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2D Tensor Parallelism
Author: Zhengda Bian, Yongbin Li
Prerequisite
Example Code
Related Paper
Introduction
1D tensor parallelism does not partition activations, which can also consume a great amount of memory in terms of large-scale models. To evenly distribute the computation and memory load, an efficient 2D tensor parallelism algorithm was introduced based on SUMMA (Scalable Universal Matrix Multiplication Algorithm).
Let's still take a linear layer Y = XA as an example.
Given P=q\times q processors (necessary condition), e.g. q=2, we split both the input X and weight A into
\left[\begin{matrix} X_{00} & X_{01} \\ X_{10} & X_{11} \end{matrix} \right]
\text{~and~}
\left[\begin{matrix} A_{00} & A_{01} \\ A_{10} & A_{11} \end{matrix} \right].
The calculation includes q steps. When t=1, X_{i0} is broadcasted in its row, and A_{0j} is broadcasted in its column. So, we have
\left[\begin{matrix} X_{00},A_{00} & X_{00},A_{01} \\ X_{10},A_{00} & X_{10},A_{01} \end{matrix} \right].
Then we multiply X_{i0} and A_{0j} on each processor (i, j) as
\left[\begin{matrix} X_{00}A_{00} & X_{00}A_{01} \\ X_{10}A_{00} & X_{10}A_{01} \end{matrix} \right] (1).
Similarly, when t=2, X_{i1} is broadcasted in its row, A_{1j} is broadcasted in its column, and we multiply them as
\left[\begin{matrix} X_{01}A_{10} & X_{01}A_{11} \\ X_{11}A_{10} & X_{11}A_{11} \end{matrix} \right] (2).
By adding (1) and (2) up, we have
Y = XA = \left[\begin{matrix} X_{00}A_{00}+X_{01}A_{10} & X_{00}A_{01}+X_{01}A_{11} \\ X_{10}A_{00}+X_{11}A_{10} & X_{10}A_{01}+X_{11}A_{11} \end{matrix} \right].
Efficiency
Given P=q\times q processors, we present the theoretical computation and memory cost, as well as the communication cost based on the ring algorithm in both the forward and backward pass of 2D tensor parallelism.
| Computation | Memory (parameters) | Memory (activations) | Communication (bandwidth) | Communication (latency) |
|---|---|---|---|---|
O(1/q^2) |
O(1/q^2) |
O(1/q^2) |
O(6(q-1)/q) |
O(6(q-1)) |
Usage
To enable 2D tensor parallelism for our model, e.g. on 4 GPUs, we need to configure the parallelism setting as below.
CONFIG = dict(parallel=dict(
data=1,
pipeline=1,
tensor=dict(size=4, mode='2d'),
))
Then Colossal-AI will automatically apply 2D parallelism to all the layers from colossalai.nn.
Let's define a model that consists of a two-layer multi-layer perceptron (MLP) as below.
import colossalai
import colossalai.nn as col_nn
import torch
from colossalai.utils import print_rank_0
class MLP(torch.nn.Module):
def __init__(self, dim: int = 256):
super().__init__()
intermediate_dim = dim * 4
self.dense_1 = col_nn.Linear(dim, intermediate_dim)
print_rank_0(f'Weight of the first linear layer: {self.dense_1.weight.shape}')
self.activation = torch.nn.GELU()
self.dense_2 = col_nn.Linear(intermediate_dim, dim)
print_rank_0(f'Weight of the second linear layer: {self.dense_2.weight.shape}')
self.dropout = col_nn.Dropout(0.1)
def forward(self, x):
x = self.dense_1(x)
print_rank_0(f'Output of the first linear layer: {x.shape}')
x = self.activation(x)
x = self.dense_2(x)
print_rank_0(f'Output of the second linear layer: {x.shape}')
x = self.dropout(x)
return x
Launch Colossal-AI on 4 GPUs and build the model
parser = colossalai.get_default_parser()
colossalai.launch(config=CONFIG,
rank=args.rank,
world_size=args.world_size,
local_rank=args.local_rank,
host=args.host,
port=args.port)
m = MLP()
We will see the shapes of partitioned parameters(e.g. weights) in the MLP model.
Weight of the first linear layer: torch.Size([128, 512])
Weight of the second linear layer: torch.Size([512, 128])
The complete weight of the first linear layer is supposed to have the shape [256, 1024]. After the partitioning of 2D parallelism, it becomes [128, 512] on each GPU.
Similarly, the second layer partitions the weight [1024, 256] into [512, 128].
We can run the model with some random inputs.
from colossalai.context import ParallelMode
from colossalai.core import global_context as gpc
from colossalai.utils import get_current_device
x = torch.randn((16, 256), device=get_current_device())
# partition input
torch.distributed.broadcast(x, src=0)
x = torch.chunk(x, 2, dim=0)[gpc.get_local_rank(ParallelMode.PARALLEL_2D_COL)]
x = torch.chunk(x, 2, dim=-1)[gpc.get_local_rank(ParallelMode.PARALLEL_2D_ROW)]
print_rank_0(f'Input: {x.shape}')
x = m(x)
Then we can see the shapes of activation results.
Input: torch.Size([8, 128])
Output of the first linear layer: torch.Size([8, 512])
Output of the second linear layer: torch.Size([8, 128])
The activation tensors in 2D parallelism are all split in both row and column.
E.g. the output of the first linear layer has the shape [8, 512], while the second layer has the output of [8, 128].