A Problem aboud using multi-gpu with a two-stage CNN model
I design a CNN model which has two stages. First stage is generating proposals like RPN in Faster RCNN and the second feeds these proposals into the following part.
It causes error in the second step.
Accroding the below error information, it seems like the second input is not correctly assigned to the multi GPU.
However, The model works file with single gpu.
  File &quot;/home/f523/guazai/sdb/rsy/cornerPoject/myCornerNet6/exp/train.py&quot;, line 212, in run_epoch
    cls, rgr = self.model([proposal, fm], stage='two')
  File &quot;/home/f523/anaconda3/envs/rsy/lib/python3.6/site-packages/torch/nn/modules/module.py&quot;, line 722, in _call_impl
    result = self.forward(*input, **kwargs)
  File &quot;/home/f523/anaconda3/envs/rsy/lib/python3.6/site-packages/torch/nn/parallel/data_parallel.py&quot;, line 156, in forward
    return self.gather(outputs, self.output_device)
  File &quot;/home/f523/anaconda3/envs/rsy/lib/python3.6/site-packages/torch/nn/parallel/data_parallel.py&quot;, line 168, in gather
    return gather(outputs, output_device, dim=self.dim)
  File &quot;/home/f523/anaconda3/envs/rsy/lib/python3.6/site-packages/torch/nn/parallel/scatter_gather.py&quot;, line 68, in gather
    res = gather_map(outputs)
  File &quot;/home/f523/anaconda3/envs/rsy/lib/python3.6/site-packages/torch/nn/parallel/scatter_gather.py&quot;, line 63, in gather_map
    return type(out)(map(gather_map, zip(*outputs)))
  File &quot;/home/f523/anaconda3/envs/rsy/lib/python3.6/site-packages/torch/nn/parallel/scatter_gather.py&quot;, line 55, in gather_map
    return Gather.apply(target_device, dim, *outputs)
  File &quot;/home/f523/anaconda3/envs/rsy/lib/python3.6/site-packages/torch/nn/parallel/_functions.py&quot;, line 68, in forward
    return comm.gather(inputs, ctx.dim, ctx.target_device)
  File &quot;/home/f523/anaconda3/envs/rsy/lib/python3.6/site-packages/torch/cuda/comm.py&quot;, line 166, in gather
    return torch._C._gather(tensors, dim, destination)
RuntimeError: CUDA error: an illegal memory access was encountered


PS
my model sciprt shows below. I want my two-stage model can support for multi batch. e.g. the batch size is 4 and every img output 128 proposal, so the proposal size in here is (4*128, 5)
def _stage2(self, xs):
    proposal, fm = xs
    if proposal.dim()==2 and proposal.size(1) == 5:
        # train mode
        roi = roi_align(fm, proposal, output_size=[15, 15])
    elif proposal.dim()==3 and proposal.size(2) == 4:
        # eval mode
        roi = roi_align(fm, [proposal[0]], output_size=[15, 15])
    else:
        assert AssertionError(&quot; The boxes tensor shape should be Tensor[K, 5] in train or Tensor[N, 4] in eval&quot;)
    x = self.big_kernel(roi)
    cls = self.cls_fm(x)
    rgr = self.rgr_fm(x)
    return cls, rgr


I know where I am wrong. Here’s my second stage to feed input
cls, offset = self.model([proposal, fm], stage='two')

proposal is the ROI whose shape is [N, 5], the 1th dim is the batch index. e.g. The batch size is 4, the range of index is [0,1,2,3]. And fm is the feature map.
When I use the mult-gpu like 2 gpu. the proposal and fm will be split into two branch and feed into two gpu. However the batch index range still be [0,1,2,3], then cause a index error and raise gpu error.
What I do is add a line before roi_align like below:
from torchvision.ops import roi_align
proposal[:, 0] = proposal[:, 0] % fm.size(0) # this make multi-gpu work
roi = roi_align(fm, proposal, output_size=[15, 15])

