Multiple Nodes

Data Parallel

Request 3 nodes with at least 4 GPUs each.

#!/bin/bash
# Number of Nodes
#SBATCH --nodes=3

# Number of tasks. 3 (1 per node)
#SBATCH --ntasks=3

# Number of GPU per node
#SBATCH --gres=gpu:4
#SBATCH --gpus-per-node=4

# 16 CPUs per node
#SBATCH --cpus-per-gpu=4

# 16Go per nodes (4Go per GPU)
#SBATCH --mem=16G

# we need all nodes to be ready at the same time
#SBATCH --wait-all-nodes=1

# Total resources:
#   CPU: 16 * 3 = 48
#   RAM: 16 * 3 = 48 Go
#   GPU:  4 * 3 = 12

# Setup our rendez-vous point
RDV_ADDR=$(hostname)
WORLD_SIZE=$SLURM_JOB_NUM_NODES
# -----

srun --label torchrun \
   --nproc_per_node=$SLURM_GPUS_PER_NODE\
   --nnodes=$WORLD_SIZE\
   --rdzv_id=$SLURM_JOB_ID\
   --rdzv_backend=c10d\
   --rdzv_endpoint=$RDV_ADDR\
   training_script.py

You can find below a pytorch script outline on what a multi-node trainer could look like.

import os
import torch.distributed as dist

class Trainer:
   def __init__(self):
      self.local_rank = None
      self.chk_path = ...
      self.model = ...

   @property
   def device_id(self):
      return self.local_rank

   def load_checkpoint(self, path):
      self.chk_path = path
      # ...

   def should_checkpoint(self):
      # Note: only one worker saves its weights
      return self.global_rank == 0 and self.local_rank == 0

   def save_checkpoint(self):
      if self.chk_path is None:
            return

      # Save your states here
      # Note: you should save the weights of self.model not ddp_model
      # ...

   def initialize(self):
      self.global_rank = int(os.environ.get("RANK", -1))
      self.local_rank = int(os.environ.get("LOCAL_RANK", -1))

      assert self.global_rank >= 0, 'Global rank should be set (Only Rank 0 can save checkpoints)'
      assert self.local_rank >= 0, 'Local rank should be set'

      dist.init_process_group(backend="gloo|nccl")

   def sync_weights(self, resuming=False):
      if resuming:
            # in the case of resuming all workers need to load the same checkpoint
            self.load_checkpoint()

            # Wait for everybody to finish loading the checkpoint
            dist.barrier()
            return

      # Make sure all workers have the same initial weights
      # This makes the leader save his weights
      if self.should_checkpoint():
            self.save_checkpoint()

      # All workers wait for the leader to finish
      dist.barrier()

      # All followers load the leader's weights
      if not self.should_checkpoint():
            self.load_checkpoint()

      # Leader waits for the follower to load the weights
      dist.barrier()

   def dataloader(self, dataset, batch_size):
      train_sampler = ElasticDistributedSampler(dataset)
      train_loader = DataLoader(
            dataset,
            batch_size=batch_size,
            num_workers=4,
            pin_memory=True,
            sampler=train_sampler,
      )
      return train_loader

   def train_step(self):
      # Your batch processing step here
      # ...
      pass

   def train(self, dataset, batch_size):
      self.sync_weights()

      ddp_model = torch.nn.parallel.DistributedDataParallel(
            self.model,
            device_ids=[self.device_id],
            output_device=self.device_id
      )

      loader = self.dataloader(dataset, batch_size)

      for epoch in range(100):
            for batch in iter(loader):
               self.train_step(batch)

               if self.should_checkpoint():
                  self.save_checkpoint()

def main():
   trainer = Trainer()
   trainer.load_checkpoint(path)
   tainer.initialize()

   trainer.train(dataset, batch_size)

Note

To bypass Python GIL (Global interpreter lock) pytorch spawn one process for each GPU. In the example above this means at least 12 processes are spawn, at least 4 on each node.

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