Closed jasperzhong closed 3 years ago
jasperzhong
commented
3 years ago megatron/arguments.py
先确定tensor-model-parallel size (t)，然后确定pipeline-model-parallel size (p)，得到model-parallel size (t p). 最后确定data-parallel size (d = n // (tp))
log里面可以看到使用的parallel configuration.
jasperzhong
commented
3 years ago megatron/training.py
pretrain函数. 很多训练脚本的入口.
通信分两种: torchDDP, localDDP. localDDP就是不带overlap的通信. 不是很懂为什么搞一个这样的东西.文档说在某些情况下，localDDP还更好. 额.
注意: 如果使用了pipeline parallelism，必须使用localDDP.
fp16训练会使用这个classFloat16OptimizerWithFloat16Params. 看上去把apex里面做的事情又写了一遍？不过看上去支持bf16.
loss用到了lm_loss和sop_loss. sop_loss参见 https://github.com/vycezhong/read-papers/issues/19
train函数主要逻辑在train_step函数. 比较核心的逻辑. 流程无非是: forward + backward (overlapped with communication) + backward-embedding-all-reduce + update.
对于backward-embedding-all-reduce注释是这么说的:
# All-reduce word_embeddings' grad across first and last stages to ensure
# that word_embeddings parameters stay in sync.
# This should only run for models that support pipelined model parallelism
# (BERT and GPT-2).一般来讲，embedding layer和最后一层(MLP?)做一个weight sharing，但是在pipelined model parallelism里面，这两层不在一个worker上，所以需要在最后一个stage上创建一个word_embedding copy，而且这两个copy需要一个额外的all-reduce操作(对grad)保证参数一致. 确实.
jasperzhong
commented
3 years ago mpu: model parallel utilitympu/mappings.py
Megatron-LM论文里面描述过两个共轭的函数:
一种是forward的时候copy，backward的时候all-reduce.
class _CopyToModelParallelRegion(torch.autograd.Function):
"""Pass the input to the model parallel region."""
@staticmethod
def symbolic(graph, input_):
return input_
@staticmethod
def forward(ctx, input_):
return input_
@staticmethod
def backward(ctx, grad_output):
return _reduce(grad_output)一种是forward的时候all-reduce，backward的时候copy.
class _ReduceFromModelParallelRegion(torch.autograd.Function):
"""All-reduce the input from the model parallel region."""
@staticmethod
def symbolic(graph, input_):
return _reduce(input_)
@staticmethod
def forward(ctx, input_):
return _reduce(input_)
@staticmethod
def backward(ctx, grad_output):
return grad_output这两个函数背后的原因其实很好理解. 可以把all-reduce理解为一个sum操作，直接return _input理解为copy操作. copy操作的结果分到两个worker上，梯度来自于两部分，所以梯度需要做sum. 而sum操作的结果的梯度直接copy就行了.
还有一种函数是forward的时候all-gather，backward的时候scatter.
class _GatherFromModelParallelRegion(torch.autograd.Function):
"""Gather the input from model parallel region and concatinate."""
@staticmethod
def symbolic(graph, input_):
return _gather(input_)
@staticmethod
def forward(ctx, input_):
return _gather(input_)
@staticmethod
def backward(ctx, grad_output):
return _split(grad_output)mpu/layers.py
ColumnParallelLinear语义: Y = XA + b. A沿column partition. 这样Y的column也被partition了. 可选是否gather_output. 如果是True，则会调用all-gather使得每个GPU都有完整的Y; 否则，每个GPU都只拿到对应的partition.
Broadcast x Split(1) -> Split(1)
代码如下:
def forward(self, input_):
# Set up backprop all-reduce.
input_parallel = copy_to_tensor_model_parallel_region(input_)
# Matrix multiply.
bias = self.bias if not self.skip_bias_add else None
output_parallel = F.linear(input_parallel, self.weight, bias)
if self.gather_output:
# All-gather across the partitions.
output = gather_from_tensor_model_parallel_region(output_parallel)
else:
output = output_parallel
output_bias = self.bias if self.skip_bias_add else None
return output, output_bias注意:
gather_output为True，就需要forward的时候把Y_i做all-gather，backward的时候，完整的grad需要scatter到对应worker上. RowParallelLinear语义: Y = XA + b. A沿row partition，X沿column partition. 这样XA的结果需要做all-reduce.
Split(1) x Split(0) -> PartialSum
代码如下:
def forward(self, input_):
# Set up backprop all-reduce.
if self.input_is_parallel:
input_parallel = input_
else:
input_parallel = scatter_to_tensor_model_parallel_region(input_)
# Matrix multiply.
output_parallel = F.linear(input_parallel, self.weight)
# All-reduce across all the partitions.
output_ = reduce_from_tensor_model_parallel_region(output_parallel)
if not self.skip_bias_add:
output = output_ + self.bias if self.bias is not None else output_
output_bias = None
else:
output = output_
output_bias = self.bias
return output, output_bias注意:
VocabParallelEmbedding为了memory均衡，embedding也会做shard，而不是放在一个GPU上. 按照vocab维度做partition，每个worker拿到embedding的一部分，所以一部分输入找不到对应的embedding，所以embedding输出需要做一个all-reduce使得结果完整.
代码如下:
def forward(self, input_):
if self.tensor_model_parallel_size > 1:
# Build the mask.
input_mask = (input_ < self.vocab_start_index) | \
(input_ >= self.vocab_end_index)
# Mask the input.
masked_input = input_.clone() - self.vocab_start_index
masked_input[input_mask] = 0
else:
masked_input = input_
# Get the embeddings.
output_parallel = F.embedding(masked_input, self.weight,
self.padding_idx, self.max_norm,
self.norm_type, self.scale_grad_by_freq,
self.sparse)
# Mask the output embedding.
if self.tensor_model_parallel_size > 1:
output_parallel[input_mask, :] = 0.0
# Reduce across all the model parallel GPUs.
output = reduce_from_tensor_model_parallel_region(output_parallel)
return output注意:
mpu/cross_entropy.py
这个理解起来有点难度，有些细节没看懂，大致idea是可以理解的. idea在Megatron-LM论文里面有提到: all-reduce loss (size = b x s)而不是logits (size = b x s x v).
mpu/random.py
CheckpointFunction: gradient checkpoint实现. 注意要保存forward时候的rng_state. 在backward的时候，首先，为了保证recompute的结果和forward时候是一样的，需要restoreforward时候rng_state. 然后再restore backward的rng_state，最后做backward.
mpu/data.py
broadcast_data: 把input_data从rank 0 broadcast到所有tensor-model-parallel ranks上. 在pretrain_bert.py的get_batch函数里面调用了这个函数.
这和data parallelism不一样，data parallelism是不同rank load不同数据. tensor-model-parallel不同rank必须load同样的数据.
不过我不是很清楚为什么要通过broadcast来实现. 有点奇怪.
mpu/initialize.py
定义了几个global data:
# Intra-layer model parallel group that the current rank belongs to.
_TENSOR_MODEL_PARALLEL_GROUP = None
# Inter-layer model parallel group that the current rank belongs to.
_PIPELINE_MODEL_PARALLEL_GROUP = None
# Model parallel group (both intra- and pipeline) that the current rank belongs to.
_MODEL_PARALLEL_GROUP = None
# Embedding group.
_EMBEDDING_GROUP = None
# Data parallel group that the current rank belongs to.
_DATA_PARALLEL_GROUP = None其中最重要的函数是initialize_model_parallel，主要是初始化上述几个global data. . 其他函数基本是上面几个global data的getter/setter.
一个group大小就是parallelism dim size. 这个parallelism group的数量就是 #GPU / parallelism dim size. 比如，n = 16，p = 4，那么一个pipeline group有4张卡，一共有4个pipeline groups. 以此类推.
分配group的逻辑在 https://github.com/vycezhong/read-papers/issues/188 这篇论文有所讲述. 比如对于(p, t, d) = (4, 2, 2)的一个parallel configuration，分配结果如下:
那篇论文没有给出具体的分配group算法. 通过这部分实现代码可以略窥一二.
# Build the data-parallel groups.
global _DATA_PARALLEL_GROUP
assert _DATA_PARALLEL_GROUP is None, \
'data parallel group is already initialized'
all_data_parallel_group_ranks = []
for i in range(pipeline_model_parallel_size):
start_rank = i * num_pipeline_model_parallel_groups
end_rank = (i + 1) * num_pipeline_model_parallel_groups
for j in range(tensor_model_parallel_size):
ranks = range(start_rank + j, end_rank,
tensor_model_parallel_size)
all_data_parallel_group_ranks.append(list(ranks))
group = torch.distributed.new_group(ranks)
if rank in ranks:
_DATA_PARALLEL_GROUP = group首先，分为p个pipeline stages，stage 1的rank范围是 [0, n//p)，stage 2的rank范围是[n//p, 2n//p) .... stage p的rank范围是[(p-1)n//p, n). 每个pipeline stage有n//p个GPUs.
对于每一个stage，ranks = range(start_rank + j, end_rank, tensor_model_parallel_size)代表从n//p个GPUs中，隔t个取一个作为data-parallel group. 所以每个data-parallel group大小为 n // p // t = d.
# Build the tensor model-parallel groups.
global _TENSOR_MODEL_PARALLEL_GROUP
assert _TENSOR_MODEL_PARALLEL_GROUP is None, \
'tensor model parallel group is already initialized'
for i in range(num_tensor_model_parallel_groups):
ranks = range(i * tensor_model_parallel_size,
(i + 1) * tensor_model_parallel_size)
group = torch.distributed.new_group(ranks)
if rank in ranks:
_TENSOR_MODEL_PARALLEL_GROUP = group从中可以看出，tensor-model-parallel group的rank一定是相邻的，比如(0, 1), (2, 3)等.
global _PIPELINE_MODEL_PARALLEL_GROUP
global _PIPELINE_GLOBAL_RANKS
assert _PIPELINE_MODEL_PARALLEL_GROUP is None, \
'pipeline model parallel group is already initialized'
global _EMBEDDING_GROUP
assert _EMBEDDING_GROUP is None, \
'embedding group is already initialized'
for i in range(num_pipeline_model_parallel_groups):
ranks = range(i, world_size,
num_pipeline_model_parallel_groups)
group = torch.distributed.new_group(ranks)
if rank in ranks:
_PIPELINE_MODEL_PARALLEL_GROUP = group
_PIPELINE_GLOBAL_RANKS = ranks
# Setup embedding group (to exchange gradients between
# first and last stages).
if len(ranks) > 1:
embedding_ranks = [ranks[0], ranks[-1]]
else:
embedding_ranks = ranks
group = torch.distributed.new_group(embedding_ranks)
if rank in embedding_ranks:
_EMBEDDING_GROUP = grouppipeline-model-parallel group是隔p个取一个，比如[0, 4, 8, 12].
jasperzhong
commented
3 years ago p2p_communication.py
pipeline parallelism需要inter-stage的P2P通信. 主要实现是_communnicate函数. 其他几个API都是调用该函数.
这个函数的注释写得不错，解释得非常清楚. 实现的功能是在stages中双向send/recv.
def _communicate(tensor_send_next, tensor_send_prev, recv_prev, recv_next,
use_ring_exchange=False, tensor_shape=None,
override_scatter_gather_tensors_in_pipeline=False,
dtype_=None):
"""Communicate tensors between stages. Used as helper method in other
communication methods that are used in megatron/schedules.py.
Takes the following arguments:
tensor_send_next: tensor to send to next rank (no tensor sent if
set to None).
tensor_send_prev: tensor to send to prev rank (no tensor sent if
set to None).
recv_prev: boolean for whether tensor should be received from
previous rank.
recv_next: boolean for whether tensor should be received from
next rank.
use_ring_exchange: boolean for whether torch.distributed.ring_exchange()
API should be used.
tensor_shape: optional, use when the input sequence contains less
tokens than the default sequence length
override_scatter_gather_tensors_in_pipeline: optional, this is used
when tensor_shape is
provided to overwide
scatter gather tensors
dtype_: optional, this is used when tensor_shape is provied and what
is the type of tensor_shape
Returns:
(tensor_recv_prev, tensor_recv_next)
"""实现:
if recv_prev:
tensor_recv_prev = torch.empty(tensor_chunk_shape,
requires_grad=requires_grad,
device=torch.cuda.current_device(),
dtype=dtype)
if recv_next:
tensor_recv_next = torch.empty(tensor_chunk_shape,
requires_grad=requires_grad,
device=torch.cuda.current_device(),
dtype=dtype)torch.distributed.batch_isend_irecv，批量做异步send/recv (实际上就是个for循环...) 然后调用wait()进行同步. ops = []
if tensor_send_prev is not None:
send_prev_op = torch.distributed.P2POp(
torch.distributed.isend, tensor_send_prev,
mpu.get_pipeline_model_parallel_prev_rank())
ops.append(send_prev_op)
if tensor_recv_prev is not None:
recv_prev_op = torch.distributed.P2POp(
torch.distributed.irecv, tensor_recv_prev,
mpu.get_pipeline_model_parallel_prev_rank())
ops.append(recv_prev_op)
if tensor_send_next is not None:
send_next_op = torch.distributed.P2POp(
torch.distributed.isend, tensor_send_next,
mpu.get_pipeline_model_parallel_next_rank())
ops.append(send_next_op)
if tensor_recv_next is not None:
recv_next_op = torch.distributed.P2POp(
torch.distributed.irecv, tensor_recv_next,
mpu.get_pipeline_model_parallel_next_rank())
ops.append(recv_next_op)
if len(ops) > 0:
reqs = torch.distributed.batch_isend_irecv(ops)
for req in reqs:
req.wait() # Split tensor into smaller chunks if using scatter-gather optimization.
if not override_scatter_gather_tensors_in_pipeline and \
args.scatter_gather_tensors_in_pipeline:
if tensor_send_next is not None:
tensor_send_next = mpu.split_tensor_into_1d_equal_chunks(tensor_send_next)
if tensor_send_prev is not None:
tensor_send_prev = mpu.split_tensor_into_1d_equal_chunks(tensor_send_prev)
...
# If using scatter-gather optimization, gather smaller chunks.
if not override_scatter_gather_tensors_in_pipeline and \
args.scatter_gather_tensors_in_pipeline:
if recv_prev:
tensor_recv_prev = mpu.gather_split_1d_tensor(
tensor_recv_prev).view(tensor_shape).requires_grad_()
if recv_next:
tensor_recv_next = mpu.gather_split_1d_tensor(
tensor_recv_next).view(tensor_shape).requires_grad_()_communicate函数支持双向的send/recv，实际的用法如下:
| API | send_next | send_prev | recv_prev | recv_next |
|---|---|---|---|---|
recv_forward |
✓ | |||
recv_backward |
✓ | |||
send_forward |
✓ | |||
send_backward |
✓ | |||
send_forward_recv_backward |
✓ | ✓ | ||
send_backward_recv_forward |
✓ | ✓ |
另外对于pipeline的first stage和last stage需要特判一下. 比如recv_forward对于first stage就不需要做，recv_backward对于last stage也不需要做.
schedules.py
get_forward_backward_func函数用于选择何种pipeline schedule.
def get_forward_backward_func():
args = get_args()
if mpu.get_pipeline_model_parallel_world_size() > 1:
if args.virtual_pipeline_model_parallel_size is not None:
forward_backward_func = forward_backward_pipelining_with_interleaving
assert get_num_microbatches() % args.pipeline_model_parallel_size == 0, \
'number of microbatches is not divisible by pipeline-parallel ' \
'size when using interleaved schedule'
else:
forward_backward_func = forward_backward_pipelining_without_interleaving
else:
forward_backward_func = forward_backward_no_pipelining
return forward_backward_funcNVIDIA在SC '21上的论文描述了一种interleaving pipeline schedule，能够进一步降低bubble size，但是增加了一定的通信开销. 本文暂不考虑这种特殊的schedule，主要研究PipeDream-Flush的实现，即上面代码里面的forward_backward_pipelining_without_interleaving函数.
PipeDream-Flush的一个iteration分为三个阶段:
BTW，1F1B schedule是memory-efficient的. 因为1F1B schedule将in-flight microbatches的数量限制到pipeline depth (p)，而不是number of microbatches (m) (e.g., GPipe). 一般来讲，为了降低bubble time，m >> p.
首先确定每个worker在warm-up phase的microbatches数量，为m - rank - 1，即随着rank依次递减. last stage warm-up所需的microbatches数量为零，即直接开始steady阶段.
num_warmup_microbatches = \
(mpu.get_pipeline_model_parallel_world_size() -
mpu.get_pipeline_model_parallel_rank() - 1)
num_microbatches_remaining = \
num_microbatches - num_warmup_microbatches每个worker还需要建立一个FIFO队列，用于保存来自上游的activation(input_tensor)和向下游发送的activation (output_tensor). 这些保存的activations将用于反向传播.
# Input, output tensors only need to be saved when doing backward passes
input_tensors = None
output_tensors = None
if not forward_only:
input_tensors = []
output_tensors = []warm-up phase
# Run warmup forward passes.
for i in range(num_warmup_microbatches):
input_tensor = p2p_communication.recv_forward(timers=timers)
output_tensor = forward_step(forward_step_func, data_iterator, model,
input_tensor, losses_reduced)
p2p_communication.send_forward(output_tensor, timers=timers)
if not forward_only:
input_tensors.append(input_tensor)
output_tensors.append(output_tensor)注意:
recv_forward直接返回None. steady phase 逻辑: forward -> send forward & recv backward -> backward -> send backward & recv forward
# Before running 1F1B, need to receive first forward tensor.
# If all microbatches are run in warmup / cooldown phase, then no need to
# receive this tensor here.
if num_microbatches_remaining > 0:
input_tensor = p2p_communication.recv_forward(timers=timers)
# Run 1F1B in steady state.
for i in range(num_microbatches_remaining):
last_iteration = (i == (num_microbatches_remaining - 1))
output_tensor = forward_step(forward_step_func, data_iterator, model,
input_tensor, losses_reduced)
if forward_only:
p2p_communication.send_forward(output_tensor, timers=timers)
if not last_iteration:
input_tensor = p2p_communication.recv_forward(timers=timers)
else:
output_tensor_grad = \
p2p_communication.send_forward_recv_backward(output_tensor,
timers=timers)
# Add input_tensor and output_tensor to end of list.
input_tensors.append(input_tensor)
output_tensors.append(output_tensor)
# Pop input_tensor and output_tensor from the start of the list for
# the backward pass.
input_tensor = input_tensors.pop(0)
output_tensor = output_tensors.pop(0)
input_tensor_grad = \
backward_step(optimizer, input_tensor, output_tensor,
output_tensor_grad)
if last_iteration:
input_tensor = None
p2p_communication.send_backward(input_tensor_grad, timers=timers)
else:
input_tensor = \
p2p_communication.send_backward_recv_forward(
input_tensor_grad, timers=timers)注意:
input_tensor_grad)，这个梯度需要回传到上游进行进一步反向传播. input_tensor_grad回传，而不需要接收来自上游的forward的activation了. 另外论文的示意图有点小问题，部分forward pass的时间应当后移到backward后. 已提issue.
和warm-up phase对称，执行次数也是num_warmup_microbatches，只不过是专门做backward.
# Run cooldown backward passes.
if not forward_only:
for i in range(num_warmup_microbatches):
input_tensor = input_tensors.pop(0)
output_tensor = output_tensors.pop(0)
output_tensor_grad = p2p_communication.recv_backward(timers=timers)
input_tensor_grad = \
backward_step(optimizer, input_tensor, output_tensor,
output_tensor_grad)
p2p_communication.send_backward(input_tensor_grad, timers=timers)注意: 这个phase清理未完成的backward，所以只需要pop队列就行了.
上述三个phase执行结束后，input_tensors和output_tensors都应为空.
注意到，这里对于pipeline的实现，单个worker的通信和计算是没有overlap的. 因为send和recv都是阻塞的，发送的消息必须被上下游接收后才能进行下一步计算.
model/distributed.py
使用Pipeline parallelism，data parallelism的optimizer只能用LocalDDP. (见training.py)
# We only support local DDP with multiple micro-batches.
if len(model) > 1 or mpu.get_pipeline_model_parallel_world_size() > 1:
assert args.DDP_impl == 'local'实现在DistributedDataParallel类. 从arguments.py的默认参数来看，use_contiguous_buffers这个flag默认是True. 那么会开一份连续的buffer用于通信，记作main_grad. 并且会注册一个backward hook，用于accumulate gradients. 相关代码如下:
class DistributedDataParallel(DistributedDataParallelBase):
def __init__(self, module,
accumulate_allreduce_grads_in_fp32,
use_contiguous_buffers):
...
self._grad_buffers = None
if self.use_contiguous_buffers:
self._grad_buffers = {}
...
# Allocate the buffer.
for dtype, num_elements in type_num_elements.items():
self._grad_buffers[dtype] = MemoryBuffer(num_elements, dtype)
...
# Backward hook.
# Accumalation function for the gradients. We need
# to store them so they don't go out of scope.
self.grad_accs = []
# Loop over all the parameters in the model.
for param in self.module.parameters():
if param.requires_grad:
# Expand so we get access to grad_fn.
param_tmp = param.expand_as(param)
# Get the gradient accumulator functtion.
grad_acc = param_tmp.grad_fn.next_functions[0][0]
grad_acc.register_hook(self._make_param_hook(param))
self.grad_accs.append(grad_acc)
def _make_param_hook(self, param):
"""Create the all-reduce hook for backprop."""
# Hook used for back-prop.
def param_hook(*unused):
# Add the gradient to the buffer.
if param.grad.data is not None:
param.main_grad.add_(param.grad.data)
# Now we can deallocate grad memory.
param.grad = None
return param_hook
def allreduce_gradients(self):
"""Reduce gradients across data parallel ranks."""
# If we have buffers, simply reduce the data in the buffer.
if self._grad_buffers is not None:
for _, buffer_ in self._grad_buffers.items():
buffer_.data /= mpu.get_data_parallel_world_size()
torch.distributed.all_reduce(
buffer_.data, group=mpu.get_data_parallel_group())
...其中，MemoryBuffer就是torch.zeros创建的一个in-memory buffer on GPU.
所以LocalDDP是没有计算和通信的overlap的. 其实pipeline parallelism完全可以用torchDDP，设置下gradient accumulation steps为m就行了. 因为pipeline parallelism把一个minibatch拆成m个microbatches，和gradient accumulation没有区别.
另外我不清楚为什么要搞一个continuous buffer. 注释里面有一句:
has the potential to reduce memory fragmentation.
还是不理解...
model/transformer.py model/language_model.py model/bert_model.py
pipeline parallelism最后一块 —— model partition. 其实也没啥好说的，因为现在的大模型都是repetitive的，所以直接按照层数切分，每一层是一模一样的transformer layer. 代码见model/transformer.py.
self.num_layers = args.num_layers // mpu.get_pipeline_model_parallel_world_size()不过first stage和last stage有点特殊.
pre_process. 代码见model/language_model.py. # Embeddings.
if self.pre_process:
self.embedding = Embedding(self.hidden_size,
args.padded_vocab_size,
args.max_position_embeddings,
args.hidden_dropout,
self.init_method,
self.num_tokentypes)
self._embedding_key = 'embedding'post_preprocess. 代码见model/bert_model.py if self.post_process:
self.lm_head = BertLMHead(
self.word_embeddings_weight().size(0),
args.hidden_size, init_method, args.layernorm_epsilon, parallel_output)
self._lm_head_key = 'lm_head'
self.binary_head = None
if self.add_binary_head:
self.binary_head = get_linear_layer(args.hidden_size, 2,
init_method)
self._binary_head_key = 'binary_head'training.py
最后回过头看下train_step函数，看看single training step的流程:
# Set grad to zero.
if args.DDP_impl == 'local' and args.use_contiguous_buffers_in_local_ddp:
for partition in model:
partition.zero_grad_buffer()
optimizer.zero_grad() forward_backward_func = get_forward_backward_func()
losses_reduced = forward_backward_func(
forward_step_func, data_iterator, model,
optimizer, timers, forward_only=False)到这里整个pipeline都已经forward完成并且backward完成了，loss和grad都算好了.
# All-reduce if needed.
if args.DDP_impl == 'local':
timers('backward-params-all-reduce').start()
for model_module in model:
model_module.allreduce_gradients()
timers('backward-params-all-reduce').stop()到这里才开始做data parallelism的all-reduce.
因为embedding做了weight sharing. first stage和last stage都有一份embedding，为了保证参数一致，需要对二者的grad做all-reduce. 其他stage忽略此过程.
调用optimizer.step()更新参数.
注意: pipeline中不同stage更新参数是不同步的，完全可以有先后. 其实并没有一个explicit pipeline flush. 这个同步推迟到了下一个iteration recv等待的时候 (看来first stage应该是bottleneck).
最后分析下各个阶段时间. 配置: (p, t, d) = (2, 2, 2), gpu_per_node = 2, m = 2.
注意，是last_rank的worker print log，同时也是pipeline last stage.
从log来看，看上去recv_forward占了很长的时间，这其实是warm-up phase的时间，因为这是pipeline last stage的log.
另外backward-embedding-all-reduce也很高，也主要是等待时间，等待first stage做完backward做完data parallelism的all-reduce.
只有backward-send-forward-recv和backward-send能代表真实的通信时间. 可以看到时间非常短，比data parallelism的all-reduce时间短很多!
optimizer的时间有点出乎意料. 70%的时间用来处理mixed precision的问题. 哈.
最后，backward其实不比forward慢多少，forward:backward = 1:2夸张了，这里差不多是forward:backward = 7:10.
https://github.com/NVIDIA/Megatron-LM
刻不容缓.