Optuna HPO & Lightning Multi-GPU Training using DDP on SLURM - ValueError: World Size does not Match

[eTuDpy]

I'm trying to utilize all the computational resources on a SLURM cluster to speed up my hyperparameter optimization using Optuna and Pytorch Lightning.

My code works fine solely with the Pytorch Lightning trainer and without the Optuna HPO loop, however, when using all together the world_size does seem to fail to be set to the correct value.

I know, that Optuna preferce the "ddp_spawn", however, as far as I can tell, there is not much performance gain of the additional GPUs.

But when it comes to start the Pytorch Lightning Trainer, I get the following ValueError:

"ValueError: You set `devices=4` and `num_nodes=1` in Lightning, but the product (4 * 1) does not match the world size (1)."

As the code runs smoothly without Optuna AND runs smoothly with Optuna but just 1 GPU / no DDP, I assume it must be something else missing or be wrong.

My Slurm File looks like this:

#!/bin/bash -l
#SBATCH --partition=alpha
#SBATCH --nodes=1
#SBATCH --ntasks-per-node=1
#SBATCH --gres=gpu:4
#SBATCH --cpus-per-task=4
#SBATCH --mem-per-gpu=40Gb
#SBATCH --time=72:00:00
#SBATCH --output ./output/%A.out
#SBATCH --error ./error/%A.out

# Activate My Environment
module load Anaconda3/2022.05
#module load Python/3.10.4
#module load CUDA/12.0.0
source activate $HOME/user-kernel/dl_p310

# run script
srun torchrun start_pl_hpo_ddp.py

My Trainer looks like this:

# Class Definition for better Optuna Pruning
class OptunaPruning(PyTorchLightningPruningCallback, pl.Callback):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

# Code Snippet
pl_pruner = OptunaPruning(trial, monitor="val_loss")
dist.init_process_group(backend="nccl")
torch.set_float32_matmul_precision("medium")
trainer = pl.Trainer(accelerator="gpu",
                     strategy='ddp', 
                     #num_nodes=1
                     devices=4, 
                     max_epochs=20,
                     logger=True,
                     check_val_every_n_epoch=1,
                     callbacks=[pl_pruner], 
                     log_every_n_steps=100,
                     num_sanity_val_steps=0,
                     enable_progress_bar=False,
                     enable_model_summary = False,
                     enable_checkpointing=False,
                    )

Small Notes:

• I commented out num_nodes as this post mentioned it is unnecessary.
• setting #SBATCH --ntasks-per-node=1 to 4 does not do the trick

[inonoo123]

Ran into the same issue

[btebbutt]

same

[Les1ie]

Same problem.

[bursica]

Same issue:

• ddp without optuna OK,
• optuna without ddp OK ,
• optuna and ddp/several GPU's KO