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Oct. 17, 2017
Aug. 28, 2017


@musyoku had a very nice SRU implementaion in chainer, where he reports 30x speed-up over LSTM in some cases!

Check the speed comparison he did here.


SRU is a recurrent unit that can run over 10 times faster than cuDNN LSTM, without loss of accuracy tested on many tasks.

Average processing time of LSTM, conv2d and SRU, tested on GTX 1070

For example, the figure above presents the processing time of a single mini-batch of 32 samples. SRU achieves 10 to 16 times speed-up compared to LSTM, and operates as fast as (or faster than) word-level convolution using conv2d.


Training RNNs as Fast as CNNs

  title={Training RNNs as Fast as CNNs},
  author={Lei, Tao and Zhang, Yu},
  journal={arXiv preprint arXiv:1709.02755},


Install requirements via pip install -r requirements.txt. CuPy and pynvrtc needed to compile the CUDA code into a callable function at runtime. Only single GPU training is supported.


The usage of SRU is similar to nn.LSTM. SRU likely requires more stacking layers than LSTM. We recommend starting by 2 layers and use more if necessary (see our report for more experimental details).

import torch
from torch.autograd import Variable
from cuda_functional import SRU, SRUCell

# input has length 20, batch size 32 and dimension 128
x = Variable(torch.FloatTensor(20, 32, 128).cuda())

input_size, hidden_size = 128, 128

rnn = SRU(input_size, hidden_size,
    num_layers = 2,          # number of stacking RNN layers
    dropout = 0.0,           # dropout applied between RNN layers
    rnn_dropout = 0.0,       # variational dropout applied on linear transformation
    use_tanh = 1,            # use tanh?
    use_relu = 0,            # use ReLU?
    bidirectional = False    # bidirectional RNN ?

output, hidden = rnn(x)      # forward pass

# output is (length, batch size, hidden size * number of directions)
# hidden is (layers, batch size, hidden size * number of directions)

Make sure and the shared library cuda/lib64 can be found by the system, e.g.

export LD_LIBRARY_PATH=/usr/local/cuda/lib64
export PYTHONPATH=path_to_repo/sru

Instead of using PYTHONPATH, the SRU module now can be installed as a regular package via python install or pip install. See this PR.



  • ReLU activation
  • support multi-GPU (context change)
  • Layer normalization, residual, batch norm, etc
  • support packed sequence