Speeding up training¶

Author: Stephen Roller

This tutorial walks you through a few ways to massively speed up your training runs in ParlAI. These tricks tend to work best with generative models, but some can also be used with others.

:::{warning} Retuning-hyperparameters Many of these options are effectively tricks for changing the batchsize. Note that you will want to freshly tune your learning rate when using these tricks, or simply use these options in the first place.

This tutorial is only for illustration purposes of how to speed up training, and may not get you the best performing model. You will need to tune other hyperparameters like Learning Rate. :::

A summary of the speedups, for an 8 layer Transformer, is in this table:

Method	Train	Eval	Total	Speedup
Baseline	579s	60s	639s	1.00x
Skip generation	579s	20s	599s	1.07x
Dynamic batching	289s	18s	307s	2.08x
FP16	212s	11s	223s	2.60x
Larger batchsize (FP16)	168s	10s	178s	3.59x
Background preprocessing	144s	10s	154s	4.15x
Using 4 GPUs	63s	4s	67s	8.64x

Setting a baseline¶

We’ll start with an example training command, which trains a transformer/generator on ConvAI2 for 1 epoch, using a batchsize of 64, with a roughly 20M parameter model. We’ll train using ADAM optimizer, with a learning rate of 1e-3. We’ll build the dictionary ahead of time to ensure it’s kept the same. This will be our baseline.

parlai build_dict --task convai2 --dict-file dictfile
parlai train --dict-file dictfile --model transformer/generator \
    --task convai2 --num-epochs 1.0 --batchsize 64 --n-layers 8  \
    --embedding-size 250 --ffn-size 1000 --optimizer adam --learningrate 1e-3

On my computer using a 16gb V100, this takes about 550s. 500s is spend in training, and another 50s is spent in evaluation.

We will continuously modify this training command in this tutorial, but you are free to mix and match options.

Skip generation & larger eval batchsize¶

You may notice your model is taking a long time to evaluate, even though the evaluation dataset is much smaller. This is because we are doing full generation through the model, including beam search. You can get a massive speedup by turning off this generation step, with --skip-generation true.

Note that during evaluation, we don’t need to keep activations around for backpropagation, so we can also afford to increase the batchsize. About 2x commonly works, but 1.5x is more conservative.

parlai train --dict-file dictfile --model transformer/generator \
    --task convai2 --num-epochs 1.0 --batchsize 64 --n-layers 8  \
    --embedding-size 250 --ffn-size 1000 --optimizer adam --learningrate 1e-3 \
    --skip-generation true --eval-batchsize 128

This brings evaluation time down to 16s, but doesn’t affect training time. Just remember you will need to turn --skip-generation back off if you want statistics like BLEU or F1. Also, --skip-generation is only an option in generative models. Ranking models have similar options like -cands batch.

:::{warning} Persistence Models trained with --skip-generation True will remember this option when loading back up. You will need to manually set it back to false whenever you need evaluations with generations. :::

Dynamic batching¶

Dynamic batching groups conversations of the same length at the same time, to minimize the amount of unnecessary padding in the tensors. Furthermore, dynamic batching actually increases the batch size to use the maximum amount of memory available on your GPU.

Add --dynamic-batching full (-dynb full) to your training command. Note that in order to use dynamic batching, we must also set a --truncate option. We’ll use 256, since that is longer than almost all the conversations in our data. Note you can use the clen metric to help you set a good truncation.

parlai train --dict-file dictfile --model transformer/generator \
    --task convai2 --num-epochs 1.0 --batchsize 64 --n-layers 8  \
    --embedding-size 250 --ffn-size 1000 --optimizer adam --learningrate 1e-3 \
    --skip-generation true --eval-batchsize 128 \
    --dynamic-batching full --truncate 256

You should notice that your memory utilization is much higher in this mode. This actually has an advantage, since you can more easily find your maximum batchsize. To get the full benefits of dynamic batching, make sure to use the largest batchsize you can.

Overall, this results in a large increase in speed: about 2x, bringing training down to 250s and evaluation to 11s.

:::{warning} WARNING You may find perplexity is quite a bit worse than without dynamic batching. This is because we use larger batches, and take fewer steps. You can usually increase your learning rate pretty substantially when using dynamic batching, to compensate for the fewer steps. :::

FP16¶

If you have access to an NVIDIA GPU with FP16 CUDA Cores (V100, GTX 2080, etc), then you can get large speedups by switching on the option --fp16 true. There are two version of FP16 you may use: --fp16-impl safe and --fp16-impl mem_efficient. The latter trades some numerical stability in exhange for lower memory consumption, and typically works well in practice.

Note that in order to get the full benefit of fp16, we need to make sure all our hidden dimensions are multiples of 8, otherwise the hardware won’t use CUDA cores. We will slightly adjust the size of the network to support this. We’ll slightly adjust the network parameters (–embedding-size and –ffn-size) to conform to this.

parlai train --dict-file dictfile --model transformer/generator \
    --task convai2 --num-epochs 1.0 --batchsize 64 --n-layers 8  \
    --embedding-size 256 --ffn-size 1024 --optimizer adam --learningrate 1e-3 \
    --skip-generation true --eval-batchsize 128 \
    --dynamic-batching full --truncate 256
    --fp16 true --fp16-impl mem_efficient

Further notice that FP16 often significantly lowers the memory size of your model and activations (almost by a factor of 2). This means you can usually get away with significantly increasing the batchsize (and eval batchsize).

parlai train --dict-file dictfile --model transformer/generator \
    --task convai2 --num-epochs 1.0 --batchsize 64 --n-layers 8  \
    --embedding-size 256 --ffn-size 1024 --optimizer adam --learningrate 1e-3 \
    --skip-generation true \
    --dynamic-batching full \
    --fp16 true --fp16-impl mem_efficient --batchsize 128 --eval-batchsize 256

In this example, we see about a 25% speedup. Generally you can expect a larger speedup with larger models, with models of >300M often getting a ~50% speedup. With the increased batch size, this can often be brought to 2.5x faster.

:::{warning} FP16 requires modern GPUs Without a GPU with FP16 CUDA cores, you may find that FP16 actually slows your program. You may still see a benefit from the reduced memory usage though. :::

Background preprocessing¶

We can further speed things up by doing some preprocessing, such as tokenization and dialogue state tracking, in the background thread. This can be enabled by setting --num-workers to a value greater than 0. A good rule of thumb is to set --num-workers to the number of CPU cores you have PER GPU. On my server, there are 8 cores per GPU, so I will set it to 8.

parlai train --dict-file dictfile --model transformer/generator \
    --task convai2 --num-epochs 1.0 --batchsize 64 --n-layers 8  \
    --embedding-size 256 --ffn-size 1024 --optimizer adam --learningrate 1e-3 \
    --skip-generation true \
    --dynamic-batching full \
    --fp16 true --fp16-impl mem_efficient --batchsize 128 --eval-batchsize 256 \
    --num-workers 8

Use multiple GPUs¶

If you have multiple GPUs, you can utilize them by switching from train to multiprocessing_train. If you have 4 GPUs, you’ll find training should be roughly 3.5x faster. The arguments for the training are left otherwise the same.

parlai multiprocessing_train \
    --dict-file dictfile --model transformer/generator \
    --task convai2 --num-epochs 1.0 --batchsize 64 --n-layers 8  \
    --embedding-size 256 --ffn-size 1024 --optimizer adam --learningrate 1e-3 \
    --skip-generation true \
    --dynamic-batching full \
    --fp16 true --fp16-impl mem_efficient --batchsize 128 --eval-batchsize 256 \
    --num-workers 8

Note that we leave batchsize the same: we use the batchsize PER GPU. We expect the batchsize will be effectively 4x’d by using 4 GPUs.

Note that launching a multiprocessing train can take time to “warm up” and it may take time for you to realize speed improvements when using small, toy training runs like in this document. If we train for 4 epochs instead, our FP16 large-batch run takes 614 seconds and the multi-GPU training takes 222 seconds. Also, if you have access to a SLURM cluster, distributed_train is sometimes faster than multiprocessing_train. With SLURM, multi-GPU training takes 167 seconds for 4 epochs, or about a 3.7x speedup compared to single GPU.

Similarly, we also have the multiprocessing_eval command, for using multiple GPUs in evaluation.

:::{danger} This should never be mixed with options like --model-parallel true or --data-parallel true, as those options use different GPUs without multiprocessing. The BlenderBot3B and BlenderBot9B models both use those options, so this should be used with care. :::