forward_step.py

# Copyright (c) 2024, NVIDIA CORPORATION. All rights reserved.

"""Forward step utilities."""

from collections.abc import Iterable

import torch

from megatron.training import get_args
from megatron.core import mpu, InferenceParams
from .communication import (
    send_to_next_pipeline_rank,
    recv_from_prev_pipeline_rank_)


class ForwardStep:
    """Forward step function with all the communications.
    We use a class here to hide the inference parameters
    from the outside caller."""

    def __init__(self, model, max_batch_size, max_sequence_length):
        """Set values so we don't need to do it multiple times."""
        # Make sure model is in eval mode.
        assert not isinstance(model, Iterable), \
            'interleaving schedule is not supported for inference'
        model.eval()
        self.model = model
        # Initialize inference parameters.
        self.inference_params = InferenceParams(max_batch_size,
                                                max_sequence_length)
        # Pipelining arguments.
        args = get_args()
        self.pipeline_size_larger_than_one = (
            args.pipeline_model_parallel_size > 1)
        # Threshold for whether we split up the batch for pipelining.
        self.pipelining_batch_x_seqlen = \
            args.inference_batch_times_seqlen_threshold

    def _forward(self, tokens, position_ids, attention_mask):
        return self.model(tokens, position_ids, attention_mask, inference_params=self.inference_params)

    def __call__(self, tokens, position_ids, attention_mask, recv_buffer_seq_length=None):
        """Invocation of the forward methods. Note that self.inference_params
        is being modified by the forward step."""
        # Pipelining case.
        # This runs only if current_batch_x_seqlen > args.inference_batch_times_seqlen_threshold
        # and requires setting args.pipeline_model_parallel > 1. The batch will be split into
        # smaller microbatches to be pipelined through the stages.
        if self.pipeline_size_larger_than_one:
            seq_len = tokens.size(1) if recv_buffer_seq_length is None else recv_buffer_seq_length
            current_batch_x_seqlen = tokens.size(0) * seq_len
            if current_batch_x_seqlen >= self.pipelining_batch_x_seqlen:
                micro_batch_size = \
                    max(1, self.pipelining_batch_x_seqlen // seq_len)
                return self._with_pipelining_forward_step(tokens,
                                                          position_ids,
                                                          attention_mask,
                                                          micro_batch_size,
                                                          recv_buffer_seq_length=recv_buffer_seq_length)

        recv_buffer = None
        if recv_buffer_seq_length is not None:
            recv_buffer = _allocate_recv_buffer(tokens.size(0), recv_buffer_seq_length)

        return self._no_pipelining_forward_step(tokens,
                                                position_ids,
                                                attention_mask,
                                                recv_buffer=recv_buffer)


    def _forward_step_helper(self, tokens, position_ids, attention_mask, recv_buffer=None):
        """Single forward step. Update the allocate memory flag so
        only the first time the memory is allocated."""
        batch_size = tokens.size(0)
        sequence_length = tokens.size(1)

        if recv_buffer is None:
            recv_buffer = _allocate_recv_buffer(batch_size, sequence_length)

        # Receive from previous stage.
        if recv_buffer is not None and torch.numel(recv_buffer) > 0:
            recv_from_prev_pipeline_rank_(recv_buffer)

        # Forward pass through the model.
        if not mpu.is_pipeline_first_stage():
            self.model.set_input_tensor(recv_buffer)
        output_tensor = self._forward(tokens, position_ids, attention_mask)
        if isinstance(output_tensor, tuple):
            output_tensor = output_tensor[0]

        # Send output to the next stage.
        send_to_next_pipeline_rank(output_tensor)

        return output_tensor



    def _no_pipelining_forward_step(self, tokens, position_ids, attention_mask,
                                    recv_buffer=None):
        """If recv_buffer is none, we will allocate one on the fly."""
        # Run a simple forward pass.
        output_tensor = self._forward_step_helper(tokens, position_ids,
                                                  attention_mask, recv_buffer=recv_buffer)
        # Update the sequence length offset.
        self.inference_params.sequence_len_offset += tokens.size(1)

        logits = None
        if mpu.is_pipeline_last_stage():
            logits = output_tensor

        return logits


    def _with_pipelining_forward_step(self, tokens, position_ids, attention_mask, micro_batch_size, recv_buffer_seq_length=None):
        """No interleaving is supported."""
        batch_size = tokens.size(0)
        sequence_length = tokens.size(1) if recv_buffer_seq_length is None else recv_buffer_seq_length

        # Divide the batch dimension into micro batches.
        num_micro_batches, last_chunk = divmod(batch_size,
                                            micro_batch_size)
        if last_chunk > 0:
            num_micro_batches += 1

        # Preallocate memory for output logits.
        logits = None
        if mpu.is_pipeline_last_stage():
            args = get_args()
            logits = torch.empty(
                (batch_size, sequence_length, args.padded_vocab_size),
                dtype=torch.float32, device=torch.cuda.current_device())

        # Preallocate recv buffer.
        recv_buffer = _allocate_recv_buffer(micro_batch_size, sequence_length)

        for micro_batch_index in range(num_micro_batches):
            # Slice among the batch dimenion.
            start = micro_batch_index * micro_batch_size
            end = min(start + micro_batch_size, batch_size)
            this_micro_batch_size = end - start
            tokens2use = tokens[start:end, ...]
            position_ids2use = position_ids[start:end, ...]

            # Run a simple forward pass.
            if this_micro_batch_size != micro_batch_size:
                recv_buffer = None
            output = self._forward_step_helper(tokens2use, position_ids2use, attention_mask, recv_buffer=recv_buffer)

            # Adjust the batch size offset to account for the micro-batch.
            self.inference_params.batch_size_offset += this_micro_batch_size

            # Copy logits.
            if mpu.is_pipeline_last_stage():
                logits[start:end, ...] = output

        # Once we are done with all the micro-batches, we can
        # adjust the sequence length offset.
        self.inference_params.sequence_len_offset += tokens.size(1)
        # and reset the batch size offset
        self.inference_params.batch_size_offset = 0

        return logits


def _get_recv_buffer_dtype(args):
    """Receive happens between the layers."""
    if args.fp32_residual_connection:
        return torch.float
    return args.params_dtype

def _allocate_recv_buffer(batch_size, sequence_length):
    """Receive happens between the layers with size [s, b, h]."""
    if mpu.is_pipeline_first_stage():
        return None
    args = get_args()
    recv_size = (sequence_length, batch_size, args.hidden_size)
    return torch.empty(recv_size,
                       dtype=_get_recv_buffer_dtype(args),
                       device=torch.cuda.current_device())