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Tensor Parallelism

Introduction

Tensor parallelism is a strategy employed to train and inference from very large language models by splitting the actual computations/tensors across multiple compute devices. It is a critical technique for the continued growth and application of massive deep learning models and offers a path to unlocking unprecedented model capacities.

When we use tensor parallelism to partition and compute large language models, there are various ways in which we can perform the partitioning algorithm. In 1D algorithms, we can split by rows or columns. For a row-major order matrix, if you split by column, data rearrangement is required, which is a factor affecting performance. However, splitting a row-major order matrix by rows does not consume time. In our TP implementation, we adopt the method of pre-splitting the corresponding weights, so the time consumed for this part is one-time and does not affect inference performance. Meanwhile, another major factor impacting performance is 'all reduce'. Since each node computes partial and incomplete results, it is necessary to perform 'all reduce' on the output data. But all reduce is relatively time-consuming, interestingly, by using a reasonable splitting and combining method, primitives can be operated independently across nodes, which is very helpful for performance optimization. Thus, a rational splitting method becomes extremely important.

Taking the FFN module as an example, if the first matmul splits by column and computes the matmul with input, it will result in two unrelated sub-matrices on each node. These two sub-matrices, when performing the second matmul operation, can proceed directly without having to perform 'all reduce' if splitting by rows. Thus, the entire FFN module only requires one 'all reduce', meaning that with properly tailored split implementation, even with multiple matmul operations, only one 'all reduce' operation may be needed.

FFN split


The scenario for the attention module is more complex. As shown in the following figure, a rational split can make it so that the entire attention module only requires one 'all reduce' operation, thus greatly saving synchronization time.

Attention split


Prerequisites

Multi-node and Multi-socket communications are needed in tensor parallelism, we use oneCCL for the distributed communications.

Enable Customized Model

Taking "llama" as an example, we need three modifications:

  • First, we need to split the weight used in the matmul calculation.
  • Second, determine the size of n_head after splitting, based on the total number of nodes in parallel(world_size).
  • Finally, insert the all_reduce operator after the final matmul.

For instance, when splitting the FFN (feed-forward network) module, it is visible in the FFN flowchart that both feed_forward.w1.weigh and feed_forward.w3.weight are part of the first matmul computation. This portion of the matmul should be split by column, considering the weight in ITREX is already transposed. We only need to set the split_type of these two weights to TP_1D_ROW within the calc_split_type() function in 'models/modelutils/model_files.h'.

The feed_forward.w1.weight belongs to the second part of the matmul calculation in the FFN flowchart, the split_type should be set to TP_1D_COLUMN. This ensures that the partial results from the first matmul calculation can be independently used for the second matmul calculation. There are also some primitives between the two matmul operations, and since these primitives are element-wise, they are also calculated independently on their respective nodes.

For the attention module, there are four weights: attention.wq.weight, attention.wk.weight, attention.wv.weight, and attention.wo.weight. The split_type for attention.wq.weight, attention.wk.weightand attention.wv.weight should be set to TP_1D_ROW. In contrast, attention.wo.weight should be set to TP_1D_COLUMN. The calculations for the primitives in between can be done independently.

Once the weight splitting is complete, the actual n_head computed by each node when running the model is correspondingly reduced, so it is necessary to reset the size of n_head. Code is simple like:

n_head /= world_size;
n_head_kv /= world_size;

Finally, after the last matmul calculation, insert the all_reduce operator to sum up the partial computation results, thereby obtaining the complete computational outcome.

cur = ne_all_reduce(ctx0, cur);

Build the oneCCL and setup the env

git clone https://github.com/oneapi-src/oneCCL.git
cd oneCCL
sed -i 's/cpu_gpu_dpcpp/./g' cmake/templates/oneCCLConfig.cmake.in
mkdir build
cd build
cmake ..
make -j install
source <path_to_build_dir>/_install/env/setvars.sh

To confirm that the oneCCL installation is successful, use command:

mpirun --version

If the command line prints log like below, means the oneCCL env is ready.

Intel(R) MPI Library for Linux* OS, Version 2021.9 Build 20230306 (id: d82b3071db)
Copyright 2003-2023, Intel Corporation.

Enable the CMake option and build executable file

Compile an executable file that supports tensor parallelism by enabling the CMake option NE_TP. You can build the executable file like below.

mkdir build
cd build
cmake -DNE_TP=ON .. 
make -j

Download the model weights and quantize to q4_0 format.

First you should download and convert the model to f32 format. You can also quantize the model to q4_0 format, but it is optional.

python scripts/convert.py --outtype f32 --outfile EleutherAI/gpt-j-6b

Then quantize the model to q4_0 format(optional).

python scripts/quantize.py --model_name gptj --model_file /path/to/your/ne-f32.bin --out_file ne-q4_0.bin --weight_dtype int4

Examples

We can config the mpirun to start parallel programs. Here is an example about running tensor pallelsim on 2 sockets in CPU.

mpirun -np 2 -bind-to=socket ./build/bin/main_gptj -m ne-q4_0.bin --seed 1234 -t 56 -c 68 -n 32 -p "Once upon a time, there existed a little girl, who liked to have adventures. She wanted to go to places and meet new people, and have fun." --no_mmap

We only add mpirun -np 2 -bind-to=socket to the original command to enable 2 processes to run parallel. If you want to bind specific core to each process. You can write the original command to a shell script and use command like below.

mpirun -n 1 taskset -c 0-47 sh run.sh : -n 1 taskset -c 48-95 sh run.sh

NOTICE: tensor parallelsim strategy will split the model to specific node/socket, each device already use part of the original weights differently. So we should not use shared-memory of weights to avoid cross numa weight movement. Use option --no-mmap to disable shared weights between processes.