spaCy/website/usage/_training/_tips.jade

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2017-11-07 03:05:37 +03:00
//- 💫 DOCS > USAGE > TRAINING > OPTIMIZATION TIPS AND ADVICE
p
| There are lots of conflicting "recipes" for training deep neural
| networks at the moment. The cutting-edge models take a very long time to
| train, so most researchers can't run enough experiments to figure out
| what's #[em really] going on. For what it's worth, here's a recipe seems
| to work well on a lot of problems:
+code("Batch heuristic").
def get_batches(train_data, model_type):
max_batch_sizes = {'tagger': 32, 'parser': 16, 'ner': 16, 'textcat': 64}
max_batch_size = max_batch_sizes[model_type]
if len(train_data) < 1000:
max_batch_size /= 2
if len(train_data) < 500:
max_batch_size /= 2
batch_size = compounding(1, max_batch_size, 1.001)
batches = minibatch(train_data, size=batch_size)
return batches
p
| This will set the batch size to start at #[code 1], and increase each
| batch until it reaches a maximum size. The tagger, parser and entity
| recognizer all take whole sentences as input, so they're learning a lot
| of labels in a single example. You therefore need smaller batches for
| them. The batch size for the text categorizer should be somewhat larger,
| especially if your documents are long.
p
| The trick of increasing the batch size is starting to become quite
| popular (see #[+a("https://arxiv.org/abs/1711.00489") Smith et al., 2017]).
| Their recipe is quite different from how spaCy's models are being
| trained, but there are some similarities. In training the various spaCy
| models, we haven't found much advantage from decaying the learning
| rate but starting with a low batch size has definitely helped. You
| should try it out on your data, and see how you go.
+h(3, "tips-hyperparams") Learning rate, regularization and gradient clipping
p
| By default spaCy uses the Adam solver, with default settings
| (learning rate #[code 0.001], #[code beta1=0.9], #[code beta2=0.999]).
| Some researchers have said they found these settings terrible on their
| problems but they've always performed very well in training spaCy's
| models, in combination with the rest of our recipe. You can change these
| settings directly, by modifying the corresponding attributes on the
| #[code optimizer] object. You can also set environment variables, to
| adjust the defaults.
p
| There are two other key hyper-parameters of the solver: #[code L2]
| #[strong regularization], and #[strong gradient clipping]
| (#[code max_grad_norm]). Gradient clipping is a hack that's not discussed
| often, but everybody seems to be using. It's quite important in helping
| to ensure the network doesn't diverge, which is a fancy way of saying
| "fall over during training". The effect is sort of similar to setting the
| learning rate low. It can also compensate for a large batch size (this is
| a good example of how the choices of all these hyper-parameters
| intersect).
+h(3, "tips-dropout") Dropout rate
p
| For small datasets, it's useful to set a
| #[strong high dropout rate at first], and #[strong decay] it down towards
| a more reasonable value. This helps avoid the network immediately
| overfitting, while still encouraging it to learn some of the more
| interesting things in your data. spaCy comes with a
| #[+api("top-level#util.decaying") #[code decaying]] utility function to
| facilitate this. You might try setting:
+code.
from spacy.util import decaying
dropout = decaying(0.6, 0.2, 1e-4)
p
| You can then draw values from the iterator with #[code next(dropout)],
| which you would pass to the #[code drop] keyword argument of
| #[+api("language#update") #[code nlp.update]]. It's pretty much always a
| good idea to use at least #[strong some dropout]. All of the models
| currently use Bernoulli dropout, for no particularly principled reason
| we just haven't experimented with another scheme like Gaussian dropout
| yet.
+h(3, "tips-param-avg") Parameter averaging
p
| The last part of our optimisation recipe is #[strong parameter averaging],
| an old trick introduced by
| #[+a("https://cseweb.ucsd.edu/~yfreund/papers/LargeMarginsUsingPerceptron.pdf") Freund and Schapire (1999)],
| popularised in the NLP community by
| #[+a("http://www.aclweb.org/anthology/P04-1015") Collins (2002)],
| explained in more detail by
| #[+a("http://leon.bottou.org/projects/sgd") Leon Botto]. Just about the
| only other people who seem to be using this for neural network training
| are the SyntaxNet team (one of whom is Michael Collins) but it really
| seems to work great on every problem.
p
| The trick is to store the moving average of the weights during training.
| We don't optimise this average we just track it. Then when we want to
| actually use the model, we use the averages, not the most recent value.
| In spaCy (and #[+a(gh("thinc")) Thinc]) this is done by using a
| context manager, #[+api("language#use_params") #[code use_params]], to
| temporarily replace the weights:
+code.
with nlp.use_params(optimizer.averages):
nlp.to_disk('/model')
p
| The context manager is handy because you naturally want to evaluate and
| save the model at various points during training (e.g. after each epoch).
| After evaluating and saving, the context manager will exit and the
| weights will be restored, so you resume training from the most recent
| value, rather than the average. By evaluating the model after each epoch,
| you can remove one hyper-parameter from consideration (the number of
| epochs). Having one less magic number to guess is extremely nice so
| having the averaging under a context manager is very convenient.
+h(3, "tips-transfer-learning") Transfer learning
p
| Finally, if you're training from a small data set, it's very useful to
| start off with some knowledge already in the model. #[strong Word vectors]
| are an easy and reliable way to do that, but depending on the
| application, you may also be able to start with useful knowledge from one
| of spaCy's #[+a("/models") pre-trained models], such as the parser,
| entity recogniser and tagger. If you're adapting a pre-trained model and
| you want it to retain accuracy on the tasks it was originally trained
| for, you should consider the "catastrophic forgetting" problem.
| #[+a("https://explosion.ai/blog/pseudo-rehearsal-catastrophic-forgetting", true) See this blog post]
| to read more about the problem and our suggested solution,
| pseudo-rehearsal.