Train 1 trillion+ parameter models¶
When training large models, fitting larger batch sizes, or trying to increase throughput using multi-GPU compute, Lightning provides advanced optimized distributed training strategies to support these cases and offer substantial improvements in memory usage.
Note that some of the extreme memory saving configurations will affect the speed of training. This Speed/Memory trade-off in most cases can be adjusted.
Some of these memory-efficient strategies rely on offloading onto other forms of memory, such as CPU RAM or NVMe. This means you can even see memory benefits on a single GPU, using a strategy such as DeepSpeed ZeRO Stage 3 Offload.
Check out this amazing video explaining model parallelism and how it works behind the scenes:
Choosing an Advanced Distributed GPU Strategy¶
If you would like to stick with PyTorch DDP, see DDP Optimizations.
Unlike DistributedDataParallel (DDP) where the maximum trainable model size and batch size do not change with respect to the number of GPUs, memory-optimized strategies can accommodate bigger models and larger batches as more GPUs are used. This means as you scale up the number of GPUs, you can reach the number of model parameters you’d like to train.
There are many considerations when choosing a strategy as described below. In addition, check out the visualization of various strategy benchmarks using minGPT here.
Pre-training vs Fine-tuning¶
When fine-tuning, we often use a magnitude less data compared to pre-training a model. This is important when choosing a distributed strategy as usually for pre-training, we are compute-bound. This means we cannot sacrifice throughput as much as if we were fine-tuning, because in fine-tuning the data requirement is smaller.
Overall:
When fine-tuning a model, use advanced memory efficient strategies such as Fully Sharded Training, DeepSpeed ZeRO Stage 3 or DeepSpeed ZeRO Stage 3 Offload, allowing you to fine-tune larger models if you are limited on compute
When pre-training a model, use simpler optimizations such as DeepSpeed ZeRO Stage 2, scaling the number of GPUs to reach larger parameter sizes
For both fine-tuning and pre-training, use DeepSpeed Activation Checkpointing as the throughput degradation is not significant
For example when using 128 GPUs, you can pre-train large 10 to 20 Billion parameter models using DeepSpeed ZeRO Stage 2 without having to take a performance hit with more advanced optimized multi-gpu strategy.
But for fine-tuning a model, you can reach 10 to 20 Billion parameter models using DeepSpeed ZeRO Stage 3 Offload on a single GPU. This does come with a significant throughput hit, which needs to be weighed accordingly.
When Shouldn’t I use an Optimized Distributed Strategy?¶
Sharding techniques help when model sizes are fairly large; roughly 500M+ parameters is where we’ve seen benefits. However, in the following cases, we recommend sticking to ordinary distributed strategies
When your model is small (ResNet50 of around 80M Parameters), unless you are using unusually large batch sizes or inputs.
Due to high distributed communication between devices, if running on a slow network/interconnect, the training might be much slower than expected and then it’s up to you to determince the tradeoff here.
Cutting-edge and third-party Strategies¶
Cutting-edge Lightning strategies are being developed by third-parties outside of Lightning.
If you want to try some of the latest and greatest features for model-parallel training, check out the Colossal-AI Strategy integration.
Another integration is Bagua Strategy, deep learning training acceleration framework for PyTorch, with advanced distributed training algorithms and system optimizations.
For training on unreliable mixed GPUs across the internet check out the Hivemind Strategy integration.
Efficient initialization¶
Instantiating a nn.Module in PyTorch creates all parameters on CPU in float32 precision by default.
To speed up initialization, you can force PyTorch to create the model directly on the target device and with the desired precision without changing your model code.
fabric = Trainer(accelerator="cuda", precision="16-true")
with trainer.init_module():
# models created here will be on GPU and in float16
model = MyModel()
trainer.fit(model)
This eliminates the waiting time to transfer the model parameters from the CPU to the device.
When loading a model from a checkpoint, for example when fine-tuning, set empty_init=True to avoid expensive and redundant memory initialization:
with trainer.init_module(empty_init=True):
# creation of the model is very fast
model = MyModel.load_from_checkpoint("my/checkpoint/path.ckpt")
trainer.fit(model)
For strategies that handle large sharded models (FSDP, DeepSpeed), the init_module()
should not be used, instead override the configure_model() hook:
class MyModel(LightningModule):
def __init__(self):
super().__init__()
# don't instantiate layers here
# move the creation of layers to `configure_model`
def configure_model(self):
# create all your layers here
self.layers = nn.Sequential(...)
This makes it possible to work with models that are larger than the memory of a single device.
Fully Sharded Training¶
PyTorch has it’s own version of FSDP which is upstreamed from their fairscale project. It was introduced in their v1.11.0 release but it is recommended to use it with PyTorch v1.12 or more and that’s what Lightning supports.
경고
This is an experimental feature.
Auto Wrapping¶
Model layers should be wrapped in FSDP in a nested way to save peak memory and enable communication and computation overlapping. The
simplest way to do it is auto wrapping, which can serve as a drop-in replacement for DDP without changing the rest of the code. You don’t
have to wrap layers manually as in the case of manual wrapping.
참고
For users of PyTorch < 2.0: While initializing the optimizers inside configure_optimizers hook, make sure to use self.trainer.model.parameters(), else
PyTorch will raise an error. This is required because when you use auto-wrap, the model layers are sharded and your
lightning_module.parameters() will return a generator with no params.
model = BoringModel()
trainer = Trainer(accelerator="gpu", devices=4, strategy="fsdp", precision=16)
trainer.fit(model)
You can customize the strategy configuration by adjusting the arguments of FSDPStrategy and pass that to the strategy argument inside the Trainer.
from lightning.pytorch import Trainer
from lightning.pytorch.strategies import FSDPStrategy
# equivalent to passing `"fsdp_cpu_offload"`
fsdp = FSDPStrategy(cpu_offload=True)
trainer = pl.Trainer(strategy=fsdp, accelerator="gpu", devices=4)
# configure the wrapping condition
fsdp = FSDPStrategy(auto_wrap_policy={MyTransformerBlock})
trainer = pl.Trainer(strategy=fsdp, accelerator="gpu", devices=4)
Read more here.
Manual Wrapping¶
Manual wrapping can be useful to explore complex sharding strategies by applying wrap selectively to some parts of the model. To activate
parameter sharding with manual wrapping, you can wrap your model using the wrap function. Internally in Lightning, we enable a context manager around the configure_model hook to make sure the wrap parameters are passed correctly.
When not using Fully Sharded, these wrap calls are a no-op. This means once the changes have been made, there is no need to remove the changes for other strategies.
wrap simply wraps the module with a Fully Sharded Parallel class with the correct parameters from the Lightning context manager.
Here’s an example using that uses wrap to create your model:
import torch
import torch.nn as nn
import lightning.pytorch as pl
from lightning.pytorch import Trainer
from torch.distributed.fsdp.wrap import wrap
class MyModel(pl.LightningModule):
def configure_model(self):
self.linear_layer = nn.Linear(32, 32)
self.block = nn.Sequential(nn.Linear(32, 32), nn.Linear(32, 32))
# modules are sharded across processes
# as soon as they are wrapped with `wrap`.
# During the forward/backward passes, weights get synced across processes
# and de-allocated once computation is complete, saving memory.
# Wraps the layer in a Fully Sharded Wrapper automatically
linear_layer = wrap(self.linear_layer)
for i, layer in enumerate(self.block):
self.block[i] = wrap(layer)
self.model = nn.Sequential(linear_layer, nn.ReLU(), self.block)
def configure_optimizers(self):
return torch.optim.AdamW(self.model.parameters())
model = MyModel()
trainer = Trainer(accelerator="gpu", devices=4, strategy="fsdp", precision=16)
trainer.fit(model)
In this case, Lightning will not re-wrap your model, so you don’t need to set FSDPStrategy(auto_wrap_policy=...).
Check out this tutorial to learn more about it.
Activation Checkpointing¶
Activation checkpointing reduces GPU memory usage by avoiding the storage of intermediate activation tensors in selected layers. The tradeoff is that computation cost for the backpropagation increases, as the dropped activations need to be recomputed.
Enable checkpointing on large layers (like Transformers) by providing a policy:
from lightning.pytorch.strategies import FSDPStrategy
fsdp = FSDPStrategy(activation_checkpointing_policy={MyTransformerBlock})
trainer = pl.Trainer(strategy=fsdp, accelerator="gpu", devices=4)
You could also configure activation checkpointing manually inside the configure_model hook:
from torch.distributed.algorithms._checkpoint.checkpoint_wrapper import apply_activation_checkpointing
class MyModel(pl.LightningModule):
...
def configure_model(self):
# Same code as in the "Manual wrapping" snippet above
...
apply_activation_checkpointing(self.model)
In this case, Lightning will not re-configure activation checkpointing, so you don’t need to set FSDPStrategy(activation_checkpointing=...).
DeepSpeed¶
DeepSpeed is a deep learning training optimization library, providing the means to train massive billion parameter models at scale. Using the DeepSpeed strategy, we were able to train model sizes of 10 Billion parameters and above, with a lot of useful information in this benchmark and the DeepSpeed docs. DeepSpeed also offers lower level training optimizations, and efficient optimizers such as 1-bit Adam. We recommend using DeepSpeed in environments where speed and memory optimizations are important (such as training large billion parameter models).
경고
This is an experimental feature.
Below is a summary of all the configurations of DeepSpeed.
DeepSpeed ZeRO Stage 1 - Shard optimizer states, remains at speed parity with DDP whilst providing memory improvement
DeepSpeed ZeRO Stage 2 - Shard optimizer states and gradients, remains at speed parity with DDP whilst providing even more memory improvement
DeepSpeed ZeRO Stage 2 Offload - Offload optimizer states and gradients to CPU. Increases distributed communication volume and GPU-CPU device transfer, but provides significant memory improvement
DeepSpeed ZeRO Stage 3 - Shard optimizer states, gradients, parameters and optionally activations. Increases distributed communication volume, but provides even more memory improvement
DeepSpeed ZeRO Stage 3 Offload - Offload optimizer states, gradients, parameters and optionally activations to CPU. Increases distributed communication volume and GPU-CPU device transfer, but even more significant memory improvement.
DeepSpeed Activation Checkpointing - Free activations after forward pass. Increases computation, but provides memory improvement for all stages.
To use DeepSpeed, you first need to install DeepSpeed using the commands below.
pip install deepspeed
If you run into an issue with the install or later in training, ensure that the CUDA version of the PyTorch you’ve installed matches your locally installed CUDA (you can see which one has been recognized by running nvcc --version).
참고
DeepSpeed currently only supports single optimizer, single scheduler within the training loop.
When saving a checkpoint we rely on DeepSpeed which saves a directory containing the model and various components.
DeepSpeed ZeRO Stage 1¶
DeepSpeed ZeRO Stage 1 partitions your optimizer states (Stage 1) across your GPUs to reduce memory.
It is recommended to skip Stage 1 and use Stage 2, which comes with larger memory improvements and still remains efficient. Stage 1 is useful to pair with certain optimizations such as Torch ORT.
from lightning.pytorch import Trainer
model = MyModel()
trainer = Trainer(accelerator="gpu", devices=4, strategy="deepspeed_stage_1", precision=16)
trainer.fit(model)
DeepSpeed ZeRO Stage 2¶
DeepSpeed ZeRO Stage 2 partitions your optimizer states (Stage 1) and your gradients (Stage 2) across your GPUs to reduce memory. In most cases, this is more efficient or at parity with DDP, primarily due to the optimized custom communications written by the DeepSpeed team.
As a result, benefits can also be seen on a single GPU. Do note that the default bucket sizes allocate around 3.6GB of VRAM to use during distributed communications, which can be tweaked when instantiating the strategy described in a few sections below.
from lightning.pytorch import Trainer
model = MyModel()
trainer = Trainer(accelerator="gpu", devices=4, strategy="deepspeed_stage_2", precision=16)
trainer.fit(model)
python train.py --strategy deepspeed_stage_2 --precision 16 --accelerator 'gpu' --devices 4
DeepSpeed ZeRO Stage 2 Offload¶
Below we show an example of running ZeRO-Offload. ZeRO-Offload leverages the host CPU to offload optimizer memory/computation, reducing the overall memory consumption.
from lightning.pytorch import Trainer
model = MyModel()
trainer = Trainer(accelerator="gpu", devices=4, strategy="deepspeed_stage_2_offload", precision=16)
trainer.fit(model)
This can also be done via the command line using a PyTorch Lightning script:
python train.py --strategy deepspeed_stage_2_offload --precision 16 --accelerator 'gpu' --devices 4
You can also modify the ZeRO-Offload parameters via the strategy as below.
from lightning.pytorch import Trainer
from lightning.pytorch.strategies import DeepSpeedStrategy
model = MyModel()
trainer = Trainer(
accelerator="gpu",
devices=4,
strategy=DeepSpeedStrategy(offload_optimizer=True, allgather_bucket_size=5e8, reduce_bucket_size=5e8),
precision=16,
)
trainer.fit(model)
참고
We suggest tuning the allgather_bucket_size parameter and reduce_bucket_size parameter to find optimum parameters based on your model size.
These control how large a buffer we limit the model to using when reducing gradients/gathering updated parameters. Smaller values will result in less memory, but tradeoff with speed.
DeepSpeed allocates a reduce buffer size multiplied by 1.5x so take that into consideration when tweaking the parameters.
The strategy sets a reasonable default of 2e8, which should work for most low VRAM GPUs (less than 7GB), allocating roughly 3.6GB of VRAM as buffer. Higher VRAM GPUs should aim for values around 5e8.
For even more speed benefit, DeepSpeed offers an optimized CPU version of ADAM called DeepSpeedCPUAdam to run the offloaded computation, which is faster than the standard PyTorch implementation.
import lightning.pytorch
from lightning.pytorch import Trainer
from deepspeed.ops.adam import DeepSpeedCPUAdam
class MyModel(pl.LightningModule):
...
def configure_optimizers(self):
# DeepSpeedCPUAdam provides 5x to 7x speedup over torch.optim.adam(w)
return DeepSpeedCPUAdam(self.parameters())
model = MyModel()
trainer = Trainer(accelerator="gpu", devices=4, strategy="deepspeed_stage_2_offload", precision=16)
trainer.fit(model)
DeepSpeed ZeRO Stage 3¶
DeepSpeed ZeRO Stage 3 shards the optimizer states, gradients and the model parameters (also optionally activations). Sharding model parameters and activations comes with an increase in distributed communication, however allows you to scale your models massively from one GPU to multiple GPUs. The DeepSpeed team report the ability to fine-tune models with over 40B parameters on a single GPU and over 2 Trillion parameters on 512 GPUs. For more information we suggest checking the DeepSpeed ZeRO-3 Offload documentation.
We’ve ran benchmarks for all these features and given a simple example of how all these features work in Lightning, which you can see at minGPT.
To reach the highest memory efficiency or model size, you must:
Use the DeepSpeed strategy with the stage 3 parameter
Use CPU Offloading to offload weights to CPU, plus have a reasonable amount of CPU RAM to offload onto
Use DeepSpeed Activation Checkpointing to shard activations
Below we describe how to enable all of these to see benefit. With all these improvements we reached 45 Billion parameters training a GPT model on 8 GPUs with ~1TB of CPU RAM available.
Also please have a look at our DeepSpeed ZeRO Stage 3 Tips which contains a lot of helpful information when configuring your own models.
참고
When saving a model using DeepSpeed and Stage 3, model states and optimizer states will be saved in separate sharded states (based on the world size). See Collating Single File Checkpoint for DeepSpeed ZeRO Stage 3 to obtain a single checkpoint file.
from lightning.pytorch import Trainer
from deepspeed.ops.adam import FusedAdam
class MyModel(pl.LightningModule):
...
def configure_optimizers(self):
return FusedAdam(self.parameters())
model = MyModel()
trainer = Trainer(accelerator="gpu", devices=4, strategy="deepspeed_stage_3", precision=16)
trainer.fit(model)
trainer.test()
trainer.predict()
You can also use the Lightning Trainer to run predict or evaluate with DeepSpeed once the model has been trained.
from lightning.pytorch import Trainer
class MyModel(pl.LightningModule):
...
model = MyModel()
trainer = Trainer(accelerator="gpu", devices=4, strategy="deepspeed_stage_3", precision=16)
trainer.test(ckpt_path="my_saved_deepspeed_checkpoint.ckpt")
DeepSpeed ZeRO Stage 3 Offload¶
DeepSpeed ZeRO Stage 3 Offloads optimizer state, gradients to the host CPU to reduce memory usage as ZeRO Stage 2 does, however additionally allows you to offload the parameters as well for even more memory saving.
참고
When saving a model using DeepSpeed and Stage 3, model states and optimizer states will be saved in separate sharded states (based on the world size). See Collating Single File Checkpoint for DeepSpeed ZeRO Stage 3 to obtain a single checkpoint file.
from lightning.pytorch import Trainer
from lightning.pytorch.strategies import DeepSpeedStrategy
# Enable CPU Offloading
model = MyModel()
trainer = Trainer(accelerator="gpu", devices=4, strategy="deepspeed_stage_3_offload", precision=16)
trainer.fit(model)
# Enable CPU Offloading, and offload parameters to CPU
model = MyModel()
trainer = Trainer(
accelerator="gpu",
devices=4,
strategy=DeepSpeedStrategy(
stage=3,
offload_optimizer=True,
offload_parameters=True,
),
precision=16,
)
trainer.fit(model)
DeepSpeed Infinity (NVMe Offloading)¶
Additionally, DeepSpeed supports offloading to NVMe drives for even larger models, utilizing the large memory space found in NVMes. DeepSpeed reports the ability to fine-tune 1 Trillion+ parameters using NVMe Offloading on one 8 GPU machine. Below shows how to enable this, assuming the NVMe drive is mounted in a directory called /local_nvme.
from lightning.pytorch import Trainer
from lightning.pytorch.strategies import DeepSpeedStrategy
# Enable CPU Offloading
model = MyModel()
trainer = Trainer(accelerator="gpu", devices=4, strategy="deepspeed_stage_3_offload", precision=16)
trainer.fit(model)
# Enable CPU Offloading, and offload parameters to CPU
model = MyModel()
trainer = Trainer(
accelerator="gpu",
devices=4,
strategy=DeepSpeedStrategy(
stage=3,
offload_optimizer=True,
offload_parameters=True,
remote_device="nvme",
offload_params_device="nvme",
offload_optimizer_device="nvme",
nvme_path="/local_nvme",
),
precision=16,
)
trainer.fit(model)
When offloading to NVMe you may notice that the speed is slow. There are parameters that need to be tuned based on the drives that you are using. Running the aio_bench_perf_sweep.py script can help you to find optimum parameters. See the issue for more information on how to parse the information.
DeepSpeed Activation Checkpointing¶
Activation checkpointing frees activations from memory as soon as they are not needed during the forward pass. They are then re-computed for the backwards pass as needed.
Activation checkpointing is very useful when you have intermediate layers that produce large activations.
This saves memory when training larger models, however requires using a checkpoint function to run modules as shown below.
경고
Ensure to not wrap the entire model with activation checkpointing. This is not the intended usage of activation checkpointing, and will lead to failures as seen in this discussion.
from lightning.pytorch import Trainer
import deepspeed
class MyModel(LightningModule):
...
def __init__(self):
super().__init__()
self.block_1 = nn.Sequential(nn.Linear(32, 32), nn.ReLU())
self.block_2 = torch.nn.Linear(32, 2)
def forward(self, x):
# Use the DeepSpeed checkpointing function instead of calling the module directly
# checkpointing self.block_1 means the activations are deleted after use,
# and re-calculated during the backward passes
x = deepspeed.checkpointing.checkpoint(self.block_1, x)
return self.block_2(x)
from lightning.pytorch import Trainer
from lightning.pytorch.strategies import DeepSpeedStrategy
import deepspeed
class MyModel(pl.LightningModule):
...
def configure_model(self):
self.block_1 = nn.Sequential(nn.Linear(32, 32), nn.ReLU())
self.block_2 = torch.nn.Linear(32, 2)
def forward(self, x):
# Use the DeepSpeed checkpointing function instead of calling the module directly
x = deepspeed.checkpointing.checkpoint(self.block_1, x)
return self.block_2(x)
model = MyModel()
trainer = Trainer(accelerator="gpu", devices=4, strategy="deepspeed_stage_3_offload", precision=16)
# Enable CPU Activation Checkpointing
trainer = Trainer(
accelerator="gpu",
devices=4,
strategy=DeepSpeedStrategy(
stage=3,
offload_optimizer=True, # Enable CPU Offloading
cpu_checkpointing=True, # (Optional) offload activations to CPU
),
precision=16,
)
trainer.fit(model)
DeepSpeed ZeRO Stage 3 Tips¶
Here is some helpful information when setting up DeepSpeed ZeRO Stage 3 with Lightning.
If you’re using Adam or AdamW, ensure to use FusedAdam or DeepSpeedCPUAdam (for CPU Offloading) rather than the default torch optimizers as they come with large speed benefits
Treat your GPU/CPU memory as one large pool. In some cases, you may not want to offload certain things (like activations) to provide even more space to offload model parameters
When offloading to the CPU, make sure to bump up the batch size as GPU memory will be freed
We also support sharded checkpointing. By passing
save_full_weights=Falseto theDeepSpeedStrategy, we’ll save shards of the model which allows you to save extremely large models. However to load the model and run test/validation/predict you must use the Trainer object.
Collating Single File Checkpoint for DeepSpeed ZeRO Stage 3¶
After training using ZeRO Stage 3, you’ll notice that your checkpoints are a directory of sharded model and optimizer states. If you’d like to collate a single file from the checkpoint directory please use the below command, which handles all the Lightning states additionally when collating the file.
from lightning.pytorch.utilities.deepspeed import convert_zero_checkpoint_to_fp32_state_dict
# lightning deepspeed has saved a directory instead of a file
save_path = "lightning_logs/version_0/checkpoints/epoch=0-step=0.ckpt/"
output_path = "lightning_model.pt"
convert_zero_checkpoint_to_fp32_state_dict(save_path, output_path)
경고
This single file checkpoint does not include the optimizer/lr-scheduler states. This means we cannot restore training via the trainer.fit(ckpt_path=) call. Ensure to keep the sharded checkpoint directory if this is required.
Custom DeepSpeed Config¶
In some cases you may want to define your own DeepSpeed Config, to access all parameters defined. We’ve exposed most of the important parameters, however, there may be debugging parameters to enable. Also, DeepSpeed allows the use of custom DeepSpeed optimizers and schedulers defined within a config file that is supported.
참고
All strategy default parameters will be ignored when a config object is passed. All compatible arguments can be seen in the DeepSpeed docs.
from lightning.pytorch import Trainer
from lightning.pytorch.strategies import DeepSpeedStrategy
deepspeed_config = {
"zero_allow_untested_optimizer": True,
"optimizer": {
"type": "OneBitAdam",
"params": {
"lr": 3e-5,
"betas": [0.998, 0.999],
"eps": 1e-5,
"weight_decay": 1e-9,
"cuda_aware": True,
},
},
"scheduler": {
"type": "WarmupLR",
"params": {
"last_batch_iteration": -1,
"warmup_min_lr": 0,
"warmup_max_lr": 3e-5,
"warmup_num_steps": 100,
},
},
"zero_optimization": {
"stage": 2, # Enable Stage 2 ZeRO (Optimizer/Gradient state partitioning)
"offload_optimizer": {"device": "cpu"}, # Enable Offloading optimizer state/calculation to the host CPU
"contiguous_gradients": True, # Reduce gradient fragmentation.
"overlap_comm": True, # Overlap reduce/backward operation of gradients for speed.
"allgather_bucket_size": 2e8, # Number of elements to all gather at once.
"reduce_bucket_size": 2e8, # Number of elements we reduce/allreduce at once.
},
}
model = MyModel()
trainer = Trainer(accelerator="gpu", devices=4, strategy=DeepSpeedStrategy(config=deepspeed_config), precision=16)
trainer.fit(model)
We support taking the config as a json formatted file:
from lightning.pytorch import Trainer
from lightning.pytorch.strategies import DeepSpeedStrategy
model = MyModel()
trainer = Trainer(
accelerator="gpu", devices=4, strategy=DeepSpeedStrategy(config="/path/to/deepspeed_config.json"), precision=16
)
trainer.fit(model)
You can use also use an environment variable via your PyTorch Lightning script:
PL_DEEPSPEED_CONFIG_PATH=/path/to/deepspeed_config.json python train.py --strategy deepspeed
DDP Optimizations¶
DDP Static Graph¶
DDP static graph assumes that your model employs the same set of used/unused parameters in every iteration, so that it can deterministically know the flow of training and apply special optimizations during runtime.
참고
DDP static graph support requires PyTorch>=1.11.0
from lightning.pytorch import Trainer
from lightning.pytorch.strategies import DDPStrategy
trainer = Trainer(devices=4, strategy=DDPStrategy(static_graph=True))
When Using DDP on a Multi-node Cluster, Set NCCL Parameters¶
NCCL is the NVIDIA Collective Communications Library that is used by PyTorch to handle communication across nodes and GPUs. There are reported benefits in terms of speedups when adjusting NCCL parameters as seen in this issue. In the issue, we see a 30% speed improvement when training the Transformer XLM-RoBERTa and a 15% improvement in training with Detectron2.
NCCL parameters can be adjusted via environment variables.
참고
AWS and GCP already set default values for these on their clusters. This is typically useful for custom cluster setups.
export NCCL_NSOCKS_PERTHREAD=4
export NCCL_SOCKET_NTHREADS=2
Gradients as Bucket View¶
Enabling gradient_as_bucket_view=True in the DDPStrategy will make gradients views point to different offsets of the allreduce communication buckets. See DistributedDataParallel for more information.
This can reduce peak memory usage and throughput as saved memory will be equal to the total gradient memory + removes the need to copy gradients to the allreduce communication buckets.
참고
When gradient_as_bucket_view=True you cannot call detach_() on gradients. If hitting such errors, please fix it by referring to the zero_grad() function in torch/optim/optimizer.py as a solution (source).
from lightning.pytorch import Trainer
from lightning.pytorch.strategies import DDPStrategy
model = MyModel()
trainer = Trainer(accelerator="gpu", devices=4, strategy=DDPStrategy(gradient_as_bucket_view=True))
trainer.fit(model)
DDP Communication Hooks¶
DDP Communication hooks is an interface to control how gradients are communicated across workers, overriding the standard allreduce in DistributedDataParallel. This allows you to enable performance improving communication hooks when using multiple nodes.
Enable FP16 Compress Hook for multi-node throughput improvement:
from lightning.pytorch import Trainer
from lightning.pytorch.strategies import DDPStrategy
from torch.distributed.algorithms.ddp_comm_hooks import default_hooks as default
model = MyModel()
trainer = Trainer(accelerator="gpu", devices=4, strategy=DDPStrategy(ddp_comm_hook=default.fp16_compress_hook))
trainer.fit(model)
Enable PowerSGD for multi-node throughput improvement:
참고
PowerSGD typically requires extra memory of the same size as the model’s gradients to enable error feedback, which can compensate for biased compressed communication and improve accuracy (source).
from lightning.pytorch import Trainer
from lightning.pytorch.strategies import DDPStrategy
from torch.distributed.algorithms.ddp_comm_hooks import powerSGD_hook as powerSGD
model = MyModel()
trainer = Trainer(
accelerator="gpu",
devices=4,
strategy=DDPStrategy(
ddp_comm_state=powerSGD.PowerSGDState(
process_group=None,
matrix_approximation_rank=1,
start_powerSGD_iter=5000,
),
ddp_comm_hook=powerSGD.powerSGD_hook,
),
)
trainer.fit(model)
Combine hooks for accumulated benefit:
from lightning.pytorch import Trainer
from lightning.pytorch.strategies import DDPStrategy
from torch.distributed.algorithms.ddp_comm_hooks import (
default_hooks as default,
powerSGD_hook as powerSGD,
)
model = MyModel()
trainer = Trainer(
accelerator="gpu",
devices=4,
strategy=DDPStrategy(
ddp_comm_state=powerSGD.PowerSGDState(
process_group=None,
matrix_approximation_rank=1,
start_powerSGD_iter=5000,
),
ddp_comm_hook=powerSGD.powerSGD_hook,
ddp_comm_wrapper=default.fp16_compress_wrapper,
),
)
trainer.fit(model)
When using Post-localSGD, you must also pass model_averaging_period to allow for model parameter averaging:
from lightning.pytorch import Trainer
from lightning.pytorch.strategies import DDPStrategy
from torch.distributed.algorithms.ddp_comm_hooks import post_localSGD_hook as post_localSGD
model = MyModel()
trainer = Trainer(
accelerator="gpu",
devices=4,
strategy=DDPStrategy(
ddp_comm_state=post_localSGD.PostLocalSGDState(
process_group=None,
subgroup=None,
start_localSGD_iter=8,
),
ddp_comm_hook=post_localSGD.post_localSGD_hook,
model_averaging_period=4,
),
)
trainer.fit(model)
