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257 lines
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ReStructuredText
257 lines
11 KiB
ReStructuredText
How spaCy Works
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===============
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The following are some hasty preliminary notes on how spaCy works. The short
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story is, there are no new killer algorithms. The way that the tokenizer works
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is novel and a bit neat, and the parser has a new feature set, but otherwise
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the key algorithms are well known in the recent literature.
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Some might also wonder how I get Python code to run so fast. I don't --- spaCy
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is written in `Cython`_, an optionally statically-typed language that compiles
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to C or C++, which is then loaded as a C extension module.
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This makes it `easy to achieve the performance of native C code`_, but allows the
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use of Python language features, via the Python C API. The Python unicode
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library was particularly useful to me. I think it would have been much more
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difficult to write spaCy in another language.
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.. _Cython: http://cython.org/
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.. _easy to achieve the performance of native C code: https://honnibal.wordpress.com/2014/10/21/writing-c-in-cython/
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Tokenizer and Lexicon
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---------------------
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Tokenization is the task of splitting a string into meaningful pieces, called
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tokens, which you can then compute with. In practice, the task is usually to
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match the tokenization performed in some treebank, or other corpus. If we want
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to apply a tagger, entity recogniser, parser etc, then we want our run-time
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text to match the training conventions. If we want to use a model that's been
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trained to expect "isn't" to be split into two tokens, ["is", "n't"], then that's
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how we need to prepare our data.
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In order to train spaCy's models with the best data available, I therefore
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tokenize English according to the Penn Treebank scheme. It's not perfect, but
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it's what everybody is using, and it's good enough.
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What we don't do
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################
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The Penn Treebank was distributed with a script called tokenizer.sed, which
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tokenizes ASCII newswire text roughly according to the Penn Treebank standard.
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Almost all tokenizers are based on these regular expressions, with various
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updates to account for unicode characters, and the fact that it's no longer
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1986 --- today's text has URLs, emails, emoji, etc.
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Usually, the resulting regular expressions are applied in multiple passes, which
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is quite inefficient. Often no care is taken to preserve indices into the original
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string. If you lose these indices, it'll be difficult to calculate mark-up based
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on your annotations.
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Tokenizer Algorithm
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###################
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spaCy's tokenizer assumes that no tokens will cross whitespace --- there will
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be no multi-word tokens. If we want these, we can post-process the
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token-stream later, merging as necessary. This assumption allows us to deal
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only with small chunks of text. We can cache the processing of these, and
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simplify our expressions somewhat.
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Here is what the outer-loop would look like in Python. (You can see the
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production implementation, in Cython, here.)
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.. code:: python
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cache = {}
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def tokenize(text):
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tokens = []
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for substring in text.split(' '):
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if substring in cache:
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tokens.extend(cache[substring])
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else:
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subtokens = _tokenize_substring(substring)
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tokens.extend(subtokens)
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cache[substring] = subtokens
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return tokens
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The actual work is performed in _tokenize_substring. For this, I divide the
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tokenization rules into three pieces:
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1. A prefixes expression, which matches from the start of the string;
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2. A suffixes expression, which matches from the end of the string;
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3. A special-cases table, which matches the whole string.
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The algorithm then proceeds roughly like this (consider this like pseudo-code;
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this was written quickly and has not been executed):
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.. code:: python
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# Tokens which can be attached at the beginning or end of another
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prefix_re = _make_re([",", '"', '(', ...])
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suffix_re = _make_re(s[",", "'", ":", "'s", ...])
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# Contractions etc are simply enumerated, since they're a finite set. We
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# can also specify anything we like here, which is nice --- different data
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# has different quirks, so we want to be able to add ad hoc exceptions.
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special_cases = {
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"can't": ("ca", "n't"),
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"won't": ("wo", "n't"),
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"he'd've": ("he", "'d", "'ve"),
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...
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":)": (":)",) # We can add any arbitrary thing to this list.
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}
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def _tokenize_substring(substring):
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prefixes = []
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suffixes = []
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while substring not in special_cases:
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prefix, substring = _apply_re(substring, prefix_re)
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if prefix:
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prefixes.append(prefix)
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else:
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suffix, substring = _apply_re(substring, suffix_re)
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if suffix:
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suffixes.append(suffix)
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else:
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break
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This procedure splits off tokens from the start and end of the string, at each
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point checking whether the remaining string is in our special-cases table. If
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it is, we stop splitting, and return the tokenization at that point.
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The advantage of this design is that the prefixes, suffixes and special-cases
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can be declared separately, in easy-to-understand files. If a new entry is
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added to the special-cases, you can be sure that it won't have some unforeseen
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consequence to a complicated regular-expression grammar.
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Coupling the Tokenizer and Lexicon
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##################################
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As mentioned above, the tokenizer is designed to support easy caching. If all
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we were caching were the matched substrings, this would not be so advantageous.
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Instead, what we do is create a struct which houses all of our lexical
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features, and cache *that*. The tokens are then simply pointers to these rich
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lexical types.
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In a sample of text, vocabulary size grows exponentially slower than word
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count. So any computations we can perform over the vocabulary and apply to the
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word count are very efficient.
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Part-of-speech Tagger
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---------------------
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.. _how to write a good part of speech tagger: https://honnibal.wordpress.com/2013/09/11/a-good-part-of-speechpos-tagger-in-about-200-lines-of-python/ .
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In 2013, I wrote a blog post describing `how to write a good part of speech
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tagger`_.
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My recommendation then was to use greedy decoding with the averaged perceptron.
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I think this is still the best approach, so it's what I implemented in spaCy.
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The tutorial also recommends the use of Brown cluster features, and case
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normalization features, as these make the model more robust and domain
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independent. spaCy's tagger makes heavy use of these features.
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Dependency Parser
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-----------------
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.. _2014 blog post: https://honnibal.wordpress.com/2013/12/18/a-simple-fast-algorithm-for-natural-language-dependency-parsing/
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The parser uses the algorithm described in my `2014 blog post`_.
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This algorithm, shift-reduce dependency parsing, is becoming widely adopted due
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to its compelling speed/accuracy trade-off.
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Some quick details about spaCy's take on this, for those who happen to know
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these models well. I'll write up a better description shortly.
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1. I use greedy decoding, not beam search;
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2. I use the arc-eager transition system;
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3. I use the Goldberg and Nivre (2012) dynamic oracle.
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4. I use the non-monotonic update from my CoNLL 2013 paper (Honnibal, Goldberg
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and Johnson 2013).
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So far, this is exactly the configuration from the CoNLL 2013 paper, which
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scored 91.0. So how have I gotten it to 92.4? The following tweaks:
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1. I use Brown cluster features --- these help a lot;
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2. I redesigned the feature set. I've long known that the Zhang and Nivre
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(2011) feature set was suboptimal, but a few features don't make a very
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compelling publication. Still, they're important.
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3. When I do the dynamic oracle training, I also make
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the upate cost-sensitive: if the oracle determines that the move the parser
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took has a cost of N, then the weights for the gold class are incremented by
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+N, and the weights for the predicted class are incremented by -N. This
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only made a small (0.1-0.2%) difference.
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Implementation
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##############
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I don't do anything algorithmically novel to improve the efficiency of the
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parser. However, I was very careful in the implementation.
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A greedy shift-reduce parser with a linear model boils down to the following
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loop:
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.. code:: python
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def parse(words, model, feature_funcs, n_classes):
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state = init_state(words)
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for _ in range(len(words) * 2):
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features = [templ(state) for templ in feature_funcs]
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scores = [0 for _ in range(n_classes)]
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for feat in features:
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weights = model[feat]
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for i, weight in enumerate(weights):
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scores[i] += weight
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class_ = max(enumerate(scores), key=lambda item: item[1])
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transition(state, class_)
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The parser makes 2N transitions for a sentence of length N. In order to select
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the transition, it extracts a vector of K features from the state. Each feature
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is used as a key into a hash table managed by the model. The features map to
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a vector of weights, of length C. We then dot product the feature weights to the
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scores vector we are building for that instance.
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The inner-most loop here is not so bad: we only have a few dozen classes, so
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it's just a short dot product. Both of the vectors are in the cache, so this
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is a snack to a modern CPU.
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The bottle-neck in this algorithm is the 2NK look-ups into the hash-table that
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we must make, as these almost always have to hit main memory. The feature-set
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is enormously large, because all of our features are one-hot boolean
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indicators. Some of the features will be common, so they'll lurk around in the
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CPU's cache hierarchy. But a lot of them won't be, and accessing main memory
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takes a lot of cycles.
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.. _Jeff Preshing's excellent post: http://preshing.com/20130107/this-hash-table-is-faster-than-a-judy-array/ .
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I used to use the Google dense_hash_map implementation. This seemed a solid
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choice: it came from a big brand, it was in C++, and it seemed very
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complicated. Later, I read `Jeff Preshing's excellent post`_ on open-addressing
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with linear probing.
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This really spoke to me. I had assumed that a fast hash table implementation
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would necessarily be very complicated, but no --- this is another situation
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where the simple strategy wins.
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I've packaged my Cython implementation separately from spaCy, in the package
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`preshed`_ --- for "pre-hashed", but also as a nod to Preshing. I've also taken
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great care over the feature extraction and perceptron code, which I'm distributing
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in a package named `thinc`_ (since it's for learning very sparse models with
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Cython).
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.. _preshed: https://github.com/syllog1sm/preshed
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.. _thinc: https://github.com/honnibal/thinc
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By the way: from comparing notes with a few people, it seems common to
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implement linear models in a way that's suboptimal for multi-class
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classification. The mistake is to store in the hash-table one weight per
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(feature, class) pair, rather than mapping the feature to a vector of weights,
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for all of the classes. This is bad because it means you need to hit the table
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C times, one per class, as you always need to evaluate a feature against all of
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the classes. In the case of the parser, this means the hash table is accessed
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2NKC times, instead of the 2NK times if you have a weights vector. You should
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also be careful to store the weights contiguously in memory --- you don't want
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a linked list here. I use a block-sparse format, because my problems tend to
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have a few dozen classes.
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