PyTorch - contiguous()

There are few operations on Tensor in PyTorch that do not really change the content of the tensor, but only how to convert indices in to tensor to byte location. These operations include:

narrow(), view(), expand() and transpose()

For example: when you call transpose(), PyTorch doesn't generate new tensor with new layout, it just modifies meta information in Tensor object so offset and stride are for new shape. The transposed tensor and original tensor are indeed sharing the memory!

x = torch.randn(3,2)
y = torch.transpose(x, 0, 1)
x[0, 0] = 42
print(y[0,0])
# prints 42

This is where the concept of contiguous comes in. Above x is contiguous but y is not because its memory layout is different than a tensor of same shape made from scratch. Note that the word "contiguous" is bit misleading because its not that the content of tensor is spread out around disconnected blocks of memory. Here bytes are still allocated in one block of memory but the order of the elements is different!

When you call contiguous(), it actually makes a copy of tensor so the order of elements would be same as if tensor of same shape created from scratch.

Normally you don't need to worry about this. If PyTorch expects contiguous tensor but if its not then you will get RuntimeError: input is not contiguous and then you just add a call to contiguous().

From the [pytorch documentation][1]:

contiguous() → Tensor

Returns a contiguous tensor containing the same data as self 

tensor. If self tensor is contiguous, this function returns the self tensor.

Where contiguous here means not only contiguous in memory, but also in the same order in memory as the indices order: for example doing a transposition doesn't change the data in memory, it simply changes the map from indices to memory pointers, if you then apply contiguous() it will change the data in memory so that the map from indices to memory location is the canonical one. [1]: http://pytorch.org/docs/master/tensors.html

tensor.contiguous() will create a copy of the tensor, and the element in the copy will be stored in the memory in a contiguous way. The contiguous() function is usually required when we first transpose() a tensor and then reshape (view) it. First, let's create a contiguous tensor:

aaa = torch.Tensor( [[1,2,3],[4,5,6]] )
print(aaa.stride())
print(aaa.is_contiguous())
#(3,1)
#True

The stride() return (3,1) means that: when moving along the first dimension by each step (row by row), we need to move 3 steps in the memory. When moving along the second dimension (column by column), we need to move 1 step in the memory. This indicates that the elements in the tensor are stored contiguously.

Now we try apply come functions to the tensor:

bbb = aaa.transpose(0,1)
print(bbb.stride())
print(bbb.is_contiguous())

#(1, 3)
#False


ccc = aaa.narrow(1,1,2)   ## equivalent to matrix slicing aaa[:,1:3]
print(ccc.stride())
print(ccc.is_contiguous())

#(3, 1)
#False


ddd = aaa.repeat(2,1)   # The first dimension repeat once, the second dimension repeat twice
print(ddd.stride())
print(ddd.is_contiguous())

#(3, 1)
#True


## expand is different from repeat.
## if a tensor has a shape [d1,d2,1], it can only be expanded using "expand(d1,d2,d3)", which
## means the singleton dimension is repeated d3 times
eee = aaa.unsqueeze(2).expand(2,3,3)
print(eee.stride())
print(eee.is_contiguous())

#(3, 1, 0)
#False


fff = aaa.unsqueeze(2).repeat(1,1,8).view(2,-1,2)
print(fff.stride())
print(fff.is_contiguous())

#(24, 2, 1)
#True

Ok, we can find that transpose(), narrow() and tensor slicing, and expand() will make the generated tensor not contiguous. Interestingly, repeat() and view() does not make it discontiguous. So now the question is: what happens if I use a discontiguous tensor?

The answer is it the view() function cannot be applied to a discontiguous tensor. This is probably because view() requires that the tensor to be contiguously stored so that it can do fast reshape in memory. e.g:

bbb.view(-1,3)

we will get the error:

---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)
<ipython-input-63-eec5319b0ac5> in <module>()
----> 1 bbb.view(-1,3)

RuntimeError: invalid argument 2: view size is not compatible with input tensor's size and stride (at least one dimension spans across two contiguous subspaces). Call .contiguous() before .view(). at /pytorch/aten/src/TH/generic/THTensor.cpp:203

To solve this, simply add contiguous() to a discontiguous tensor, to create contiguous copy and then apply view()

bbb.contiguous().view(-1,3)
#tensor([[1., 4., 2.],
        [5., 3., 6.]])