# TensorFlow LSTM benchmark¶

There are multiple LSTM implementations/kernels available in TensorFlow, and we also have our own kernel. In this benchmark, we try to compare the runtime performance during training for each of the kernels. We try to measure in a way that it should be generic and not be specific for our Returnn framework. You can run this benchmark yourself with this script.

In Returnn with the TensorFlow backend, the rec layer (TFNetworkRecLayer.RecLayer) you can use these LSTM kernels via the unit argument:

• BasicLSTM (GPU and CPU).
Uses tf.contrib.rnn.BasicLSTMCell via dynamic_rnn. I.e. the cell itself is pure TensorFlow, and the loop over time is done via tf.while_loop.
• StandardLSTM (GPU and CPU).
Uses tf.contrib.rnn.LSTMCell via dynamic_rnn. I.e. the cell itself is pure TensorFlow, and the loop over time is done via tf.while_loop. This has some more options compared to BasicLSTM.
• LSTMBlock (GPU and CPU).
Uses tf.contrib.rnn.LSTMBlockCell via dynamic_rnn. The time-step operation is implemented as a single TF operation, and the loop over time is done via tf.while_loop. Thus this should be faster than BasicLSTM and StandardLSTM.
• LSTMBlockFused (GPU and CPU).
Uses tf.contrib.rnn.LSTMBlockFusedCell. The loop over time is part of the op (“fused” in TF terminology), thus this is like NativeLSTM and CudnnLSTM a single op for the whole calculation. This is based on LSTMBlock and should thus be faster than LSTMBlock.
• CudnnLSTM (GPU only).
Uses the LSTM kernel from cuDNN, via tf.contrib.cudnn_rnn.CudnnLSTM. The loop over time is done internally (“fused”). If you import such a model on CPU, it will automatically convert it into a LSTMBlockFused.
• NativeLSTM (GPU and CPU).
Uses our own CUDA kernel which can also be compiled on CPU. The loop over time is also done via C++ code inside the op (“fused”). See TFNativeOp.

If you just use LSTM, it will currently use NativeLSTM by default. Except of NativeLSTM, all of these kernels are part of the official TensorFlow framework. Note that these kernels are always use for a single direction in time and a single layer.

The cuDNN LSTM kernel can also work bidirectional and do multiple layers at once but tf.contrib.cudnn_rnn.CudnnLSTM currently does not support batches with sequences of different length, thus this is normally not an option to use. Note that most frameworks with cuDNN bindings do not support this correctly (see here), where CNTK is currently the only exception. In TensorFlow, this is issue 6633. Note that you still can use the cuDNN kernel in the way we do in Returnn, i.e. for a single layer in one time-direction.

For the benchmark, we build a multi-layer bidirectional network. Example of a 3 layer bidirectional LSTM:

network = {
"lstm1_fwd" : { "class": "rec", "unit": "lstm", "n_out" : 500, "direction": 1 },
"lstm1_bwd" : { "class": "rec", "unit": "lstm", "n_out" : 500, "direction": -1 },

"lstm2_fwd" : { "class": "rec", "unit": "lstm", "n_out" : 500, "direction": 1, "from" : ["lstm1_fwd", "lstm1_bwd"] },
"lstm2_bwd" : { "class": "rec", "unit": "lstm", "n_out" : 500, "direction": -1, "from" : ["lstm1_fwd", "lstm1_bwd"] },

"lstm3_fwd" : { "class": "rec", "unit": "lstm", "n_out" : 500, "direction": 1, "from" : ["lstm2_fwd", "lstm2_bwd"] },
"lstm3_bwd" : { "class": "rec", "unit": "lstm", "n_out" : 500, "direction": -1, "from" : ["lstm2_fwd", "lstm2_bwd"] },

"output" :   { "class" : "softmax", "loss" : "ce", "from" : ["lstm3_fwd", "lstm3_bwd"] }
}


We use framewise cross entropy as a loss for training, and we use a very simple artificial dataset (GeneratingDataset.Task12AXDataset) with dense input with a very low number of dimensions (9) and single output class indices (sparse) with a very low number of class labels (2), so that the overhead of the final softmax layer should be minimal, as well as the whole input pipeline. We are not interested in the error performance on this task in this benchmark, as in theory the results should all be the same. In practice, they are not due to different implementations, and also the initialization is currently not the same in all cases. However, that has no effect on the runtime performance.

By default, we use chunking, i.e. not the full sequences but only slices of it of fixed size (50 frames), to reduce the amount of padding in a mini-batch and also to keep the maximum sequence length of a batch fixed, and also to be able to increase the amount of sequences in a batch to allow more parallelism (40 sequences). See our paper for more details about chunking. Thus, our mini-batch has in total 2000 frames.

## Comparison¶

For a 5 layer bidirectional LSTM with dimension 500 in each time direction, on a GeForce GTX 980, using 8 CPU threads, we got these results:

GPU:CudnnLSTM: 0:00:08.8151
GPU:NativeLSTM: 0:00:08.8440
GPU:LSTMBlockFused: 0:00:16.9765
GPU:LSTMBlock: 0:00:33.4895
GPU:StandardLSTM: 0:00:39.5170
GPU:BasicLSTM: 0:00:41.7282
CPU:NativeLSTM: 0:04:05.4365
CPU:LSTMBlockFused: 0:04:35.1702
CPU:StandardLSTM: 0:04:57.7977
CPU:BasicLSTM: 0:05:00.5334
CPU:LSTMBlock: 0:05:07.5613


On a GeForce GTX 1080 Ti, using 8 CPU threads, for the same experiment we got:

GPU:NativeLSTM: 0:00:05.2728
GPU:CudnnLSTM: 0:00:05.3645
GPU:LSTMBlockFused: 0:00:09.3915
GPU:LSTMBlock: 0:00:15.3071
GPU:StandardLSTM: 0:00:17.8279
GPU:BasicLSTM: 0:00:22.3976
CPU:NativeLSTM: 0:05:09.6268
CPU:LSTMBlockFused: 0:07:45.5984
CPU:StandardLSTM: 0:08:02.5465
CPU:BasicLSTM: 0:08:16.3543
CPU:LSTMBlock: 0:08:18.1589


And on a GeForce GTX 1070, with 4 CPU threads, we got:

GPU:NativeLSTM: 0:00:03.9989
GPU:CudnnLSTM: 0:00:05.4496
GPU:LSTMBlockFused: 0:00:07.5233
GPU:LSTMBlock: 0:00:11.1515
GPU:StandardLSTM: 0:00:12.0605
GPU:BasicLSTM: 0:00:12.0833
CPU:LSTMBlockFused: 0:02:53.6482
CPU:BasicLSTM: 0:03:00.8289
CPU:StandardLSTM: 0:03:01.6320
CPU:LSTMBlock: 0:03:04.8836
CPU:NativeLSTM: 0:03:18.5375


On a CPU-only system with a single CPU thread, we got:

CPU:NativeLSTM: 0:15:55.7625
CPU:LSTMBlockFused: 0:24:53.1451
CPU:BasicLSTM: 0:26:28.2804
CPU:StandardLSTM: 0:27:10.0493
CPU:LSTMBlock: 0:27:58.8870


Each of those are executed on different hardware, so there might be small other differences due to that. Also the number of available CPU threads differs. Each of those were run on Ubuntu 16.04 with TensorFlow 1.2 (installed via pip), CUDA 8.0 and cuDNN 5.1.

## Analysis and discussion¶

We are quite proud that our own LSTM kernel (NativeLSTM) has a similar runtime than the cuDNN LSTM kernel (CudnnLSTM), sometimes even better. The implementation of it is quite straight-forward.

As expected, on GPU, both NativeLSTM and CudnnLSTM are faster than LSTMBlockFused (sometimes twice as fast).

Also as expected, on GPU, LSTMBlockFused is faster than LSTMBlock (up to 50%).

On GPU, LSTMBlock seems slightly faster than BasicLSTM/StandardLSTM but the difference is not so big.

Interestingly, on all experiments, on GPU, StandardLSTM seems to be slightly faster than BasicLSTM, which is not expected, as the BasicLSTM is simpler and also recommended by TensorFlow if you don’t need the extended options which are available for StandardLSTM.

On CPU, it again looks different, and not as clear. This depends also on how much CPU threads will be used, and on the hardware. For example, NativeLSTM is currently not well optimized to use multiple threads (intra op parallelism). See also TFUtil.setup_tf_thread_pools() about intra and inter op parallelism.

We see that with a very low number of threads, on CPU, NativeLSTM can be the fastest, but not necessarily. Increasing the number of threads, NativeLSTM can become the slowest.

On CPU, LSTMBlockFused seems to be the fastest despite NativeLSTM, no matter the number of threads.

On CPU, interestingly, BasicLSTM and StandardLSTM seem to be slightly faster than LSTMBlock.