I want to gather tensors from specific ranks  in each rank (For example, I want gather ranks=[0,1] in rank0&rank1,  and gather ranks=[2,3] in rank2&3). I implement by initial new group:
import os
import random
import torch
import torch.nn as nn
import torch.multiprocessing as mp
import torch.distributed as dist

import torch.utils.data
import torch.utils.data.distributed
from torch.multiprocessing import Process


from absl import flags
from absl import app

FLAGS = flags.FLAGS

flags.DEFINE_integer('nodes_num', 1, 'machine num')
flags.DEFINE_integer('ngpu', 4, 'ngpu per node')
flags.DEFINE_integer('world_size', 4, 'FLAGS.nodes_num*FLAGS.ngpu')
flags.DEFINE_integer('node_rank', 0, 'rank of machine, 0 to nodes_num-1')
flags.DEFINE_integer('rank', 0, 'rank of total threads, 0 to FLAGS.world_size-1, will be re-compute in main_worker func')

@torch.no_grad()
def group_gather(tensor, rank, ngpu_per_node):
    #ranks = [0,1]
    if rank == 0 or rank == 1:
        ranks = [0,1]
    if rank == 2 or rank == 3:
        ranks = [2,3]
    print('ranks: ', ranks)
    group = dist.new_group(ranks = ranks)
    tensors_gather = [torch.ones_like(tensor) for _ in range(2)]
    torch.distributed.all_gather(tensors_gather, tensor, group, async_op=False)
    output = torch.cat(tensors_gather, dim=0)
    print('gather out shape: ', output.shape)
    return output


class ToyModel(nn.Module):
    def __init__(self):
        super(ToyModel, self).__init__()
        self.fc = nn.Linear(3,2)

    def forward(self, x, rank, ngpu_per_node):
        x_gather = group_gather(x, rank, ngpu_per_node)
        out = self.fc(x_gather)
        return out 

def main(argv):
    del argv
    FLAGS.append_flags_into_file('tmp.cfg')
    os.environ['MASTER_ADDR'] = '127.0.0.1'
    os.environ['MASTER_PORT'] = str(random.randint(1,100000))
    mp.spawn(main_worker, nprocs=FLAGS.ngpu, args=())

def main_worker(gpu_rank):
    FLAGS._parse_args(FLAGS.read_flags_from_files(['--flagfile=./tmp.cfg']), True)
    FLAGS.mark_as_parsed()
    FLAGS.rank = FLAGS.node_rank * FLAGS.ngpu + gpu_rank # rank among FLAGS.world_size
    dist.init_process_group(
        backend='nccl',
        init_method='env://',
        world_size=FLAGS.world_size,
        rank=FLAGS.rank)
    model = ToyModel()
    torch.cuda.set_device(gpu_rank)
    model.cuda()
    model = torch.nn.parallel.DistributedDataParallel(
        model, device_ids=[gpu_rank])

    x = torch.randn(4,3).cuda()
    model(x, FLAGS.rank, FLAGS.ngpu)


if __name__ == '__main__':
    app.run(main)



In group_gather(…), I init new group according to thread’s rank.
But this scripts can not always work well, It may crash in some times, and raise error:
Traceback (most recent call last):
  File "/root/anaconda3/lib/python3.6/site-packages/torch/multiprocessing/spawn.py", line 19, in _wrap
    fn(i, *args)
  File "/root/test_distcomm/test_group_gather.py", line 78, in main_worker
    model(x, FLAGS.rank, FLAGS.ngpu)
  File "/root/anaconda3/lib/python3.6/site-packages/torch/nn/modules/module.py", line 532, in __call__
    result = self.forward(*input, **kwargs)
  File "/root/anaconda3/lib/python3.6/site-packages/torch/nn/parallel/distributed.py", line 447, in forward
    output = self.module(*inputs[0], **kwargs[0])
  File "/root/anaconda3/lib/python3.6/site-packages/torch/nn/modules/module.py", line 532, in __call__
    result = self.forward(*input, **kwargs)
  File "/root/test_distcomm/test_group_gather.py", line 48, in forward
    x_gather = group_gather(x, gpu_rank, ngpu_per_node)
  File "/root/anaconda3/lib/python3.6/site-packages/torch/autograd/grad_mode.py", line 49, in decorate_no_grad
    return func(*args, **kwargs)
  File "/root/test_distcomm/test_group_gather.py", line 35, in group_gather
    torch.distributed.all_gather(tensors_gather, tensor, group, async_op=False)
  File "/root/anaconda3/lib/python3.6/site-packages/torch/distributed/distributed_c10d.py", line 1153, in all_gather
    work = group.allgather([tensor_list], [tensor])
RuntimeError: NCCL error in: /pytorch/torch/lib/c10d/ProcessGroupNCCL.cpp:410, unhandled system error, NCCL version 2.4.8


I think the logic in code is correct, and I can not figure out where is wrong.
I run this code with 4 nvidia-t4 gpus with cuda10.1, my pytorch version is 1.4.0.
You can simply run this code with ‘python main.py’ (may need pip install absl-py)