Run on an on-prem cluster (advanced)¶
Run on a SLURM managed cluster¶
Lightning automates the details behind training on a SLURM-powered cluster. In contrast to the general purpose cluster above, the user does not start the jobs manually on each node and instead submits it to SLURM which schedules the resources and time for which the job is allowed to run.
Design your training script¶
To train a model using multiple nodes, do the following:
Design your LightningModule (no need to add anything specific here).
Enable DDP in the trainer
# train on 32 GPUs across 4 nodes trainer = Trainer(accelerator="gpu", devices=8, num_nodes=4, strategy="ddp")
It’s a good idea to structure your training script like this:
# train.py def main(args): model = YourLightningModule(args) trainer = Trainer(accelerator="gpu", devices=8, num_nodes=4, strategy="ddp") trainer.fit(model) if __name__ == "__main__": args = ... # you can use your CLI parser of choice, or the `LightningCLI` # TRAIN main(args)
Create the appropriate SLURM job:
# (submit.sh) #!/bin/bash -l # SLURM SUBMIT SCRIPT #SBATCH --nodes=4 # This needs to match Trainer(num_nodes=...) #SBATCH --gres=gpu:8 #SBATCH --ntasks-per-node=8 # This needs to match Trainer(devices=...) #SBATCH --mem=0 #SBATCH --time=0-02:00:00 # activate conda env source activate $1 # debugging flags (optional) export NCCL_DEBUG=INFO export PYTHONFAULTHANDLER=1 # on your cluster you might need these: # set the network interface # export NCCL_SOCKET_IFNAME=^docker0,lo # might need the latest CUDA # module load NCCL/2.4.7-1-cuda.10.0 # run script from above srun python3 train.py
If you want auto-resubmit (read below), add this line to the submit.sh script
Submit the SLURM job
Enable auto wall-time resubmitions¶
When you use Lightning in a SLURM cluster, it automatically detects when it is about to run into the wall time and does the following:
Saves a temporary checkpoint.
Requeues the job.
When the job starts, it loads the temporary checkpoint.
To get this behavior make sure to add the correct signal to your SLURM script
# 90 seconds before training ends SBATCH --signal=SIGUSR1@90
You can change this signal if your environment requires the use of a different one, for example
Then, when you make your trainer, pass the requeue_signal option to the
trainer = Trainer(plugins=[SLURMEnvironment(requeue_signal=signal.SIGHUP)])
If auto-resubmit is not desired, it can be turned off in the
from pytorch_lightning.plugins.environments import SLURMEnvironment trainer = Trainer(plugins=[SLURMEnvironment(auto_requeue=False)])
Build your SLURM script¶
Here is an example where you run a grid search of 9 combinations of hyperparameters. See also the multi-node examples here.
# grid search 3 values of learning rate and 3 values of number of layers for your net # this generates 9 experiments (lr=1e-3, layers=16), (lr=1e-3, layers=32), # (lr=1e-3, layers=64), ... (lr=1e-1, layers=64) parser = HyperOptArgumentParser(strategy="grid_search", add_help=False) parser.opt_list("--learning_rate", default=0.001, type=float, options=[1e-3, 1e-2, 1e-1], tunable=True) parser.opt_list("--layers", default=1, type=float, options=[16, 32, 64], tunable=True) hyperparams = parser.parse_args() # Slurm cluster submits 9 jobs, each with a set of hyperparams cluster = SlurmCluster( hyperparam_optimizer=hyperparams, log_path="/some/path/to/save", ) # OPTIONAL FLAGS WHICH MAY BE CLUSTER DEPENDENT # which interface your nodes use for communication cluster.add_command("export NCCL_SOCKET_IFNAME=^docker0,lo") # see the output of the NCCL connection process # NCCL is how the nodes talk to each other cluster.add_command("export NCCL_DEBUG=INFO") # setting a main port here is a good idea. cluster.add_command("export MASTER_PORT=%r" % PORT) # ************** DON'T FORGET THIS *************** # MUST load the latest NCCL version cluster.load_modules(["NCCL/2.4.7-1-cuda.10.0"]) # configure cluster cluster.per_experiment_nb_nodes = 12 cluster.per_experiment_nb_gpus = 8 cluster.add_slurm_cmd(cmd="ntasks-per-node", value=8, comment="1 task per gpu") # submit a script with 9 combinations of hyper params # (lr=1e-3, layers=16), (lr=1e-3, layers=32), (lr=1e-3, layers=64), ... (lr=1e-1, layers=64) cluster.optimize_parallel_cluster_gpu( main, nb_trials=9, job_name="name_for_squeue" # how many permutations of the grid search to run )
The other option is that you generate scripts on your own via a bash command or use our native solution.
The Trainer is stuck initializing at startup, what is causing this?
You are seeing a message like this in the logs but nothing happens:
Initializing distributed: GLOBAL_RANK: 0, MEMBER: 1/4
The most likely reasons and how to fix it:
You forgot to run the
python train.pycommand with
srun: Please have a look at the SLURM template script above which includes the
srunat the botton of the script.
The number of nodes or number of devices per node is configured incorrectly: There are two parametres in the SLURM submission script that determine how many processes will run your training, the
#SBATCH --nodes=Xsetting and
#SBATCH --ntasks-per-node=Ysettings. The numbers there need to match what is configured in your Trainer in the code:
Trainer(num_nodes=X, devices=Y). If you change the numbers, update them in BOTH places.
Setting up a cluster for distributed training is not trivial. Lightning offers lightning-grid which allows you to configure a cluster easily and run experiments via the CLI and web UI.
Try it out for free today: