  Long Short-Term Memory (LSTM) has achieved state-of-the-art performances on a
wide range of tasks. Its outstanding performance is guaranteed by the long-term
memory ability which matches the sequential data perfectly and the gating
structure controlling the information flow. However, LSTMs are prone to be
memory-bandwidth limited in realistic applications and need an unbearable
period of training and inference time as the model size is ever-increasing. To
tackle this problem, various efficient model compression methods have been
proposed. Most of them need a big and expensive pre-trained model which is a
nightmare for resource-limited devices where the memory budget is strictly
limited. To remedy this situation, in this paper, we incorporate the Sparse
Evolutionary Training (SET) procedure into LSTM, proposing a novel model dubbed
SET-LSTM. Rather than starting with a fully-connected architecture, SET-LSTM
has a sparse topology and dramatically fewer parameters in both phases,
training and inference. Considering the specific architecture of LSTMs, we
replace the LSTM cells and embedding layers with sparse structures and further
on, use an evolutionary strategy to adapt the sparse connectivity to the data.
Additionally, we find that SET-LSTM can provide many different good
combinations of sparse connectivity to substitute the overparameterized
optimization problem of dense neural networks. Evaluated on four sentiment
analysis classification datasets, the results demonstrate that our proposed
model is able to achieve usually better performance than its fully connected
counterpart while having less than 4\% of its parameters.
