg_params, aux_params, allow_missing=False, force_init=True,
allow_extra=False):
"""Assigns parameter and aux state values.
Parameters
----------
arg_params : dict
Dictionary of name to `NDArray`.
aux_params : dict
Dictionary of name to `NDArray`.
allow_missing : bool
If ``True``, params could contain missing values, and the initializer will be
called to fill those missing params.
force_init : bool
If ``True``, will force re-initialize even if already initialized.
allow_extra : boolean, optional
Whether allow extra parameters that are not needed by symbol.
If this is True, no error will be thrown when arg_params or aux_params
contain extra parameters that is not needed by the executor.
Examples
--------
>>> # An example of setting module parameters.
>>> sym, arg_params, aux_params = mx.model.load_checkpoint(model_prefix, n_epoch_load)
>>> mod.set_params(arg_params=arg_params, aux_params=aux_params)
"""
if not allow_missing:
self.init_params(initializer=None, arg_params=arg_params, aux_params=aux_params,
allow_missing=allow_missing, force_init=force_init,
allow_extra=allow_extra)
return
if self.params_initialized and not force_init:
warnings.warn("Parameters already initialized and force_init=False. "
"set_params call ignored.", stacklevel=2)
return
self._exec_group.set_params(arg_params, aux_params, allow_extra=allow_extra)
# because we didn't update self._arg_params, they are dirty now.
self._params_dirty = True
self.params_initialized = True
[docs] def bind(self, data_shapes, label_shapes=None, for_training=True,
inputs_need_grad=False, force_rebind=False, shared_module=None,
grad_req='write'):
"""Binds the symbols to construct executors. This is necessary before one
can perform computation with the module.
Parameters
----------
data_shapes : list of (str, tuple)
Typically is ``data_iter.provide_data``.
label_shapes : list of (str, tuple)
Typically is ``data_iter.provide_label``.
for_training : bool
Default is ``True``. Whether the executors should be bound for training.
inputs_need_grad : bool
Default is ``False``. Whether the gradients to the input data need to be computed.
Typically this is not needed. But this might be needed when implementing composition
of modules.
force_rebind : bool
Default is ``False``. This function does nothing if the executors are already
bound. But with this ``True``, the executors will be forced to rebind.
shared_module : Module
Default is ``None``. This is used in bucketing. When not ``None``, the shared module
essentially corresponds to a different bucket -- a module with different symbol
but with the same sets of parameters (e.g. unrolled RNNs with different lengths).
"""
# force rebinding is typically used when one want to switch from
# training to prediction phase.
if force_rebind:
self._reset_bind()
if self.binded:
self.logger.warning('Already bound, ignoring bind()')
return
self.for_training = for_training
self.inputs_need_grad = inputs_need_grad
self._grad_req = grad_req
if not for_training:
assert not inputs_need_grad
else:
pass
# this is not True, as some module might not contains a loss function
# that consumes the labels
# assert label_shapes is not None
self._data_shapes, self._label_shapes = _parse_data_desc(
self.data_names, self.label_names, data_shapes, label_shapes)
if shared_module is not None:
assert isinstance(shared_module, Module) and \
shared_module.binded and shared_module.params_initialized
shared_group = shared_module._exec_group
assert len(shared_group.execs) >= len(self._context)
else:
shared_group = None
self._exec_group = DataParallelExecutorGroup(self._symbol, self._context,
self._work_load_list, self._data_shapes,
self._label_shapes, self._param_names,
for_training, inputs_need_grad,
shared_group, logger=self.logger,
fixed_param_names=self._fixed_param_names,
grad_req=grad_req, group2ctxs=self._group2ctxs,
state_names=self._state_names)
self._total_exec_bytes = self._exec_group._total_exec_bytes
if shared_module is not None:
self.params_initialized = True
self._arg_params = shared_module._arg_params
self._aux_params = shared_module._aux_params
elif self.params_initialized:
# if the parameters are already initialized, we are re-binding
# so automatically copy the already initialized params
self._exec_group.set_params(self._arg_params, self._aux_params)
else:
assert self._arg_params is None and self._aux_params is None
param_arrays = [
zeros(shape=x[0].shape, dtype=x[0].dtype, stype=x[0].stype)
for x in self._exec_group.param_arrays
]
self._arg_params = {name:arr for name, arr in zip(self._param_names, param_arrays)}
aux_arrays = [
zeros(x[0].shape, dtype=x[0].dtype)
for x in self._exec_group.aux_arrays
]
self._aux_params = {name:arr for name, arr in zip(self._aux_names, aux_arrays)}
if shared_module is not None and shared_module.optimizer_initialized:
self.borrow_optimizer(shared_module)
self.binded = True
[docs] def reshape(self, data_shapes, label_shapes=None):
"""Reshapes the module for new input shapes.
Parameters
----------
data_shapes : list of (str, tuple)
Typically is ``data_iter.provide_data``.
label_shapes : list of (str, tuple)
Typically is ``data_iter.provide_label``.
"""
assert self.binded
self._data_shapes, self._label_shapes = _parse_data_desc(
self.data_names, self.label_names, data_shapes, label_shapes)
self._exec_group.reshape(self._data_shapes, self._label_shapes)
[docs] def init_optimizer(self, kvstore='local', optimizer='sgd',
optimizer_params=(('learning_rate', 0.01),), force_init=False):
"""Installs and initializes optimizers.
Parameters
----------
kvstore : str or KVStore
Default `'local'`.
optimizer : str or Optimizer
Default `'sgd'`
optimizer_params : dict
Default `(('learning_rate', 0.01),)`. The default value is not a dictionary,
just to avoid pylint warning of dangerous default values.
force_init : bool
Default ``False``, indicating whether we should force re-initializing the
optimizer in the case an optimizer is already installed.
"""
assert self.binded and self.params_initialized
if self.optimizer_initialized and not force_init:
self.logger.warning('optimizer already initialized, ignoring...')
return
if self._params_dirty:
self._sync_params_from_devices()
(kvstore, update_on_kvstore) = \
_create_kvstore(kvstore, len(self._context), self._arg_params)
batch_size = self._exec_group.batch_size
if kvstore and 'dist' in kvstore.type and '_sync' in kvstore.type:
batch_size *= kvstore.num_workers
rescale_grad = 1.0/batch_size
idx2name = {}
if update_on_kvstore:
idx2name.update(enumerate(self._exec_group.param_names))
else:
for k in range(len(self._context)):
idx2name.update({i*len(self._context)+k: n
for i, n in enumerate(self._exec_group.param_names)})
if isinstance(optimizer, str):
optimizer_params = dict(optimizer_params)
if 'rescale_grad' not in optimizer_params:
optimizer_params['rescale_grad'] = rescale_grad
optimizer = opt.create(optimizer,
sym=self.symbol, param_idx2name=idx2name,
**optimizer_params)
else:
assert isinstance(optimizer, opt.Optimizer)
if optimizer.rescale_grad != rescale_grad:
#pylint: disable=no-member
warnings.warn(
"Optimizer created manually outside Module but rescale_grad " +
"is not normalized to 1.0/batch_size/num_workers (%s vs. %s). "%(
optimizer.rescale_grad, rescale_grad) +
"Is this intended?", stacklevel=2)
if not optimizer.idx2name:
optimizer.idx2name = idx2name.copy()
self._optimizer = optimizer
self._kvstore = kvstore
self._update_on_kvstore = update_on_kvstore
self._updater = None
if kvstore:
if self._compression_params:
kvstore.set_gradient_compression(self._compression_params)
if update_on_kvstore:
kvstore.set_optimizer(self._optimizer)
# copy initialized local parameters to kvstore
_initialize_kvstore(kvstore=kvstore,
param_arrays=self._exec_group.param_arrays,
arg_params=self._arg_params,
param_names=self._param_names,
update_on_kvstore=update_on_kvstore)
if not update_on_kvstore:
self._updater = opt.get_updater(optimizer)
self.optimizer_initialized = True
if self._preload_opt_states is not None:
self.load_optimizer_states(self._preload_opt_states)
self._preload_opt_states = None
[docs] def borrow_optimizer(self, shared_module):
"""Borrows optimizer from a shared module. Used in bucketing, where exactly the same
optimizer (esp. kvstore) is used.
Parameters
----------
shared_module : Module
"""
assert shared_module.optimizer_initialized
self._optimizer = shared_module._optimizer
self._kvstore = shared_module._kvstore
self._update_on_kvstore = shared_module._update_on_kvstore
self._updater = shared_module._updater
self.optimizer_initialized = True
[docs] def forward(self, data_batch, is_train=None):
"""Forward computation. It supports data batches with different shapes, such as
different batch sizes or different image sizes.
If reshaping of data batch relates to modification of symbol or module, such as
changing image layout ordering or switching from training to predicting, module
rebinding is required.
See Also
----------
:meth:`BaseModule.forward`.
Parameters
----------
data_batch : DataBatch
Could be anything with similar API implemented.
is_train : bool
Default is ``None``, which means ``is_train`` takes the value of ``self.for_training``.
"""
assert self.binded and self.params_initialized
curr_data_shapes = tuple(i.shape for i in self._data_shapes)
if isinstance(data_batch, list):
assert data_batch is not None, "Encountered empty data batch"
new_data_shapes = []
for i in range(len(data_batch[0].data)):
shape = data_batch[0].data[i].shape
for db in data_batch:
assert shape == db.data[i].shape, \
"All data batches in a list need to have the same shape"
new_batch_size = len(data_batch) * shape[0]
new_data_shapes.append((new_batch_size,) + shape[1:])
new_data_shapes = tuple(new_data_shapes)
else:
new_data_shapes = tuple(i.shape for i in data_batch.data)
if curr_data_shapes != new_data_shapes:
if hasattr(data_batch, "provide_data") and data_batch.provide_data:
new_dshape = data_batch.provide_data
else:
new_dshape = [DataDesc(i.name, shape, i.dtype, i.layout) \
for i, shape in zip(self._data_shapes, new_data_shapes)]
if hasattr(data_batch, "provide_label") and data_batch.provide_label:
new_lshape = data_batch.provide_label
elif hasattr(data_batch, "label") and data_batch.label:
new_lshape = [DataDesc(i.name, j.shape, i.dtype, i.layout) \
for i, j in zip(self._label_shapes, data_batch.label)]
else:
new_lshape = None
self.reshape(new_dshape, new_lshape)
self._exec_group.forward(data_batch, is_train)
[docs] def backward(self, out_grads=None):
"""Backward computation.
See Also
----------
:meth:`BaseModule.backward`.
Parameters
----------
out_grads : NDArray or list of NDArray, optional
Gradient on the outputs to be propagated back.
This parameter is only needed when bind is called
on outputs that are not a loss function.
"""
assert self.binded and self.params_initialized
self._exec_group.backward(out_grads=out_grads)
[docs] def update(self):
"""Updates parameters according to the installed optimizer and the gradients computed
in the previous forward-backward batch.
When KVStore is used to update parameters for multi-device or multi-machine training,
a copy of the parameters are stored in KVStore. Note that for `row_sparse` parameters,
this function does update the copy of parameters in KVStore, but doesn't broadcast the
updated parameters to all devices / machines. Please call `prepare` to broadcast
`row_sparse` parameters with the next batch of data.
See Also
----------
:meth:`BaseModule.update`.
"""
assert self.binded and self.params_initialized and self.optimizer_initialized
self._params_dirty = True
if self._update_on_kvstore:
_update_params_on_kvstore(self._exec_group.param_arrays,
self._exec_group.grad_arrays,
self._kvstore, self._exec_group.param_names)
else:
_update_params(self._exec_group.param_arrays,
self._exec_group.grad_arrays,
updater=self._updater,
num_device=len(self._context),
kvstore=self._kvstore,
param_names=self._exec_group.param_names)
[docs] def get_outputs(self, merge_multi_context=True):
"""Gets outputs of the previous forward computation.
If ``merge_multi_context`` is ``True``, it is like ``[out1, out2]``. Otherwise, it
is like ``[[out1_dev1, out1_dev2], [out2_dev1, out2_dev2]]``. All the output
elements are `NDArray`. When `merge_multi_context` is `False`, those `NDArray`
might live on different devices.
Parameters
----------
merge_multi_context : bool
Default is ``True``. In the case when data-parallelism is used, the outputs
will be collected from multiple devices. A ``True`` value indicate that we
should merge the collected results so that they look like from a single
executor.
Returns
-------
list of NDArray or list of list of NDArray
Output.
"""
assert self.binded and self.params_initialized
return self._exec_group.get_outputs(merge_multi_context=merge_multi_context)
[docs] def get_states(self, merge_multi_context=True):
"""Gets states from all devices.
If `merge_multi_context` is ``True``, it is like ``[out1, out2]``. Otherwise, it
is like ``[[out1_dev1, out1_dev2], [out2_dev1, out2_dev2]]``. All the output
elements are `NDArray`.
Parameters
----------
merge_multi_context : bool
Default is ``True``. In the case when data-parallelism is used, the states
will be collected from multiple devices. A ``True`` value indicate that we
should merge the collected results so that they look like from a single
executor.
Returns
-------
list of NDArray or list of list of NDArray
States
"""
assert self.binded and self.params_initialized
return self._exec_group.get_states(merge_multi_context=merge_multi_context)
[docs] def set_states(self, states=None, value=None):
"""Sets value for states. Only one of the states & value can be specified.
Parameters
----------
states : list of list of NDArrays
source states arrays formatted like ``[[state1_dev1, state1_dev2],
[state2_dev1, state2_dev2]]``.
value : number
a single scalar value for all state arrays.
"""
assert self.binded and self.params_initialized
self._exec_group.set_states(states, value)
[docs] def update_metric(self, eval_metric, labels, pre_sliced=False):
"""Evaluates and accumulates evaluation metric on outputs of the last forward computation.
See Also
----------
:meth:`BaseModule.update_metric`.
Parameters
----------
eval_metric : EvalMetric
Evaluation metric to use.
labels : list of NDArray if `pre_sliced` parameter is set to `False`,
list of lists of NDArray otherwise. Typically `data_batch.label`.
pre_sliced: bool
Whether the labels are already sliced per device (default: False).
"""
self._exec_group.update_metric(eval_metric, labels, pre_sliced)
def _sync_params_from_devices(self):
"""Synchronizes parameters from devices to CPU. This function should be called after
calling `update` that updates the parameters on the devices, before one can read the
latest parameters from ``self._arg_params`` and ``self._aux_params``.
For row_sparse parameters on devices, ther are pulled from KVStore with all row ids.
"""
self._exec_group.get_params(self._arg_params, self._aux_params)
if self._kvstore and self._update_on_kvstore:
for param_name, param_val in sorted(self._arg_params.items()):
if param_val.stype == 'row_sparse':
row_ids = nd.arange(0, param_val.shape[0], dtype='int64')
self._kvstore.row_sparse_pull(param_name, param_val, row_ids=row_ids)
self._params_dirty = False
[docs] def save_optimizer_states(self, fname):
"""Saves optimizer (updater) state to a file.
Parameters
----------
fname : str
Path to output states file.
"""
assert self.optimizer_initialized
if self._update_on_kvstore:
self._kvstore.save_optimizer_states(fname)
else:
with open(fname, 'wb') as fout:
fout.write(self._updater.get_states())
[docs] def load_optimizer_states(self, fname):
"""Loads optimizer (updater) state from a file.
Parameters
----------
fname : str
Path to input states file.
"""
assert self.optimizer_initialized
if self._update_on_kvstore:
self._kvstore.load_optimizer_states(fname)
else:
self._updater.set_states(open(fname, 'rb').read())
[docs] def install_monitor(self, mon):
"""Installs monitor on all executors. """
assert self.binded
self._exec_group.install_monitor(mon)
[docs] def prepare(self, data_batch, sparse_row_id_fn=None):
'''Prepares the module for processing a data batch.
Usually involves switching bucket and reshaping.
For modules that contain `row_sparse` parameters in KVStore,
it prepares the `row_sparse` parameters based on the sparse_row_id_fn.
When KVStore is used to update parameters for multi-device or multi-machine training,
a copy of the parameters are stored in KVStore. Note that for `row_sparse` parameters,
the `update()` updates the copy of parameters in KVStore, but doesn't broadcast
the updated parameters to all devices / machines. The `prepare` function is used to
broadcast `row_sparse` parameters with the next batch of data.
Parameters
----------
data_batch : DataBatch
The current batch of data for forward computation.
sparse_row_id_fn : A callback function
The function takes `data_batch` as an input and returns a dict of
str -> NDArray. The resulting dict is used for pulling row_sparse
parameters from the kvstore, where the str key is the name of the param,
and the value is the row id of the param to pull.
'''
assert self.binded
if sparse_row_id_fn is not None:
if not self._kvstore or not self._update_on_kvstore:
warnings.warn(UserWarning("Parameters are not updated in the KVStore. "
"No need to call sparse_row_id_fn."))
else:
row_ids = sparse_row_id_fn(data_batch)
assert(isinstance(row_ids, dict)), "Expected dict output from sparse_row_id_fn"
for param_name, row_id in row_ids.items():
param_idx = self._exec_group.param_names.index(param_name)
param_val = self._exec_group.param_arrays[param_idx]
assert(isinstance(param_val, (tuple, list)))
if param_val[0].stype != 'row_sparse':
warnings.warn(UserWarning("%s.stype is not 'row_sparse'. No need to "
"perform row_sparse_pull." % param_name))
else:
self._kvstore.row_sparse_pull(param_name, param_val, row_ids=row_id,
priority=-param_idx)
