documentation/_model_memory_anatomy.txt

Model training anatomy
To understand performance optimization techniques that one can apply to improve efficiency of model training 
speed and memory utilization, it's helpful to get familiar with how GPU is utilized during training, and how compute 
intensity varies depending on an operation performed.
Let's start by exploring a motivating example of GPU utilization and the training run of a model. For the demonstration, 
we'll need to install a few libraries: 

pip install transformers datasets accelerate nvidia-ml-py3
The nvidia-ml-py3 library allows us to monitor the memory usage of the models from within Python. You might be familiar 
with the nvidia-smi command in the terminal - this library allows to access the same information in Python directly.
Then, we create some dummy data: random token IDs between 100 and 30000 and binary labels for a classifier. 
In total, we get 512 sequences each with length 512 and store them in a [~datasets.Dataset] with PyTorch format.

import numpy as np
from datasets import Dataset
seq_len, dataset_size = 512, 512
dummy_data = {
     "input_ids": np.random.randint(100, 30000, (dataset_size, seq_len)),
     "labels": np.random.randint(0, 1, (dataset_size)),
 }
ds = Dataset.from_dict(dummy_data)
ds.set_format("pt")

To print summary statistics for the GPU utilization and the training run with the [Trainer] we define two helper functions:

from pynvml import *
def print_gpu_utilization():
     nvmlInit()
     handle = nvmlDeviceGetHandleByIndex(0)
     info = nvmlDeviceGetMemoryInfo(handle)
     print(f"GPU memory occupied: {info.used//1024**2} MB.")
def print_summary(result):
     print(f"Time: {result.metrics['train_runtime']:.2f}")
     print(f"Samples/second: {result.metrics['train_samples_per_second']:.2f}")
     print_gpu_utilization()

Let's verify that we start with a free GPU memory:

print_gpu_utilization()
GPU memory occupied: 0 MB.

That looks good: the GPU memory is not occupied as we would expect before we load any models. If that's not the case on 
your machine make sure to stop all processes that are using GPU memory. However, not all free GPU memory can be used by 
the user. When a model is loaded to the GPU the kernels are also loaded, which can take up 1-2GB of memory. To see how 
much it is we load a tiny tensor into the GPU which triggers the kernels to be loaded as well.

import torch
torch.ones((1, 1)).to("cuda")
print_gpu_utilization()
GPU memory occupied: 1343 MB.

We see that the kernels alone take up 1.3GB of GPU memory. Now let's see how much space the model uses.
Load Model
First, we load the google-bert/bert-large-uncased model. We load the model weights directly to the GPU so that we can check 
how much space just the weights use.

from transformers import AutoModelForSequenceClassification
model = AutoModelForSequenceClassification.from_pretrained("google-bert/bert-large-uncased").to("cuda")
print_gpu_utilization()
GPU memory occupied: 2631 MB.

We can see that the model weights alone take up 1.3 GB of GPU memory. The exact number depends on the specific 
GPU you are using. Note that on newer GPUs a model can sometimes take up more space since the weights are loaded in an 
optimized fashion that speeds up the usage of the model. Now we can also quickly check if we get the same result 
as with nvidia-smi CLI:

nvidia-smi
```bash
Tue Jan 11 08:58:05 2022
+-----------------------------------------------------------------------------+
| NVIDIA-SMI 460.91.03    Driver Version: 460.91.03    CUDA Version: 11.2     |
|-------------------------------+----------------------+----------------------+
| GPU  Name        Persistence-M| Bus-Id        Disp.A | Volatile Uncorr. ECC |
| Fan  Temp  Perf  Pwr:Usage/Cap|         Memory-Usage | GPU-Util  Compute M. |
|                               |                      |               MIG M. |
|===============================+======================+======================|
|   0  Tesla V100-SXM2  On   | 00000000:00:04.0 Off |                    0 |
| N/A   37C    P0    39W / 300W |   2631MiB / 16160MiB |      0%      Default |
|                               |                      |                  N/A |
+-------------------------------+----------------------+----------------------+
+-----------------------------------------------------------------------------+
| Processes:                                                                  |
|  GPU   GI   CI        PID   Type   Process name                  GPU Memory |
|        ID   ID                                                   Usage      |
|=============================================================================|
|    0   N/A  N/A      3721      C   nvs/codeparrot/bin/python     2629MiB |
+-----------------------------------------------------------------------------+

We get the same number as before and you can also see that we are using a V100 GPU with 16GB of memory. So now we can 
start training the model and see how the GPU memory consumption changes. First, we set up a few standard training 
arguments:
py
default_args = {
    "output_dir": "tmp",
    "eval_strategy": "steps",
    "num_train_epochs": 1,
    "log_level": "error",
    "report_to": "none",
}

If you plan to run multiple experiments, in order to properly clear the memory between experiments, restart the Python 
 kernel between experiments.

Memory utilization at vanilla training
Let's use the [Trainer] and train the model without using any GPU performance optimization techniques and a batch size of 4:

from transformers import TrainingArguments, Trainer, logging
logging.set_verbosity_error()
training_args = TrainingArguments(per_device_train_batch_size=4, **default_args)
trainer = Trainer(model=model, args=training_args, train_dataset=ds)
result = trainer.train()
print_summary(result)

Time: 57.82
Samples/second: 8.86
GPU memory occupied: 14949 MB.
We see that already a relatively small batch size almost fills up our GPU's entire memory. However, a larger batch size 
can often result in faster model convergence or better end performance. So ideally we want to tune the batch size to our
model's needs and not to the GPU limitations. What's interesting is that we use much more memory than the size of the model. 
To understand a bit better why this is the case let's have a look at a model's operations and memory needs.
Anatomy of Model's Operations
Transformers architecture includes 3 main groups of operations grouped below by compute-intensity.

Tensor Contractions
Linear layers and components of Multi-Head Attention all do batched matrix-matrix multiplications. These operations are the most compute-intensive part of training a transformer.

Statistical Normalizations
Softmax and layer normalization are less compute-intensive than tensor contractions, and involve one or more reduction operations, the result of which is then applied via a map.

Element-wise Operators
These are the remaining operators: biases, dropout, activations, and residual connections. These are the least compute-intensive operations.

This knowledge can be helpful to know when analyzing performance bottlenecks.
This summary is derived from Data Movement Is All You Need: A Case Study on Optimizing Transformers 2020
Anatomy of Model's Memory
We've seen that training the model uses much more memory than just putting the model on the GPU. This is because there 
are many components during training that use GPU memory. The components on GPU memory are the following:

model weights
optimizer states
gradients
forward activations saved for gradient computation
temporary buffers
functionality-specific memory

A typical model trained in mixed precision with AdamW requires 18 bytes per model parameter plus activation memory. For 
inference there are no optimizer states and gradients, so we can subtract those. And thus we end up with 6 bytes per 
model parameter for mixed precision inference, plus activation memory.
Let's look at the details.
Model Weights:

4 bytes * number of parameters for fp32 training
6 bytes * number of parameters for mixed precision training (maintains a model in fp32 and one in fp16 in memory)

Optimizer States:

8 bytes * number of parameters for normal AdamW (maintains 2 states)
2 bytes * number of parameters for 8-bit AdamW optimizers like bitsandbytes
4 bytes * number of parameters for optimizers like SGD with momentum (maintains only 1 state)

Gradients

4 bytes * number of parameters for either fp32 or mixed precision training (gradients are always kept in fp32)

Forward Activations

size depends on many factors, the key ones being sequence length, hidden size and batch size.

There are the input and output that are being passed and returned by the forward and the backward functions and the 
forward activations saved for gradient computation.
Temporary Memory
Additionally, there are all kinds of temporary variables which get released once the calculation is done, but in the 
moment these could require additional memory and could push to OOM. Therefore, when coding it's crucial to think 
strategically about such temporary variables and sometimes to explicitly free those as soon as they are no longer needed.
Functionality-specific memory
Then, your software could have special memory needs. For example, when generating text using beam search, the software 
needs to maintain multiple copies of inputs and outputs.
forward vs backward Execution Speed
For convolutions and linear layers there are 2x flops in the backward compared to the forward, which generally translates 
into ~2x slower (sometimes more, because sizes in the backward tend to be more awkward). Activations are usually 
bandwidth-limited, and it’s typical for an activation to have to read more data in the backward than in the forward 
(e.g. activation forward reads once, writes once, activation backward reads twice, gradOutput and output of the forward, 
and writes once, gradInput).
As you can see, there are potentially a few places where we could save GPU memory or speed up operations. 
Now that you understand what affects GPU utilization and computation speed, refer to 
the Methods and tools for efficient training on a single GPU documentation page to learn about 
performance optimization techniques.