Is this the correct way of implementing the DDP?

username: mahesh_bhosale
cooked: <p>Can someone please verify if the below DDP implementation sample is correct? I have a feeling that data is somehow being duplicated and duplicate training is done on multiple GPUs. Is there a way to know how much data is distributed between multiple GPUs?</p>
<pre><code class="lang-auto">def setup(rank, world_size, port):
    os.environ['MASTER_ADDR'] = 'localhost'
    os.environ['MASTER_PORT'] = port

    # initialize the process group
    dist.init_process_group("gloo", rank=rank, world_size=world_size)

def cleanup():
    dist.destroy_process_group()

def run_parallel(rank, arg1, arg2, world_size):
    import time
    start = time.process_time()
    logger = create_logger(args)
    globals()['logger'] = logger
    logger.info('Starting pooling')
    p = Pool()
    logging.info(f"Running training on {rank}.")
    logging.info(args)

    logging.info(f"DEVICE COUNT {torch.cuda.device_count()}")
    logging.info(f'GPU: {args.gpu}')
    setup(rank, world_size, args.port)

    writer_logdir = os.path.join(args.logfile, 'tb')#'_'.join(['vm','un'+str(args.uncert), str(args.weight), args.logfile]) 
    tb = SummaryWriter(log_dir= writer_logdir)

    ##BUILD MODEL##
    model = RegNet(args).to(rank)
    ddp_model = DDP(model, device_ids=[rank], output_device = rank)
    train = TrainModel(ddp_model, train_dataloader, test_dataloader, args, NUM_CLASS, tb=tb, save_version=args.save_version)
        train.run()
    if tb is not None:
        tb.close()
    cleanup()

class TrainModel():
...
   def run(self):#device=torch.device("cuda:0")
          if self.args.sgd:
              optimizer = SGD(self.model.parameters(), lr = self.args.lr)
          else:
              optimizer = Adam(self.model.parameters(),lr = self.args.lr)
          for epoch in range(self.args.epoch):
              self.train_epoch(optimizer, scheduler, epoch)
              self.cur_epoch = epoch

   def train_epoch(self, optimizer, scheduler, epoch):
        self.model.train()
        idx = 0
        for n_iter, samples in enumerate(self.train_dataloader):
            fixed, fixed_label, moving, moving_label, fixed_nopad, seg_fname = self.data_extract(samples, self.model.device)
            if not self.save_seg:
                seg_fname = None
            self.global_idx += 1
            self.cur_idx = idx
            logging.info(f'iteration={idx}/{len(self.train_dataloader)}')
            idx+=1
            loss, trdice = self.trainIter(fixed, moving, fixed_label, moving_label, fixed_nopad=fixed_nopad, seg_f=seg_fname)
            optimizer.zero_grad()
            loss.backward()
            optimizer.step()

if __name__ == "__main__":
    args = get_args()
    torch.multiprocessing.set_sharing_strategy('file_system') # handle large number of files. Increase the number of open files.
    torch.cuda.empty_cache()
    os.environ['CUDA_VISIBLE_DEVICES'] = args.gpu
    gpu = [int(i) for i in range(torch.cuda.device_count())]
    world_size = len(GPU)
     mp.spawn(run_parallel,
             args=(args, world_size),
             nprocs=world_size,
             join=True)
</code></pre>

username: ptrblck
cooked: <p>I don’t see how <code>train_dataloader</code> is defined but you could check its <code>len</code> to see how many batches each rank will use. The common approach is to use a <code>DistributedSampler</code>, which will make sure no samples are duplicated between ranks (unless padding is applied to make the dataset length divisible by the ranks).</p>

username: mahesh_bhosale
cooked: <pre><code class="lang-auto">train_dataloader = torch.utils.data.DataLoader(
            train_data,
            batch_size=bsize,
            shuffle=True,
            drop_last= True,
            num_workers=num_workers)
</code></pre>
<p>this is how the train data loader is defined. I set the batch size to 2, but irrespective of how many GPUs I choose for training the the length in the print statement in the training loop below is all the same,<br>
<code>logging.info(f'iteration={idx}/{len(self.train_dataloader)}')</code></p>

username: ptrblck
cooked: <p>That makes sense since you are reusing the entire dataset in each rank.<br>
Use a <a href="https://pytorch.org/docs/stable/data.html#torch.utils.data.distributed.DistributedSampler"><code>torch.utils.data.distributed.DistributedSampler</code></a> to properly split the data.</p>